When zoology professor Daniel Baldassarre traveled to the remote Galápagos Islands of Wolf and Darwin with two other researchers, getting there proved quite a challenge. With no beaches to land a boat on, they had to approach the islands’ cliffs in a small dinghy and wait for a lull in the waves before jumping onto sharp, black lava rocks.
But their efforts paid off. On that trip, they saw a bird that very few people have ever set eyes on in the wild: the vampire ground finch. These finches are only found on Wolf and Darwin, and feed on the blood of much larger birds called Nazca boobies that also live on the islands.
Up close, Baldassarre and his colleagues noted the vampire finches’ blood-stained bills and their throats visibly engorged with blood. They also saw them in action.
“We observed scores of vampire finches clamouring all over the backs, tails, and wings of boobies, opening up substantial wounds with their sharp beaks, and drinking their fill of blood,” they wrote in an article for The Conversation.
Why only some birds drink blood
“Blood is a notoriously difficult food source to process and extract nutrients from, which probably partly explains why so few animals have evolved to feed on it,” Baldassarre tells Popular Science. He suspects that on these islands where freshwater is extremely rare, blood might serve as a water source for the finches.
Two Nazca boobies perform their species’ mating call. Vampire ground finches feed on Nazca boobies on the remote Galápagos Islands of Wolf and Darwin. Image: Getty Images / inthedark72
It’s not the only component of the vampire finches’ diet, however. Based on unpublished data collected by Baldassarre and his colleagues, blood makes up about one-tenth of the vampire finches’ diet.
“These finches are quite opportunistic, so they will eat just about anything they can find,” Baldassarre says. “We observed them feeding on everything from seeds to insects to broken seabird eggs.”
One theory is that vampire finches rely more on blood feeding in periods of extreme drought, when other resources are scarce. But more research is needed to see if this idea holds up in practice.
How this strange behavior evolved
Vampire finches weren’t always vampires, scientists think. The relationship between vampire finches and boobies probably started off with the finches eating parasites found on the skin and feathers of the boobies. This would have been mutually beneficial because it provided a cleaning service for the boobies and food for the finches.
But at some point in evolutionary history, in the process of picking off parasites, the finches started to create lesions on the boobies’ skin, and eventually began feeding on their blood.
Over time, their bills became more scalpel-like, perfect for making skin incisions. Their digestive system also changed. When Baldassarre and his colleagues studied the microbes found in vampire finches’ guts, they found that their gut microbiome was very different from that of other, non-blood-consuming finches on the Galápagos Islands, something they attribute to their unique diet.
Until recently, vampire finches were considered a subspecies of the sharp-beaked ground finch (Geospiza difficilis).
“But they have now been designated as their own species [Geospiza septentrionalis] because of the differences they have evolved because of blood feeding,” Baldassarre notes.
Vampire ground finches on Wolf island have been seen drinking nectar from Galápagos prickly pear flowers and other flowers. Image: Peter Wilton / CC BY 2.0
How the boobies cope
Baldassarre and his colleagues have seen vampire finches gang up on and kill nestling boobies,“but that was quite rare,” he points out.
“In most cases, the finches seem to inflict just enough damage to break the skin and extract some blood, but not too much to harm the boobies significantly.” This is similar to many parasite-host relationships, such as fleas, ticks, and mosquitoes on mammals. “You can kind of think of the vampire finches as giant mosquitoes,” Baldassarre says.
“For one, as in large animals being bitten by mosquitoes, often the damage is minimal and not noticeable, so it doesn’t elicit a strong reaction. Also, the boobies sometimes have a vested interest in staying put and guarding their spot in the colony, or perhaps nest or nestlings, so may not want to fly away. Finally, the finches seem to target hard-to-reach areas on the boobies like the base of tail feathers, so it’s likely difficult for the boobies to deter them.”
I, for one, was feeling very sorry for the boobies being constantly hounded by these small vampire birds. Until I learnt that Nazca boobies practice something called “obligate siblicide,” which essentially means that the first-hatched chick routinely attacks and kills its younger sibling. It’s a harsh, bird-eat-bird world in the Galápagos Islands.
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An extensive new analysis further confirms the oldest known human footprints in North America date back to the last ice age (about 23,500–21,700 years). To pinpoint their origins, researchers at the University of Arizona relied on cutting-edge methods at the United States Geological Survey (USGS) Radiocarbon Laboratory to examine hundreds of thousands of prehistoric pollen grains extracted from the tracks. According to researchers writing this week in the journal Science Advances, the team can even describe some of the dramatic environmental fluctuations experienced by these prehistoric humans within a single generation.
New Mexico’s White Sands National Park contains more than stunning vistas and desert dunes. It’s also home to one of the most controversial (and consequential) prehistoric discoveries of the last century. In 2006, archaeologists located footprints inside a dry lakebed that they later determined to be the earliest evidence of humans in North America. But there was a problem. Until then, researchers largely agreed that the first humans migrated into the continent about 16,000–12,000 years ago via Beringia, the subcontinent once connecting Alaska to Russia.
However, the White Sands footprints were estimated to be around 23,000 years old. That implied a sustained human presence on the continent at least seven millennia earlier than expected. Various archeological camps continued debating the true age of the White Sands footprints for years, while others cited similar—and sometimes older— sites in both North and South America. Meanwhile, the dry lakebed’s rarely seen geological conditions allowed researchers like University of Arizona geoscientist Jessica Tierney to learn more than each footprint’s age.
“The White Sands sediments are unique in that they allow us to reconstruct environmental conditions during the last ice age at a very high temporal resolution, so that we can see fluctuations that were relevant for the humans present at that time,” Tierney said.
Combining geochemical sediment signatures with the fossilized pollen, researchers accurately determined ancient regional details such as plant distribution, precipitation, temperature, and aridity. They soon learned that the humans who made the White Sands footprints would have experienced incredibly dramatic environmental shifts within their own lifetimes. Within only a few decades, the surrounding conifer forests transformed into plains covered with oak and sage. Lake Oterio, a giant body of water that once covered New Mexico’s national park, fluctuated in size for about 1,500 years until it receded completely. Although these shifts were large, they actually closely followed similar global climate changes at the time.
“And throughout that whole sequence, people were walking there, leaving their tracks alongside those of mammoths, giant sloths and dire wolves,” added Tierney.
Many of the cutest animals on Earth have one appendage in common. Whether you’re thinking of an adorable raccoon, a kitten, or a puppy, it’s hard to imagine them without their fluffy tail. In fact, the overwhelming majority of animals have tails. Only a select few, like us humans, don’t have an external tail.
Sarah Durn: Think of some of the cutest animals you know, a puppy, a kitten, maybe a little red panda. Now, picture them without their tails.
It’s weird, right?
Tails are such a normal part of the animal world that we barely think about them, and they’re not just adorable accessories.
A kangaroo uses its big muscular tail to help it balance. Opossums can use their tails to grip branches. Fish use theirs to propel them through the water. And your dog can communicate an entire emotional journey with just a few well-timed wags.
Tails can help animals climb, swim, steer, balance, communicate, even swat away an annoying bug.
So what happened to ours? Because humans are part of a huge evolutionary family where tails are basically standard equipment.
We even briefly start developing a tail when we’re embryos. But look behind you now, and unless something has gone very unexpectedly, you probably don’t have one.
And neither do chimpanzees, gorillas, orangutans, or any of our other ape relatives. So somewhere deep in our evolutionary past, our ancestors lost their tails. But why? And what would life be like if we still had them?
No question is too simple or too out there. I’m Sarah Durn, an editor at Popular Science.
Annie Colbert: And hi, I’m Annie Colbert, editor-in-chief
SD: Here at PopSci, we’re always pondering quirky questions
AC: And this week we’re wondering: Why don’t humans have tails? Because tails are everywhere in the animal kingdom.
SD: And they’re incredibly useful. Beavers use their big flat tails to help maneuver through the water. Some monkeys can basically use their tails like an extra limb, wrapping them around branches as they climb. And rattlesnakes, of course, turn theirs into a built-in warning system
AC: So what happened to ours? Did we just lose them somewhere?
SD: Well, technically, humans never lost them.
AC: Okay.
SD: But that said, we have to go way back in our family tree, tens of millions of years back, to find out why our butts don’t have tails anymore.
AC: I’m guessing the answer isn’t because of pants?
SD: No, you are correct. This goes way beyond pants. We’re talking back to a time when our ancient ape ancestors were still hanging out in trees
AC: So when did tails disappear then? And more importantly, why would you want to give up a perfectly good tail?
SD: That’s the big question because for lots of animals, tails are incredibly useful for balance. But the way our ancient ape ancestors moved through the trees may have actually made a tail kind of pointless.
AC: So evolution really said, “We’re not using this anymore, so just get rid of it.”
SD: Yeah, pretty much. And later we’re going to get into what those ancient apes looked like and why their particular style of tree climbing may have made tails more trouble than they’re worth.
AC: And naturally, we also have to ask the important question: Would life actually be better if humans still had tails?
SD: I have some thoughts.
AC: Same. Many thoughts.
Now, before we swing into our evolutionary past, we want to know what questions are circling in your brain hole. If there’s something you’ve always wanted to know, submit your questions by clicking the “Ask Us” link at popsci.com/ask.
Again, that’s popsci.com/ask and click the “Ask Us” link
SD: We can’t wait to hear your questions
AC: We’ll be back right after this quick break to find out how humans ended up on team no tail.
SD: And stick around to the end because we’ll be joined by Science Friday host Flora Lichtman to discuss what makes a really good tail and whether she’d want one herself.
And we’re back. Okay, Annie, before this episode, how much time have you spent thinking about your tail?
AC: Absolutely none. I can confidently say I have never yearned for a tail.
SD: Yeah. No, I mean, neither have I, but I have spent a lot of time thinking about my tailbone.
AC: Uh-oh, where is this going?
SD: Yeah. Uh, a couple years ago, I was walking my Saint Bernard, Poochie, when he caught a whiff of something and took off. And me, trying to be a responsible dog walker, tried to hold onto the leash.
AC: Oh no.
SD: Yes, yes, yes. Safe to say Poochie, the Saint Bernard, won, and I fell straight onto my butt.
AC: Oh, Sarah. Ouch
SD: Yes, it was un-ideal. I ended up bruising my tailbone pretty bad.
AC: Mm.
SD: And unfortunately for me, this happened about a week before I left for a two-week long trip to Italy.
AC: Oof.
SD: Rome, Pompeii, the Colosseum, the Amalfi Coast, Puglia, all while carrying a little donut butt pillow everywhere I went.
AC: Oh my.
SD: Sometimes I even wore it as a hat.
AC: I mean, obviously.
SD: You know, sun protection is important.
AC: Crucial.
SD: Yeah, so I have actually spent a bit of time thinking about and cursing our teeny-tiny, mostly not there human tail because out we actually do have a tail. Kinda.
AC: Hmm. All right. Tell me more
SD: That tailbone I bruised, the coccyx, is at the bottom of your spine.
It’s basically what’s left of when our ancestors actually had a proper external tail.
AC: But humans had more of a tail at one point, right? Like fetuses have tails in the womb?
SD: Correct. When human embryos are developing, we actually form a little tail that extends beyond the rest of the body. But by around eight weeks, that external tail disappears.
AC: So I briefly had a tail?
SD: You did!
AC: Ugh, RIP. Gone too soon.
SD: We should all have a, like, little tail funeral upon our birth.
AC: Eh, that’s obviously the only normal response here.
SD: I mean, obviously.
Okay, but here’s the thing. Humans didn’t suddenly decide to ditch our tails. We inherited our tailless bodies from our ape ancestors.
So if we really want to figure out why humans don’t have tails today, we have to first figure out how our furry ancestors lost theirs. And for that, we have to go way back.
AC: How far back are we talking?
SD: Well, first let’s go back about five or six million years.
That’s roughly when scientists estimate the last common ancestor of humans, chimpanzees, and bonobos lived, and that doesn’t really help us with our tail mystery
AC: Because chimps and bonobos don’t have tails either.
SD: Exactly. And neither do gorillas, orangutans, or any other modern ape. So we have to keep going back.
AC: So rewind. Whoop, rewind.
SD: And eventually we get all the way back to around 25 to 30 million years ago. Megalodon is stalking the seas. Daeodon, aka “hell pigs,” were roaming North American forests. And keep in mind, we’re still tens of millions of years before the first saber-toothed cats and mammoths pop up.
And this is where things get interesting because around this time, our ape ancestors branched off from the ancestors of modern monkeys.
AC: And monkeys do have tails.
SD: Right. Most do.
So somewhere around this part of our family tree, around 25 to 30 million years ago, we finally crossed over to the tailless side from our tailed ancestors.
The frustrating thing is the fossil record doesn’t give us a perfect family album. We can’t point to one specific ancient ape and say, “There, that’s our ancestor, that’s the one that lost the tail.” But we can find clues, and one of the best clues comes from an ancient ape called Ekembo.
AC: Tell us your secret, ancient ape
SD: Yes! Tell them. So fossils from this genus have been found in Kenya and date back about 17 to 20 million years. And these animals would’ve looked pretty different from modern apes.
One species, Ekembo heseloni, probably had arms and legs that were roughly the same size. It climbed around on tree branches on all fours, and it had a long, flexible lower back, unlike modern apes. But there’s one thing we know it didn’t have.
AC: A tail?
SD: Exactly. And we can tell because of particular bone, the sacrum.
AC: And where is my sacrum?
SD: It’s right at the base of your lower spine, and it’s actually one of the places scientists can look to to figure out whether an animal could have supported a tail or not.
If you look at the sacrum of a cat or another tailed mammal, it leads into a whole bunch of tail vertebrae.
But in humans and apes, the sacrum just ends in a, you know, little point. And scientists see that little point in Ekembo heseloni. So even though this animal lived up to 20 million years ago, we know they didn’t have a tail.
AC: So by 20 million years ago, the tail was already gone.
SD: At least in Ekembo.
AC: Mm.
SD: And it’s not alone.
Another ancient ape called Nacholapithecus shows up in Kenya around 15 million years ago, and fossils of its sacrum show the same thing. It couldn’t have supported a tail either.
So we don’t know exactly which one of these ancient apes ultimately gave rise to later hominids, aka our ancestors, but we do know that tailless apes were already walking or climbing around millions and millions of years before humans existed.
AC: Okay, so we know the tail disappeared way back in our evolutionary history, but that still leaves the biggest question, which is why? Like, why would you give up a tail? They seem kind of awesome.
SD: And this is where we get into a little bit of scientific speculation because we don’t have one definitive answer.
One possibility is that it has to do with the way these early apes moved. Think about modern animals that really benefit from tails.
If you’re a cheetah sprinting and making incredibly fast turns, a tail can help you balance. Or if you’re a monkey moving quickly through the trees, a tail can be really useful. But these early apes weren’t moving like modern monkeys.
AC: So what were they doing?
SD: Well, millions of years ago, these apes probably had arms and legs that were about the same length, and scientists think they walked on all fours.
They were also climbing through trees looking for fruit. And if you’re trying to get some delicious fruit growing way out on the edge of a branch, you don’t necessarily want to go flying through the canopy.
You want to move slowly and carefully they could support their weight across multiple branches and deliberately work their way towards the good stuff.
AC: So less swinging dramatically through the jungle…
SD: …and more very cautious trip to the fruit bowl.
AC: Got it.
SD: And if you’re moving that slowly and deliberately, a tail might not give you much of an advantage. In fact, it could actually come with costs.
Your body has to spend energy growing this whole extra appendage, and then you’re dragging it around behind you where a predator could potentially grab it.
So as the University of Missouri anatomist Carol Ward told Popular Science, for these slow-moving, careful climbers, the tail just didn’t offer an advantage.
AC: So after all of this, the answer to why don’t humans have tails is basically because our ancestors stopped needing them.
SD: That’s one theory, but the important thing is that we didn’t lose our tails because humans started walking upright or because of something that happened recently in human evolution.
By the time anything resembling a human showed up, tails had already been absent from our branch of the family tree for millions and millions of years
AC: So I never really stood a chance.
SD: Nope. You were born into a long, proud lineage of tailless apes.
AC: Yet after this whole conversation, I’m starting to feel like we kinda got a bad deal.
SD: Yeah, well, before you get too jealous of Team Tail, there is someone I want us to talk to because last year Science Friday host Flora Lichtman went looking for an answer to a very important question. If you are gonna have a tail, what’s the best one to have?
AC: Ooh, yes. Let’s talk upgrades.
SD: Yes, let’s. We’ll talk with Flora about the science of tails, whether there really is a best tail for balance, and whether she’d actually want one herself. That’s after this quick break.
And we’re back with Flora Lichtman. Flora is a podcaster extraordinaire, an Emmy-nominated writer for Bill Nye’s Netflix show, Bill Nye Saves the World, and one of the hosts of Science Friday, a public radio program and podcast about science, nature, technology, and health.
Flora, thank you so much for making the time to come on Popular Science’s Ask Us Anything podcast.
Flora Lichtman: I am delighted to be here. Thank you for the invitation, and I love tails, so glad to talk about them.
SD: So yes, as you know, throughout this episode, we’ve been talking all about why humans don’t have tails when so many animals do, and last year you actually explored an important piece of this puzzle on a Science Friday episode called “In Search of the Best Tail for Balance,” which we’ll link in the show notes.
In the episode, you talked with roboticist Dr. Talia Moore and developmental biologist Dr. Ceri Weber about their work into figuring out what’s the best tail, and how a tail could actually help us build better robots.
So obviously I have to ask: Is there actually a best tail for balance?
FL: You know, of course, if you ask scientists, they’ll never give you a best because that’s their, their whole job is to find the nuance and the complicated elements of every story. But what I did learn from them is that there are ways to design tails so that they do their job better.
SD: Hmm.
FL: And the thing that I didn’t know about tails is that they’re an inertial appendage, which is not a term I’d heard.
But the way these researchers described it is that it’s like a tightrope walker’s stick.
SD: Hmm.
FL: That you use to sort of regain your balance. Like that, for animals with tails, that is a primary reason why they have a tail.
SD: Yeah.
FL: The difference, though, between a tightrope walker stick and a tail is that most tails aren’t rigid rods, right? Like, they’re curvy and they’re twisty.
So what these researchers did is this simulation to try to compare, you know, what tails work best as this inertial appendage, and they found that design matters, and, like, you can optimize basically the bones in the tail to make them work better. And they found, it turns out if you have, like, short bones, then long bones, then short bones, that gives you a very good tail.
And it turns out that a lot of mammals in the animal kingdom have tails just like this.
SD: Wow. Amazing. So after spending all of this time thinking about tails: Would you want one?
FL: You know, Sarah, I’ve been thinking about that. Here’s the problem. I already have such a problem finding pants that I like and that fit, so I feel like the tail is just not a challenge I need.
On the other hand, I’m also very clumsy, and I do feel like maybe a tail would’ve helped me in the past.
SD: Yes.
FL: …keep my balance.
SD: Yeah. Yeah, I mean, like theoretically, pants aside, if you did have a tail as a human, what would be different? What would be harder? What would be easier?
FL: I mean, besides my look, I think, the thing that would be easier is walking on a slippery, icy day in the Northeast.
SD: Mm-hmm.
FL: I think they do really help you regain balance. I like the idea of being able to swing from the trees.
SD: Yeah.
FL: I’m at the playground a lot with my kids. I feel like it would be a real flex to get on those monkey bars with a tail.
SD: Oh my God, you’d be the coolest mom.
FL: The something-ist mom for sure.
Would you want a tail?
SD: I think, yeah, I mean, I guess, like, I think I would. I think it’d be fun. I feel like it’s kinda like having wings, you know?
You’d be definitely, like, ostracized a little bit. Like, people would look at you funny, but you could do a lot of cool stuff. I don’t know. I look at squirrels, and they’re basically flying,
FL: That’s true.
SD: Okay, so you also encountered a lot of impressive tales, I’m sure, in your reporting. Do you have a favorite tale from the animal world?
FL: I’m really love seahorse tails.
SD: Oh, yeah.
FL: They’re so iconic and twisty and beautiful and they really need them because they’re these lightweight little animals that are, like, being, you know, battered by ocean waves, and they can use them to hang on.
But I was reading that not only that, which is like, that’s all cool, but actually, while most tails are cylindrical, the seahorse tail actually has a square profile, which I thought was cool, too.
So it’s… I don’t know. I’m into the seahorse tail.
SD: Yeah, no, excellent choice. Excellent, excellent choice. Flora, thank you so much for taking the time to talk tails with us. It was absolutely lovely.
FL: Delighted to be here. I would love to come back anytime.
SD: Oh, we’d love to have you.
AC: Okay, I started this episode having never really thought about having a tail, and now I definitely feel like maybe I’m missing out. Like, when I need help holding things, give me a tail.
SD: Same. Although after my tailbone incident, maybe I shouldn’t be trusted with an additional appendage.
AC: Yeah, that’s maybe for the best.
And that’s it for this episode. But don’t worry, we’ve got more episodes of Ask Us Anything live in our feed right now. Follow or subscribe to Ask Us Anything by Popular Science wherever you enjoy your podcasts.
And if you like our show, please leave a rating and a review
SD: If you could have any animal’s tail, which one would you want? Tell us in the comments.
AC: Yes, we want to know.
SD: We wanna know. Our producer is Alan Haburchak. This week’s episode was based on an article written for Popular Science by Sarah Kiley Watson, and a big thank you to Flora Lichtman for joining us.
AC: Thank you team, thank you tails, and thank you everyone for listening
SD: And one more time, if you want something you’ve always wondered about explained on a future episode, go to popsci.com/ask and click the “Ask Us” link. Until next time, be careful with your tailbone and keep the questions coming.
AC: Wait, if we had tails, what would chairs look like? And pants, and car seats.
SD: Planes? It’d be kind of cute to see all the tails lined up.
AC: I feel like we need tails. It’s time to change up humans.
In exactly a month, on November 13, 2026, the world will end. Fittingly, our impending apocalypse falls on Friday the 13th.
At least, that was the prediction made by University of Illinois physicist Heinz von Foerster in a highly cited Science paper published in 1960. Though talks of supposed doomsday have renewed in recent months thanks to hyperbolic anxieties over runaway, sentient artificial intelligence, von Foerster had a much different calamity in mind.
For more than a century, the prevailing fear, dating back to English economist Thomas Malthus who lived between 1766 to 1834, was of a booming population outrunning its food supply. Famine would ensue. But Von Foerster broke from that view. He assumed (correctly, as it turns out) that human ingenuity would keep the world mostly fed. The problem, he argued, was that the population would keep accelerating, toward “infinity.”
In a now infamous quote, von Foerster predicted, “Our great—great—grandchildren will not starve to death,” von Foerster and his co-authors wrote. “They will be squeezed to death.” Homo sapiens’ last gasping breath, they predicted, would peter out in the fall of 2026.
It’s pretty clear that von Foerster’s estimate was dead wrong. Not only is the global population (around 8.3 billion people) not expanding near the rate it was when he was writing, but there are actually concerns it’s moving in the opposite direction. In dozens of countries around the world, from South Korea and Japan to much of Western Europe, the concern among demographers has shifted from worries about overpopulation to decliningbirth rates.
So how was a prediction so wrong and still so influential? Human population experts speaking with Popular Science say von Foerster and his contemporaries based their prediction on real trends from the past while utterly failing to see the world changing right before their eyes. Though they may have accurately measured growth rates up to that point, that only tells part of the story.
“He got the picture right just as that picture ceased to be right,” Rockefeller University and Columbia University professor and mathematical biologist Joel Cohen tells Popular Science. Cohen has studied human population changes for years and published the influential 1995 book How Many People Can the Earth Support?
“Von Foerster didn’t know at the time, and in fact, nobody knew” that the all-time population growth had already happened, Cohen adds. “The all-time peak happened in the early 1960s.”
Factory employees work at the Stepney Wheel Factory building tires for early automobiles in Wales during the Industrial Revolution. Image: Heritage Images / Contributor / Getty Images Heritage Images
Why did experts think the world would end on Friday the 13th?
Von Foerster and his colleagues published their “Doomsday” paper on November 4, 1960. The dramatic pronouncement hinges on an equation that projects the human population into the future based on past trends.
The team gathered 24 estimates of world population dating back 2,000 years. The key assumption underpinning their logic was that humans differ from other animals because of our unique ability to cooperate and innovate.
Fruit flies in a jar eventually level off as crowding and competition take their toll. Humans, on the other hand, the authors argue, work the other way. The more of us there are, the better we get at cooperating. And the better we get at cooperation, the faster our population grows.
Follow that curve forward, and the population doesn’t just grow; it rockets toward infinity, hitting the so-called mathematical wall on November 13, 2026, give or take five years.
Von Foerster wasn’t alone in sounding the alarm either. Eight years later, biologist Paul R. Ehrlich would publish The Population Bomb, arguably the most influential single text fueling overpopulation fears.
“Both of those guys were right looking backward, but wrong looking forward,” Cohen says.
At a glance, thinking the future will resemble the past seems pretty reasonable. The human population grew steadily but slowly over much of our roughly 300,000 years of humanity. But that changed radically with the onset of globalization, colonization, and the Industrial Revolution.
How fast is the human population growing?
As Darrell Bricker and John Ibbitson note in their 2020 book Empty Planet: The Shock of Global Population Decline, human population passed the one billion mark somewhere around 1800. That doubled to two billion by 1927. Then doubled again just less than 50 years later in 1974. It would then only take another 13 years to cross the five billion mark. And in 2022, we surpassed eight billion.
That remarkable explosion had many causes, Cohen says, but among the most important was the spread of cheap, calorie-dense crops like potatoes and corn from the Americas.
The proliferation of cheap, calorie-dense crops like potatoes and corn from the Americas helped fuel a population boom in the 20th century. Image: FPG / Staff / Getty Images FPG
Crucially, though, humans weren’t necessarily having way more kids. They were just far better at keeping them alive.
Human population booms weren’t because people were having more kids
“Humanity got better at keeping the people who were born alive through simple, but important innovations like the germ theory of disease, sanitation, nutrition,” University of Texas Associate professor and economic demographer Dean Spears tells Popular Science.
Demographers like Spears study human populations and how they change over time. He published a book on the topic titled After the Spike, earlier this year. Advances in nutrition and medicine “caused early life mortality rates to fall, so more of the humans who were born survived and that caused the population to grow.”
Factoring in longer life expectancy and reductions in child mortality around the world blows a huge hole in Von Foerster’s calculations. Death rates can only fall so far, and once most children survive, families tend to have fewer of them.
The population boom was a one time explosion, not an ever-accelerating loop.
Overpopulation fears are actually based on a misunderstanding
Von Foerster’s assumption of endlessly accelerating growth, Spears says, was “based on a misunderstanding of where population growth comes from.”
Experts specializing in human population studies at the time actually knew that was the case, Spears notes.
Neither von Foerster’s nor Ehrlich (who wrote The Population Bomb) were human population experts. Von Foerster’s background was in physics and philosophy. Ehrlich was a biologist. But they were both great at getting attention.
Cambridge-educated Malayan medical doctor Wu Lien Teh was among the first to start using medical face masks during a deadly outbreak of the Manchurian plague in 1910. Image: Public Domain
“There was never, even at that time, any reason to think that it would keep growing fast like that.” Spears says. “Paul Ehrlich was a biologist who studied butterflies, not human demography.”
Why people are worried about declining birth rates now
In the later 20th century, population growth actually started declining quite rapidly thanks to a convergence of a variety of factors.
The “Green Revolution” in farming meant that more food could be grown on the same plot of land, which extinguished fears of humanity suddenly starving itself. The first birth control pill, approved by the Federal Drug Administration (FDA) in the U.S. in 1957, also gave women more autonomy to decide when and how many children to have.
These trends, along with global population shifts away from rural areas and towards more urban areas, would eventually shift concerns away from overpopulation and towards declining birth rates.
Although demographers and policy experts debate how much people should worry about declining birth rates, it is generallyagreed that an older population that significantly outnumbers younger people can lead to long-term strains on the country’s economy and social services.
With fewer workers needed in the field, more and more people moved to cities. Historically, families living on farms or agricultural communities tend to have more children than those in urban areas because those children could work the land and, in a purely economic sense, provide more labor and, hopefully, make the family more money. In more urban areas, another child meant another mouth to feed rather than an extra pair of hands.
In the later part of the 20th century, women in many parts of the world gained far more autonomy through education, work, and access to contraception, which demographers agree is among the biggest drivers of demographic change over the past half century.
Some governments went further. China’s one-child policy (introduced in 1979) and India’s forced sterilization campaigns (between 1975-1977) were heavy-handed responses fuelled by overpopulation panic. What makes those coercive policies all the more tragic, Spears notes, is that birth rates likely would have fallen without them.
“What we’re seeing now, a world of low and falling birth rates, is actually nothing new, but just the latest moment in a decades-long or longer trajectory of falling global birth rates on average happening essentially everywhere,” Sears says.
“I would say that the coercive [state-sanctioned] school [of thought] was counterproductive,” Cohen adds. He went on to say he believes similar government-led initiatives encouraging families to have more children today, like the Trump administration’s proposed baby medal, are “counterproductive.” Real, meaningful generational changes in population, Cohen notes, are more often the product of society-level macro trends beyond the scope of any one politician or party.
Today, about a month shy of von Foerster’s doomsday, around 55 percent of the world’s countries, home to more than two-thirds of its people, have fertility rates below replacement level. That’s the rate (about 2.1 children per woman) demographers use to denote where a population replaces itself from one generation to the next.
In other words, not only is the human population not hurtling towards infinity, but in much of the world, it’s actually on track to eventually shrink. Experts largely agree birth rates are falling. Whether that’s a problem to be solved is less clear-cut.
At the family level, having fewer children can mean more money, resources, and personal attention invested in each one. It can also mean more independence, opportunity, and choice for those who decided parenthood just isn’t for them.
Stabilizing birth rates isn’t a fix all
At the broader economic or societal level, though, Spears argues there are good reasons to want birth rates to stabilize rather than keep falling. Pay-as-you-go social programs like Social Security depend on young workers to fund older people’s retirements. Fewer people also means fewer shared resources and services for everyone else.
“If you want to go out and eat lunch at a taco truck, but you’re the only person nearby who wants it, then the taco truck’s not going to be there,” Spears says. “And if you’re a school child wanting to go to school, there might not be enough other children for there to be a classroom nearby that’s suitable for you.”
Those are legitimate concerns, though Cohen warns that focusing too much on abstract numbers and trends can blur a more immediate issue directly in front of us. Globally, children are still regularly going underfed, so much so that the World Health Organization estimates around 22 percent of children under the age of five experience stunted growth from malnutrition.
“We are doing so poorly by the children we have that I can’t understand why the emphasis is on making more children,” Cohen says. “We’re talking about having more children when we don’t take care of the children we have. What the heck is that?”
But even for those who do worry about falling birth rates, Spears cautions against politicians promising quick fixes. This isn’t a short-term emergency, he says, but a “generational challenge.” One thing is clear though: Neither Spears nor Cohen are losing sleep over von Foerster’s November 13 death date.
“I’m sure not acting as if I’m too concerned about it,” Cohen says. “I’m still paying my bills. I’m still paying my taxes.”
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Anyone who’s seen the 1986 musical comedy, Little Shop of Horrors, is familiar with Audrey II. Audrey II looks like a towering, blood-eating Venus flytrap plant, even mimicking the jaw-like, snapping pod of real carnivorous plants. But while Venus flytraps consume insects, Audrey II feeds on humans.
On the other hand, plants that trap and digest bugs and, on occasion, small rodents are not only real, but actually quite diverse. Popular Science turned to Coogan to find out why some plants are carnivorous, while others simply soak up their necessary nutrients straight from the soil. We also asked Coogan to dig into all the details behind what makes these plants deadly, at least to birds and other small animals.
What are carnivorous plants?
Carnivorous plants are predatory flowering plants that attract, catch, and absorb animals for their nutrients. They’re both autotrophs, which are organisms that make their own food using light or chemical energy, and heterotrophs. This means that they also have to eat animal prey to survive.
“Carnivorous plants photosynthesize,” says Coogan. “They can use the sun to convert carbon dioxide and water into sugar,” he says, for substance. “They’re just getting some extra minerals or nutrients from the bugs or the organisms that they eat.”
In the 1986 film Little Shop of Horrors, a giant Venus flytrap-like plant proves deadly. Video: Little Shop of Horrors 4K | Seymour’s TV Interview Goes Horribly Wrong, Warner Bros. Entertainment
Meat-eating plants exist on every continent save Antarctica, and grow in sun-filled places with extremely poor, acidic, and nutrient-free soil. For instance, the bogs, swamps, and marshes of North Carolina and Florida, with their unique combination of sunlight and wetlands, allow these uniquely adapted flora to thrive. This includes Venus flytraps, which are native only to the U.S. Carolinas, and Tracy’s sundew, with its long, thread-like leaves that grow in spirals. It’s often seen blooming alongsidecarnivorous pitcher plants in the Florida Panhandle.
“Carnivory is primarily an adaptation for plants to get nutrients in low nutrient environments,” says Coogan. Carnivorous plants feed on insects to obtain the nitrogen, phosphorus, and other organic nutrients they need to survive, but are unable to absorb from the soil.
This isn’t to say that all plants that grow within poor-soil wetlands are carnivores. Take wiregrass, which is quite prominent in the southeast. “It has really heavy root systems,” says Coogan, which have formed a symbiotic or long-term relationship with mycorrhizal fungi. These fungi attach to the wiregrass roots and help them form nutrients from the region’s poor, sandy soils.
“There’s only a certain number of niches to be filled in an ecosystem, so some plants went one way,” meaning they became carnivores, “and then some plants went the other way.”
How did carnivorous plants evolve?
There are an estimated 800 to 860 species of carnivorous plants worldwide, with new species constantly being discovered. In fact, in the more than 140 million years that flowering plants have been around, carnivory has evolved independently at least a dozen times.
Carnivory is not inherited from a single meat-eating plant ancestor. Instead, carnivorous plants’ highly adapted leaf structures, which include sticky traps, snap traps, and pitfall traps for catching insects, have evolved and diversified more recently, largely between eight and 72 million years ago.
Carnivorous plants are a classic example of convergent evolution. This means that unrelated lineages have all developed similar traits separate from one another to adapt to their sunny, waterlogged, and nutrient-poor environments.
For example, “There are many different kinds of pitcher plants,” carnivorous plants that have hollowed-out pitcher-shaped leaves that act as pitfalls, says Coogan, and they’re completely unrelated. You’ve got low-growing Australian pitcher plants, which are evolutionarily closer to roses, beans, and broccoli, Asian pitcher plants that hang from tropical vines, and upright, trumpet-like North American pitcher plants. Despite their completely different DNA, they’ve all evolved the exact same, tube-shaped “pitfall trap,” says Coogan, to catch and digest insects.
Through convergent evolution, unrelated carnivorous plants like Venus flytraps, sundews, and pitcher plants have also independently repurposed ancient self-defense genes into ingenious digestive tools. While noncarnivorous plants have long used enzymes, called chitinases, to fight off fungal infections, carnivorous plants do the opposite. They use chitinases to break down the tough, chitin-rich exoskeletons of trapped insects. This then allows them to absorb vital nutrients like nitrogen and phosphorus, transforming what were originally passive, protective enzymes into active weaponry.
Are carnivorous plants harmful to larger organisms?
“I would say ‘no,’’ says Coogan. Almost all carnivorous plants feed exclusively on small insects like flies, mosquitoes, and spiders.
While there are some, like giant pitcher plants, that grow large enough to trap and digest small animals like rats and shrews, “the relationship is more mutualistic,” meaning both species benefit from each other.
This type of pitcher plant are the largest in the world and often consume small mammals. Image: Austin American-Statesman/Hearst Newspapers / Contributor / Getty Images Sara Diggins
Basically giant pitcher plant species produce a sweet, aromatic nectar on the underside of their pitcher lids that attracts small mammals. These mammals “sit on the lid of the pitcher plant to drink the exudate, which is a thick nectar-like substance,” says Coogan.
They’ll often defecate into the plant while feeding, and the giant pitcher absorbs nutrient-rich nitrogen and phosphorus directly from the droppings. It’s a win-win for both parties. “And one of many examples of mutualism in carnivorous plants.” If a small animal accidently falls in, the pitcher plant will then use its acidic digestive enzymes to consume it.
Some incredible carnivorous plants
Coogan’s favorite carnivorous plants run the gamut from small, semi-aquatic Genlisea, commonly known as corkscrew plants, to bladderwort, tiny, bladder-shaped traps that catch small, aquatic creatures and often resemble orchids.
“There are over 200 species of them,” he says, “and they come in three different habitat types. You have fully aquatic bladderwort that are floating in water. You have terrestrial ones that live in kind of soggy soils, and then you also have epiphytic ones that are living in trees and in moss.” They all use ultra-quick, vacuum-driven suction traps to capture prey.
In fact, bladderwort is one of the fastest predatory plants on Earth, and can suck in its prey in less than one millisecond. “The real reason I like them is because they are proliferous, meaning they flower en masse.”
Also popular: butterwarts. “Especially the Mexican butterworts or Pinguicula,” says Coogan. “The reason is that they’re excellent house plants. They look like succulents, but their leaves are sticky and they eat fruit flies and fungus gnats.” They act as a natural form of pest control.
What’s the largest carnivorous plant?
The world’s largest carnivorous plant by trap volume is the giant montane pitcher plant. It’s endemic to Malaysian Borneo and features massive urn-shaped traps that can grow up to 16 inches tall and eight inches wide. While lizards, frogs, and birds do occasionally fall victim to the plant’s digestive enzymes and acids, most wildlife is in the clear.
Humans can also count themselves safe, Audrey II aside.
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Ecuador’s Galapagos Islands are home to an incredible array of wildlife. Amid landscapes of black lava fields, scattered prickly pear cacti, lush highlands, and green mangrove fringes, you’ll spot lumbering land iguanas in striking shades of yellow, orange, and red, comical blue-footed boobies, and some of the pinkest flamingos around. But it turns out that these magnificent creatures aren’t born with their vibrant hues. They actually have to work for them!
To find out why some wildlife changes color according to their diets, and what these various tones and tints mean for survival,Popular Science turned to Jorge Alex Gutierrez Guerrero, an expedition leader on the Galapagos Islands’ Yacht Isabela II. Guerrero says it all comes down to naturally occurring pigments that play a major role in an animal’s immune system. They can even change the color of your skin!
What are carotenoids?
The colorful displays of Galapagos wildlife, including the bright cyan feet of blue-footed boobies and the impressive red throat pouch of magnificent frigatebirds that they inflate during courtship, are a direct result of their diet. Specifically, they result from the digestion of nature’s own punchy dyes known as carotenoids.
“Basically, carotenoids are natural pigments produced by plants, algae, and phytoplankton,” says Gutierrez. These organic chemical compounds play a vital role in nature. They help capture light for photosynthesis, which is how plants use sunlight to make their own food. They act as powerful antioxidants that can protect cells from damage. And they provide the vibrant colors that many animals, birds, and fish need to survive.
For instance, flamingos gradually lose their red pigmentation when their diet lacks carotenoids. According to Gutierrez, this is a perfect example of the “direct relationship between dietary carotenoids and coloration.”
Frigatebird with its inflated bright red pouch on the Genovesa island in the Galapagos. Image: @ Didier Marti / Getty Images Didier Marti
How Galapagos animals get their colors
Wildlife in the Galapagos gets carotenoids primarily through their diets, since these are pigments that they can’t produce naturally. The archipelago’s land iguanas eat cactus flowers and leaves to obtain carotenoids, while blue-footed boobies consume fresh fish like sardines and anchovies to acquire them. Pink flamingos get their fill of carotenoids through algae and brine shrimp.
Once the wildlife digests and metabolizes these natural pigments, the colors of carotenoids, which primarily produce shades of red, orange, and yellow, begin to show in everything from skin and shells to feet and feathers. In many cases, the more brilliant the shades, the healthier the creature. While carotenoids don’t naturally create the color blue, the archipelago’s iconic blue-footed boobies get their turquoise feet from a mix of yellow carotenoid pigments and the collagen structure of their skin.
How other animals get their vibrant hues
Examples of the power of carotenoids even exist in our own backyards.
The sunny yellow coloring of an American goldfinch is a direct result of its carotenoid-rich diet of summer weeds and sunflowers, while northern cardinals get their brilliant red hue through carotenoid-heavy foods such as honeysuckle, hackberries, and seeds.
Why do animals need carotenoids?
Carotenoids aren’t a core nutritional necessity. However, they are an important source of vitamin A, a key component in reproductive health, good vision, and a strong immune system.
While the bulk of birds and animals get carotenoids through their diets, a few rare invertebrates, like spider mites and gall gnats, can create their own carotenoids. They do so through a rare genetic accident, likely caused by a virus or parasite. An ancestor of the spider mite and an ancestor of the gall gnat had close encounters with ancient fungi, causing a horizontal gene transfer. Essentially, horizontal gene transfer is the transmission of DNA between different organisms—and, in this case, species.
Every animal processes these pigments slightly differently. For some animals, carotenoids completely change the color of their features. Others simply absorb carotenoids without changing color at all.
But all animals use carotenoids to fight off disease, such as boosting immune cells to destroy any harmful bacteria. Only healthy wildlife can spare excess carotenoids for bright skin, beaks, or feathers. So the more vibrantly hued birds, reptiles, and fish are, the healthier their immune system. They have enough carotenoids to spare for flashy colors.
For instance, blue-footed boobies with electric, turquoise-blue feet tend to be healthiest. In fact, boobies that go without food for a couple of days can end up with dull-colored feet. When animals get sick, they also lose their hue. First, their bodies divert carotenoids away from exterior features, like skin and feathers, and use them for fighting illness. Second, absorbing nutrients such as carotenoids becomes more difficult. Without carotenoids, an animal’s bright pigments will quickly fade.
Bright colors also come in handy during mating rituals
Carotenoids also play vital roles in mating rituals. When it comes to blue-footed boobies, “It’s very important for a female to find a male with the brightest blue feet, because she is choosing the best provider” for her offspring, says Gutierrez.
It’s a feature that these marine birds show off frequently in their oh-so-quirky mating dance, which includes an elaborate high-step strut. The bluer the feet, the more attractive males appear to their female counterparts.
Male blue-footed boobies do an elaborate mating dance for the ladies to show off their bright blue feet. Maybe there’s a reason Elvis sings about his blue suede shoes? Video: The Blue Footed Boobies Mating Dance | Wild Love, Nat Geo Animals
What’s more, a famous 2006 scientific study on blue-footed boobies showed that females lay smaller eggs for males with duller feet. Basically, a female will put in less reproductive investment if her male isn’t up to par.
Why are animals in the Galapagos so colorful?
“Carotenoids are prominent all over the world,” says Gutierrez. “But they’re especially visible among wildlife in the Galapagos, because it’s a limited environment where the ecosystems are well-marked.” What Gutierrez means is that the archipelago is so geographically isolated and far from the coast of Ecuador, many species—like marine iguanas—have nowhere else to go. Even within the Galapagos, a large percentage of wildlife stick closely to their specific islands.
Another reason the animals here are so colorful? Because an overall lack of predators has allowed the wildlife to evolve without needing dull or camouflage coats to protect them.
Most of the archipelago’s carotenoids come from the ocean, or in the case of land iguanas and yellow-beaked Darwin’s finches, through cactus nectar and pollen.
Further afield, the iconic pink-orange flesh of Pacific Northwest salmon is a direct result of their carotenoid-rich diet—in their case, chowing down on krill and small shrimp. Roseate spoonbills, found across the mangrove swamps, mudflats, and wetlands of the U.S. Gulf Coast, get their rosy pink sheen from dining on crustaceans, crabs, and crayfish.
The best known example comes from consuming carrots. These vegetables are loaded with a red-orange carotenoid called beta-carotene, which our bodies convert into vitamin A.
But when we consume more beta-carotene than we need, the excess pigment starts circulating in our bloodstream and leads to a yellow or orange glow. “It’s mostly visible in your palms,” says Gutierrez.
Still, he says, don’t worry if you have a penchant for sardines and anchovies—the main diet of blue-footed boobies. Since humans process and metabolize these compounds differently, “you’re not going to turn blue, too.”
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Driverless cars, once a distant techno-optimist’s dream, are making their way into more cities, racking up miles and attracting a loyal fan base. They’re also increasingly racking up tickets and driving in ways that would land a human driver in hot water.
During July Fourth celebrations this year, several Waymo autonomous robotaxis operating near San Francisco’s Golden Gate Bridge reportedly hit technical trouble that led to at least one vehicle driving over an exploding firework with two passengers inside. The riders (who luckily escaped harm) said they smelled smoke coming out of the vehicle and promptly bolted at the next intersection.
A couple of months earlier, a California driver claimed a pair of Waymos cut her off and blocked her from moving forward for nearly 50 minutes, causing a traffic pile-up and frantic 911 call. Halfway across the country, as many as five Waymos in Austin blocked police and emergency crews responding to an active shooting that left four people dead and a dozen injured.
“The question is not if this is going to turn into a deadly situation for someone, but when,” Austin City Council member Paige Ellis said at a meeting following that incident.
Human drivers in any of these situations would likely face a confrontation with police and the real possibility of a citation—or worse. But that calculus changes when there’s no one behind the wheel. Across the country, cities and states are still figuring out how to sanction a driverless vehicle that breaks the law, even as the well-financed companies operating them rack up thousands of dollars in parking tickets with seemingly few consequences.
To call the situation a work in progress is an understatement. Many localities that allow these vehicles on public roads still lack clear legal mechanisms for issuing moving violations. Where laws do exist, enforcement remains light-handed. Experts in autonomous vehicle (AV) policy tell Popular Science that this seemingly basic question, “what happens when a driverless car breaks the law?”, is still surprisingly complicated to answer.
“In principle, AVs are licensed drivers and someone owns and operates them,” University of Southern California Gould School of Law professor Gregory Keating tells Popular Science. “They should be treated like any other licensed driver.”
Despite spending years studying this very issue, Keating says the path forward remains hazy.
“In practice, I don’t know what’s going on,” he adds.
Driverless cars are racking up parking tickets
Cars breaking laws, whether driven by humans or algorithms, fall broadly into two categories: traffic violations and moving violations. Anyone who’s gotten a parking ticket is familiar with the former, and increasingly, so are driverless cars. In 2024, Waymo, by far the most common robotaxi operating today, accrued over 589 parking tickets in San Francisco and $65,065 in fines, according to The Washington Post. The Post reported another 75 tickets further south in Los Angeles. In Austin, where the company more recently started commercial operations, Waymo has reportedly received 20 parking-related citations as of June 23. Zoox, another robotaxi firm backed by Amazon, also received a citation for parking in a tow-away zone.
Cops issuing those tickets basically approach it the same way as they would a car with a human driver. Matthew Raifman, autonomous vehicle and traffic lead at UC Berkeley’s SafeTREC, tells Popular Science that in these cases, it’s the vehicle, not any individual behind the wheel, that’s on the hook–the same as it would be for a human-driven car. Parking tickets already work this way: If you borrow a friend’s car and get a ticket, it’s the car’s registered owner who pays, not the friend who parked it. Since an AV’s registered owner is the company that operates it, that same logic extends easily to driverless cars. A traffic cop who comes across an empty, illegally parked driverless car can simply print a ticket and place it on the windshield. At the end of the day, perhaps after picking up more passengers, the AV returns to a depot where a human employee collects the ticket and sends it up the chain for payment.
How cops are supposed to arrest a car with no driver
Ticketing driverless cars for more serious moving violations gets more complicated. Most municipalities still don’t have clear mechanisms for citing these vehicles when they block traffic, make illegal U-turns, or commit other mistakes that would normally lead to tense conversations with police. The core issue is confusion over who is held responsible when no human is actually making the mistake.
If a police officer observes an AV committing a moving violation, they generally flash their lights to signal the vehicle to pull over. Robotaxis are programmed to use their onboard sensors to detect police flashers, though some vehicles have struggled to do so in the past. If the AV pulls over, the officer must determine who the rogue car belongs to. Today, with only a handful of robotaxi firms operating, that detective work is pretty simple. But it could get much trickier in the near future as more AV companies enter the market, or if individuals begin owning and operating their own autonomous vehicles.
“Just because robotaxis are dominant right now doesn’t mean that robo taxis will dominate in the future,” University of South Carolina law professor and AV policy expert Bryant Walker Smith tells Popular Science. “We could have one company that develops the automated driving system, another company that puts it on a vehicle, another company that buys the vehicles.”
A clear example of how this confusion can play out occurred late last year in Northern California. San Bruno police, conducting a DUI operation, say they spotted a driverless Waymo make an illegal U-turn in front of them. Officers pulled the wayward Waymo over and contacted a human representative through the car’s speakers to report a “glitch.” But because there was nobody behind the wheel, no ticket or citation was issued the way it would have been if a human driver were involved.
“Since there was no human driver, a ticket couldn’t be issued (our citation books don’t have a box for ‘robot’),” a San Bruno Police Department Facebook post explained following the incident reads.
“That’s right… no driver, no hands, no clue.”
“I’ve been in a Waymo but, being a mere passenger, I’ve never poked around in the glove compartment to see if they carry their ‘papers’ around approximately the way that human beings do,” Keating says.
Worries about AVs breaking the law aren’t hypothetical. A recent CNN analysis of federal government records and social media footage found hundreds of driverless cars running red lights, driving into oncoming traffic, ignoring emergency road closures, and executing other potentially dangerous maneuvers. Around that same time, Waymo temporarily pulled its vehicles from highways following reports from the National Highway Traffic Safety Administration (NHTSA) of at least 13 instances where they drove into active construction zones. The company has since started resuming highway routes.
Waymo and independent supporters of AV rollouts generally have repeatedly claimed that, even when taking these issues into account, autonomous systems get into fewer crashes and drive safer than humans. Recent research lends credence to that argument in certain cases, but AVs are still limited in where and how they can operate.
“No technology is perfect, but while human drivers can only learn from their individual experiences, Waymo takes community feedback and applies those safety learnings to our entire fleet,” a Waymo spokesperson told CNN in June.
A new type of ‘ticket’ for autonomous vehicles
Regulators hope some creative new legislation can help clear up the confusion. California enacted a law allowing police to issue a “notice of noncompliance” to AVs. The whole point of these notices, and why they mark an important step forward, is to create an official record for when an AV makes a mistake, Raifman notes. Those notices are then updated in the California DMV’s system within 72 hours. Prior to this, police didn’t have a clear way to document when an AV violated the law, creating a gap that contributed to uncertainty about how well the technology works in the real world.
Still, actually punishing human drivers and AVs isn’t a one-to-one comparison. When humans make mistakes, they receive points on their driver’s license. Rack up enough points, and a driver risks losing their license altogether. As Raifman points out, notices of noncompliance attempt to apply a similar deterrent for AV companies. The notices are recorded, added up, and factored into whether state regulators continue granting those companies permits to operate autonomously. Too many violations, especially those causing major traffic issues or injuries, could lead to a company’s permit being revoked. But unlike humans, AV makers don’t have to worry about individual monetary fines for these moving violations.
An autonomous Waymo car drives into the blocked off Potrero Annex construction site while a construction worker watches. Image: Getty Images Thomas Hunter II/Alamy Live News
“It’s an indirect mechanism to enforcement, but it is not enforcement through money and points,” Walker Smith said.
Raifman compares this enforcement method to how regulators manage pollution from power plants. Local governments often grant permits allowing a facility to emit certain pollutants, but only up to a specified amount. If a plant exceeds that or otherwise violates its permit, regulators can issue a notice of violation, which, depending on its severity, can lead to a permit suspension or even cause operation to shut down.
That’s a different process than the one facing an individual resident who pollutes their own backyard. In that case, they’d likely just receive a citation. In both the power plant case and now with AVs, enforcement relies on an ongoing back-and-forth between corporations and regulators, rather than a single punishment to an individual.
That may make sense on paper, but it’s also frustrating for everyday drivers, many of whom are just now seeing AVs spring up in their towns and on their roads without their input. The frustration shows up in comment sections and forums, with some describing what can seem like two separate rules or laws–one for humans and another for robots and their owners.
“They pay the fine and just keep doing what they are doing,” one Reddit post on the topic reads. “This is the corporate reality we live in, where corporations are supposedly people, but have none of the responsibilities or need to deal with things like following the law that the rest of us do.”
Raifman believes enforcement and accountability for driverless cars are broadly improving, but he also empathizes with annoyed drivers who feel the status quo is unfair. One major issue contributing to that frustration, he notes, is that the “pain” felt by a human driver and an AV company violating the same law isn’t equal. A robotaxi company accruing hundreds of thousands of dollars in parking tickets can simply shrug it off as a cost of doing business. For a human driver living on a budget, however, just a few tickets can severely impact their finances.
“I think that’s where there’s an asymmetric relationship compared to humans, and I think that’s a source of frustration,” Raifman says.
Autonomous vehicles are a physical manifestation of growing AI anxiety
That pent-up frustration stems, in part, from a familiar pattern in tech: innovations move fast, maybe breaking some things along the way, and regulators end up playing catch-up. Right now, cities with the most exposure to driverless cars (San Francisco, Los Angeles, Phoenix, and Austin) have just begun to sort out what fair and consistent enforcement looks like, even as the technology keeps expanding well ahead of it.
In just the last year, Waymo has nearly quintupled its fleet, from around 700 vehicles to nearly 4,000, according to The New York Times. Zoox and Tesla are both preparing significant robotaxi launches in multiple cities, and changes to federal rules mean many of these vehicles won’t even have visible steering wheels or pedals. Driverless 18-wheelers are already barreling across Texas highways, and the prospect of individual car owners swapping on true hand-off, eyes-off driving features in their own vehicles gets closer every day.
And even if the AV supporters are right that the technology is ultimately safer than humans, sheer scale means more and more people will end up watching these vehicles behave badly. In a climate where public trust in automation and AI broadly is souring, driverless carmakers may have more incentive than ever to keep their cars from running afoul of the law, not just to satisfy regulators, but because the cars themselves have become a material stand-in for how people feel about AI writ large.
“I think people are going to put a lot of their hopes and fears about AI generally disproportionately on automated vehicles because they’re so visceral and physical and real and obvious,” Walker Smith said.
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Imagine this: You’re at the doctor’s office, and instead of one quick tap with the reflex hammer—the kind that makes your leg kick out without you meaning to—they just…keep going. Tap, tap, tap, right on that same spot below your knee, for a solid two minutes straight. It wouldn’t hurt you. But after a while, you’d probably start feeling pretty strongly about wanting it to stop—and if they didn’t, you might start wanting to kick someone on purpose.
Now imagine you’re a dog, and someone finds your “sweet spot”—that one particular patch of skin on the side or back—and won’t quit scratching it. Suddenly, your back leg is kicking without your permission, over and over, like it’s got somewhere better to be. As adorable as it looks from the outside, it’s probably not exactly a party for you.
“It’s an annoyance,” says Nancy Thomas, DVM, a veterinarian with Telavets, an online veterinary expert consultation service. “It doesn’t do any good to continue doing it. The spinal cord is trying to say ‘get off of me.’”
So what is the mechanism behind your pup’s leg acting on autopilot?
Your dog’s leg has a mind of its own
So it might look like your dog is practicing to bend it like Beckham, or maybe is angling for a spot with the Rockettes, but what is really happening is entirely out of your pet’s control. It’s something called the pruritic scratch reflex.
It’s a reflex arc, a shortcut in the nervous system that bypasses the brain altogether to allow for an instant response. When something (your hand, a flea, a burr caught in the fur) touches the skin in that spot, sensory nerves pick up on it and send a signal straight to the spinal cord. The spinal cord doesn’t pause to think it over, according to Thomas; it’s the same shortcut your body uses when you touch a hot stove, although your dog’s involuntary kicks do not involve pain.
“You touch a stove, and the signal goes from the sensory nerves up to the spinal cord, and the spinal cord immediately sends a message back to your finger to get off the stove,” Thomas says. “It’s a millisecond kind of a deal.”
When dogs twitch during pets, it’s not because they like it. Video: That’s the Spot, @TheDogRufferee
The reason dogs have the pruritic scratch reflex basically comes down to pest control: that particular area along your dog’s flank is prime real estate for fleas, ticks, mites, and burrs—the kind of small, easy-to-miss irritants that can burrow in or bite before a dog even notices. Rather than relying on conscious awareness to catch the pest, dogs’ built in a shortcut: If something touches down in that zone, the leg kicks automatically on the off chance it’s actually a threat trying to dig in.
It’s the same basic reflex that makes a horse’s skin twitch to shake off a fly, says Thomas, who also created an emergency medical ID tag for horses and their riders.
“It’s a mechanism designed to keep nefarious things off the body,” Thomas says.
It’s not the same as tickling
A very common misunderstanding around the pruritic scratch response is that it is a form of tickling, and it’s easy to see why: your dog is responding in a riotous, involuntary, and rather comical way to a physical trigger.
But as gleeful as it looks when your dog breaks into those rapid-fire leg kicks, the experience is probably neutral at best for your dog. By continuing to do it, you could be “perpetuating the annoyance,” according to Thomas. Even tickling isn’t universally and unequivocally loved.
To be clear, this doesn’t mean your dog is necessarily having a miserable time every time you find that spot. In general, the reflexive leg kick is just proof that your dog’s built-in protective wiring is working exactly the way it’s supposed to. But there’s one pattern worth keeping an eye on. If your dog reacts almost anywhere you touch them, or seems unusually sensitive even on their belly (which is normally a calmer, less reactive area due to its lack of contact with the ground), that’s worth mentioning to your vet.
“That’s a possible red flag for atopy,” Thomas says, referring to allergies or skin inflammation that leaves the whole body more sensitive. It’s similar to how a patch of poison ivy makes even light touch uncomfortable.
“That might be a bit of a sign,” Thomas says. “These dogs might be a little extra sensitive, due to a skin condition.”
To make bonding with your dog more enjoyable for both of you, Thomas advises sticking to areas not commonly associated with the pruritic scratch reflex, such as the belly. She also recommends watching your dog’s body language to ensure they’re having as much fun as you. It’s a much better way to ‘get your kicks’ without turning your dog into a full-on kickboxer.
“If you’re rubbing their belly, and they’re all spread out and relaxed-looking, that’s a sign you’re on the right track,” she says.
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
Growing up, many of us remember dutifully taking a few more bites of protein (and, of course, vegetables) to appease our moms before we were allowed to progress to the sweet, carb-heavy goodness of dessert. Lately, it seems hard to say whether our moms would be delighted or horrified to see what has happened to protein since.
Just walk into any grocery store right now, and protein is everywhere. Protein water. Protein chips. You can even request protein foam on your favorite fancy latte. Somewhere, a bodybuilder is chugging a protein shake the size of a fire extinguisher as you read this. It’s safe to say this is not your mother’s protein.
To be fair, protein deserves at least some of its mom-and-jock-endorsed reputation. It helps build muscles, keeps you feeling full, and is an essential nutrient we truly can’t live without. Older adults, especially, have been told to eat more of it as they age.
“We think of protein as beneficial,” Dr. Dudley Lamming, a professor at the University of Wisconsin-Madison and lead researcher of the review, told Popular Science, “but a number of studies have started to come out showing that when you look at large populations over the course of decades, people who were eating more protein over the course of these decades were having higher rates of age-related diseases.”
A pattern in the data sparked new questions
Lamming didn’t set out looking for a problem with protein. If anything, he was surprised by a contradiction he was noticing between population data and the conventional wisdom about protein—people who ate more protein over time were showing higher rates of cancer, diabetes, and cardiovascular disease.
“The question is, how can this be?” he remembers wondering.
Proteinmaxxing is spiking prices for whey protein. Video: ‘Proteinmaxxing’: How this trend is spiking whey protein demands, @WKYCChannel3
But a smaller slice, about five or six randomized trials, tested protein restriction directly in people. Those human trials showed consistent short-term benefits: People lost weight, their fat mass declined, and in one study of people with type 2 diabetes, insulin sensitivity improved.
Lamming adds that these results do not appear to be associated with mere calorie reduction.
“In fact, because dietary protein is satiating, people in our studies and others actually ate more calories when they were on a protein-restricted diet,” he says. “But they lost weight, and that’s because energy expenditure increases.”
One hormone may explain what’s happening
To understand the potential connection between protein restriction and increased energy expenditure, Lamming’s review explored existing mouse studies on the topic. Findings from these studies indicate that when protein intake drops, the liver releases a hormone called FGF21. That hormone signals the brain, which in turn triggers what researchers call “beiging.”
“This is a process that essentially tells your fat tissue to stop simply storing fat, but actually to start metabolizing it and burning it for energy,” says Lamming.
In practice, protein restriction doesn’t mean cutting protein out altogether. Most people get about 16 to 17 percent of their calories from protein; in the studies Lamming and his team reviewed, that’s typically cut by around half, down to roughly seven to nine percent.
“We’re not talking about restricting to an unhealthy level where you’re protein insufficient,” he says. “What we’re really talking about, in most cases, is restricting down to the recommended daily allowance as reported by the World Health Organization, which is about 0.8 grams per kilogram [of body weight].”
You don’t necessarily have to restrict protein as a whole to achieve these benefits, either; mouse studies found that cutting back on specific amino acids, including methionine, isoleucine, and valine, may provide some of the same benefits in terms of fat mass, blood sugar control, and longevity.
This doesn’t necessarily mean everyone should be in a hurry to throw away their protein waffles. Lamming is careful to note that people who exercise routinely may be an exception.
“We actually think that dietary protein has negative metabolic health outcomes for sedentary people, like the general population, since most of the population is fairly sedentary,” he says. “But maybe people who are exercising need more protein to help build muscle, and maybe it wouldn’t have these negative metabolic outcomes.”
Although these findings are based only on mouse studies so far, they may suggest that working out might protect against the downsides of higher protein intake, Lamming says.
“Fortunately,” he says, “a lot of the people who are very pro-protein are also people who are exercising.”
Lamming says his team plans to test these ideas—specifically, whether protein supplementation genuinely benefits those who exercise, and whether restriction benefits those who don’t—more directly in both mice and people over the next few years.
“For your typical person who’s not engaged in exercise, we would suspect, based on our research, that eating more calories in the form of protein would be detrimental,” he says.
Lamming’s own suggestion for sedentary people: Consider skipping the protein-added version of a product when a regular option is available.
He was quick to add, however, that those who can safely exercise certainly should.
“No one’s going to argue that more exercise would be bad,” he says.
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
A lot of us like to start our days with a cup of coffee. Whether it’s a pour-over you make at home or a latte from your favorite coffee shop, many people need caffeine to wake up and feel alert.
If you rely on coffee as a productivity hack, you might be wondering if there’s a time of day that would optimize its effects on the brain. It turns out the answer isn’t that straightforward. How the body reacts to caffeine depends on factors such as age, metabolism, and tolerance. So, here’s what the experts say about how to time that cup of joe.
What effect does coffee have on the body?
Caffeine is the central stimulant found in coffee and other foods like tea, soda, and chocolate. It helps us feel more awake by blocking adenosine, a naturally occurring chemical that builds up in the brain while we’re awake. Experiencing morning grogginess, or sleep inertia, is related to residual adenosine in the brain, though other factors like your sleep quality also play a role. As adenosine builds up throughout the day, it makes us feel increasingly tired and helps signal to the brain that it’s time for sleep.
Since caffeine has a similar chemical structure to adenosine, it fits into the same receptors in the brain. When caffeine binds to these receptors, it blocks adenosine from doing its job. As a result, the brain receives fewer of the signals that make us feel tired, helping us feel more awake, alert, and energized.
Caffeine is the central stimulant found in coffee. Video: How does caffeine keep us awake? – Hanan Qasim, TED-Ed
“But this perceived energy does come at a cost,” says Dustin Lee, an assistant professor of psychiatry and behavioral science at Johns Hopkins, in an email to Popular Science. The brain adapts to these conditions incredibly well, so over time, the brain produces more adenosine receptors.
The more receptors your brain has, the more caffeine it needs to feel alert, eventually leading to caffeine tolerance and withdrawal. “Withdrawal symptoms, which I’m sure most of us have some familiarity with, include headache, irritability, fatigue, and difficulty concentrating,” Lee explains.
If you want to quit caffeine or miss your usual cup of joe, then you’re likely to experience these symptoms very quickly. They usually start within 12 to 24 hours after stopping caffeine, peak within 48 hours and resolve within two to nine days.
When should you drink coffee?
“There’s no set answer for everyone,” says Marie-Pierre St-Onge, a professor of nutritional medicine at Columbia University Medical Center, to Popular Science. It depends on your age, metabolism, and caffeine tolerance.
“Peak caffeine absorption occurs rapidly, at about 30 to 60 minutes after ingestion,” says Lee, so a simple answer to the question would be to drink coffee about an hour before whatever activity that would benefit from caffeine’s stimulating effects. If you’re going on a run, for example, you might want to get some caffeine in before heading out, says St-Onge. That’s why its use is limited by some sports governing organizations like the National Collegiate Athletic Association.
You shouldn’t really need caffeine right when you wake up, as your body should already be alert, says St-Onge. “If you’re starting to feel a slump a few hours after waking up, that would be when to drink coffee.”
Both Lee and St-Onge stress that the answer to when you should drink coffee really depends on the individual. The ability to metabolize caffeine slows down with age, so the amount of time to clear caffeine from your bloodstream gets longer as you get older. Your circadian rhythm also changes as you get older, which means you might get tired earlier in the day.
“Compounded, [these changes] make it so that you would want to consume your caffeine earlier as you get older,” explains St-Onge.
“Caffeine dose matters just as much as the time at which you drink your cup of coffee,” adds Lee.
A cup of coffee can actually have a wide range of caffeine—the dose can depend on the ratio of coffee to water, how you brewed your cup, and even bean type. And “tolerance has a major effect on caffeine’s effects, so the morning cup of coffee might serve more to partially reverse overnight caffeine withdrawal, so that should be taken into consideration as well,” he says. Basically, that means that for regular coffee drinkers, the first cup of the day is contributing to reversing the effects of not having had any caffeine since the day before.
When should you not drink coffee?
“Not too close to bedtime,” says St-Onge. If you’re finding that you’re having trouble falling asleep, make sure that you’re not drinking coffee too late in the afternoon.
Drinking coffee on an empty stomach can also be too abrasive for some people, and may lead to side effects like increased jitters and an increased urge to go to the bathroom. (Though the idea that it can lead to stomach ulcers is a myth.) If you’re someone with a sensitive stomach, try eating something alongside your cup of coffee to be safe.
So, don’t fret too much the next time you’re thinking of when to have a cup of coffee. Just remember: not too late, and not too much. Then you might become dependent on the stuff.
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
Imagine this: A giant sperm whale (Physeter macrocephalus) has just swallowed you whole.
It might seem like a bizarre scenario to envision, but you’d hardly be the first. In the Bible’s Book of Jonah, the belly of a whale is the setting for the prophet Jonah’s three days of repentance and prayer. In the 1940 Disney film Pinocchio, Geppetto manages to rig up a cozy living space inside the stomach of Monstro the sperm whale, complete with amenities including a small fire and a fishing setup for seafood that drifts in. Even his pet goldfish, Cleo, is swimming inside the microcosm of her highly ironic fishbowl.
Surviving inside of a giant sea creature has become a classic “fish tale” of human resilience and redemption. In the latest reimagining of this theme, the upcoming movie Whalefall, a scuba diver is swallowed by a sperm whale and has exactly one hour to escape before his oxygen runs out.
But for all our fascination with extended whale-squatting, one fact remains: it has never actually happened. The few close calls with whale ingestion have involved humpback whales, a species physically incapable of swallowing a person. In all cases, the whale spat the person up as quickly as you might cough up a bug that flew into your mouth.
“I wouldn’t call that swallowed,” Shane Gero, a whale biologist and Lead Biologist at Project CETI, tells Popular Science. “I would call that a mistake by both the mammals involved.”
So, if it’s never actually happened, is it even possible?
A highly unlikely accident
Whalefall is the film adaptation of a 2023 novel of the same name by Pulitzer Prize winning author Daniel Kraus, and arrives in theaters on October 16.Protagonist Jay Gardiner is on a diving expedition to find the remains of his late father, a mariner who died at sea. Suddenly, Jay is swallowed by a giant sperm whale, with only an hour of oxygen remaining in his scuba tank. To try and survive, Jay deploys a series of highly resourceful, increasingly MacGyver-esque maneuvers, including using the beak of a squid to cut through the whale’s stomach lining, and navigating his climb up into the whale’s throat using the glow of a jellyfish.
The ordeal is an adrenaline-fueled adventure for Jay, but could any of it ever happen? Gero, who has studied sperm whales for over 20 years, thinks it’s highly improbable, starting with the swallowing part.
“To my knowledge, no one has ever been eaten by a sperm whale,” he says.
Gero hypothesizes that after some generous suspension of disbelief about sperm whales’ feeding habits and anatomy, there could be a situation where “the largest sperm whale ever” could accidentally ingest a human.
“I think anatomically, maybe you could shove a slender human down the throat of a very large, male sperm whale,” he says. “Would it happen? Very unlikely. Certainly not with scuba gear and a scuba tank making the human that much bigger.”
Yet Gero, who has read the novel, praised the book’s meticulous efforts to make the science line up, including its explanation for why a sperm whale would be anywhere near a scuba diver. Sperm whales usually feed at about 3,000-feet-deep at the bottom of the ocean, an area not typically occupied by scuba divers. The book reconciles this by having a giant squid (Architeuthis dux) struggling at the surface to escape being eaten by the sperm whale. When the whale comes to the surface to eat the squid, the squid latches onto Jay, who is then pulled into the whale’s mouth. But even after accepting this explanation, Gero notes that sperm whales use echolocation, which would likely distinguish a human as unusual prey, not a potentially tasty morsel.
“Eating a human by accident seems very unlikely, but it makes for a really exciting story,” Gero says.
The odds of surviving inside a whale
If a person somehow managed to survive entering the mouth of a sperm whale without drowning from the propulsive rush of water, Gero says that their luck would worsen almost immediately.
“More than likely, the human drowns the second they get smashed into the mouth of the whale, whether they were drawn in by a squid or accidentally brought into the mouth of a whale,” he says. “My guess is the person dies the second they enter the mouth.”
Gero raises another interesting possibility. Sperm whales usually prey on boneless squid, so consuming a scuba-outfitted human could be a choking hazard, and could choke to death.
Male sperm whale swimming at the surface, Pelagos Sanctuary, Mediterranean Sea, Italy. Image: by wildestanimal via Getty Images.
This would certainly make for a shorter movie, but Gero notes that the whale’s body could also have a more lively response to an unwanted foreign object in its mouth.
“When I swallow something that I shouldn’t have, it’s like a non-controllable response — your body just starts moving its mouth and coughing,” Gero says. “In that sort of movement, or freakout, it probably is going to do a lot of physical damage to the human.”
Although Gero acknowledges that he is not a whale anatomist, he still believes that it would be highly unlikely for any living creature to make it into the stomach of a sperm whale, let alone set up camp there.
“There are acids and gases,” he says. “The digestive tract of any mammal is not meant to be a livable place for any prey that ends up there.”
Dr. Joy Reidenberg, a comparative anatomist at the Icahn School of Medicine at Mount Sinai in New York City who consulted on the anatomy for both the Whalefall book and film, explains that a sperm whale’s stomach is divided into four chambers. These chambers are a vestige from their early days as a land-based, plant-eating mammal about 50 million years ago. The first chamber is lined with thick, muscular tissue. It’s responsible for mashing up food, since sperm whales’ teeth, found only on the lower jaw, are too pointed to chew.
If Jay needed something solid to grab onto and pull himself toward the whale’s throat, Reidenberg tells Popular Science he wouldn’t find much. It would mostly be “wet, slimy, squishy walls,” with “no hooks, no handles.” The one exception is the whale’s voice box, or larynx, which sticks up like a rigid pipe.
“If he gets his arm around that, he can pull himself out,” she explains.
Even in that situation, Reidenberg says, few people would live to tell this phenomenal tale.
“Most people, unless they have protection and air, couldn’t survive this situation. Also, a sperm whale would probably never swallow a person.”
Meet your actual nightmare
While the fictitious scenario of a massive sperm whale swallowing a human relies as much on poetic license as it does grounded science, could any other land or sea animal engulf an entire human being?
Horrifyingly, yes. The reticulated python (Malayopython reticulatus), which is the world’s longest snake species and can measure as much as 26-feet-long, has been known to take in an entire human. According to a 2025 study tracking python-related deaths in Indonesia, 17 people were preyed upon by reticulated pythons between 1927 and 2025. Astonishingly, four of these people survived, although it’s unlikely they spent any quality time inside the snake. In several of the fatal cases, the humans were swallowed whole.
Gero notes that a green anaconda (Eunectes murinus) might also be up to the task, since they can swallow animals as large as deer. Even then, this prey is already dead when it’s consumed.
“Anacondas famously wrap around and crush their prey,” Gero says. “The body is limp by the time it goes down.”
Gero also makes one key distinction about sperm whales: they have absolutely no interest in preying on humans, intentionally or otherwise. In fact, humans have spent centuries slaughtering whales for oil, an uneasy relationship that may be part of why they loom so large in our storytelling.
“If those stories exist from the whaling days, they’re more a lesson to future whalers than they are based in reality,” he says. “I’ve never seen any kind of aggression from a sperm whale. These are gentle giants of the deep.”
Gero hopes movies like Whalefall will spark genuine curiosity about the intelligent, peaceful animals behind the myth. He and the team at Project CETI are working to decipher how sperm whales communicate with each other. In the meantime, you can visit listen.projectceti.org to record what you’d say to a sperm whale, if you ever had a chance.
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.
“Do you want a treat, Milton?” “Do you want to go for a walk, Bernie?” Annnnd, cue the head tilt from your dog.
Many dogs turn their heads, perk their ears, and stand at attention when their humans say certain words. But why? Well, it’s not just cute. There’s some serious science and fascinating theories about dogs’ ability to understand language. On the latest episode of Ask Us Anything, we explain how dogs’ head tilting likely makes your furry friend able to listen and process information better.
Annie Colbert: And I’m editor-in-chief, Annie Colbert.
SD: Here at PopSci, we’re always pondering quirky questions.
AC: And this week, we’re wondering, why do dogs tilt their heads? So Sarah, tell us, why are our cute furry friends looking at us all askew?
SD: Well, scientists aren’t totally sure.
AC: Seriously? Something dogs do all the time, they don’t know why?
SD: I know, I know. There are a few theories out there, though. Maybe our pups are trying to hear us better.
AC: I guess that makes sense. I remember my dad used to kind of tilt his head when he didn’t have his hearing aids in.
SD: That might have just been a your dad thing.
AC: Yes, probably.
SD: Yeah, there’s also a chance dogs are trying to get a better look at our faces.
AC: That also feels plausible.
SD: But there’s another possibility that’s a lot more interesting.
AC: Ooh, okay.
SD: Researchers have found that some dogs are more likely to tilt their heads when they hear words they recognize.
AC: Wait, so they might be thinking?
SD: Maybe. That little head tilt could have something to do with how dogs pay attention to, and process, what we’re saying.
AC: Which somehow makes it even cuter.
SD: Right? So why do some dogs tilt more than others? What’s actually happening in their brains? And does your dog understand more of what you say than you realize?
AC: I must know.
SD: We’re gonna get into all of it. But before we do, we wanna hear from you. What questions are you head tilting, AKA thinking about?
AC: If there’s something you’ve always wanted to know, something that makes you tilt your head, submit your question by clicking the “Ask Us” link at popsci.com/ask.
SD: Send us your questions.
AC: Yes. And coming up, we’ll dig into what might actually be going on when your dog does that adorable little head tilt.
SD: And what it could tell us about how much they really understand when we talk to them.
AC: That’s after this quick break.
SD: Welcome back. Annie, now before we get into the science, I feel like we have to establish our dog credentials here. So did you grow up with dogs?
AC: Yes. We always had at least one dog growing up, everything from mixed breed dogs to cocker spaniels named after various bread products. We had both a Muffin and a Biscuit at one point.
And then we had pugs, and I think I can confidently say that pugs were definitely the biggest head tilters.
Annie’s parents’ pug, Herbie, pictured in 2007. Image: Annie Colbert / Popular Science
SD: Mm.
AC: And I always assumed it’s just because they were perpetually confused by life.
SD: I mean, aren’t we all? I grew up with Saint Bernards.
AC: Hmm.
SD: So my family has had, oh, five.
AC:Wow.
SD: And my grandmother, who started the trend, actually had something like nine throughout her life.
AC: What? That’s so many.
SD: I know. I love Saint Bernards. And Saint Bernards, at least the ones I’ve gotten to know, are not huge head tilters.
AC: Hmm, interesting. Is it maybe because their heads are just too heavy to tilt?
SD: There is a lot of head to move for a Saint Bernard. Hmm.
AC: Yeah.
SD: But our newest puppy, Tartufo, we call him Tufo, is actually a little bit of a head tilter, and sometimes if you say a word he knows, he’ll look right at you and just tip.
Tufo. Image: Sarah Durn / Popular Science
AC: Aw.
SD: I know, it’s so cute. I love him so much. He’s like a super sweet, like, expressive dog, so the head tilt just, you know, sends it over the edge.
AC: Love it.
SD: But here’s the really cool part: Tufo seems to head tilt more when he hears words he recognizes, and scientists think that might be a clue about what the head tilt actually means.
AC: All right, so what’s happening in that giant Saint Bernard brain?
SD: Potentially language processing.
AC: Excuse me?
SD: I know. Dogs have lived alongside humans for roughly 20,000 to 30,000 years, and over that time they’ve gotten remarkably good at dealing with this extremely weird thing humans do all day long.
AC: Talk at them.
SD: Constantly. And brain scans suggest dogs process parts of human speech in ways that are surprisingly similar to us. When a dog hears a familiar word, the left side of its brain becomes active, even if you change your tone of voice. But if you say an unfamiliar word using a familiar tone, the right side becomes more involved.
AC: So their brains are basically going, “Okay, I recognize that word,” versus, “I don’t know what you’re saying, but you sound excited.”
SD: Yeah, and that brings us to a 2025 study that basically asked, okay, where does the adorable head tilt fit into all of this? Researchers recruited 103 dog owners and had them film their dogs in four different situations.
First: Dog doing nothing.
AC: A challenging assignment for most dogs.
SD: Totally. Totally. And then second: Owner silently staring at dog.
AC: It’s kind of intimidating.
SD: Right. And then, this is my favorite: Owner talking to dog about ancient Egyptian civilization.
AC: Finally, that canine Egyptology study we were always waiting for.
SD: Exactly. I know everyone was on the edge of their seats. The idea was, you know, to give dogs speech that was neutral or unfamiliar. And then in the final and fourth condition, owners used familiar words with a warm, excited tone.
AC: Like, “Wanna go for a walk?”
SD: Exactly. And that is when the head tilt really showed up.
Dogs tilted much more often when their owners were saying familiar things in that enthusiastic, excited way.
AC: So they weren’t just reacting to the sound. Right.
SD: Courtney Sexton, one of the authors of the study, described it as a kind of communicative exchange. The dog isn’t necessarily going, “Ah, yes, I fully comprehend this sentence,” but something about you talking to them using words they know seems to make that little head go sideways.
AC: Which feels like watching the loading wheel appear over their little brains.
SD: Oh, yeah, totally. Like… Processing, processing, walk detected.
AC: Uh, that is exactly what it sounds like.
SD: Oh, yeah, 100%. And, you know, somehow this all gets a little weirder. Because researchers also noticed that most of the dogs who tilted went to the right.
AC: Huh. There’s a preferred head tilt direction?
SD: Apparently. And there’s a possible brain connection here. Brains process sensory information contralaterally, which is a fancy way of saying that the left side of the brain is connected to the right side of the body and vice versa. So one hypothesis is that a right head tilt could be connected to the dog engaging the left hemisphere of the brain, the side involved in processing familiar language.
AC: So we might literally be seeing the dog’s brain work.
SD: Maybe, but, you know, giant asterisk here. Researchers are not saying right tilt equals language happening. That part is still very much a hypothesis. The study also had another really weird result.
AC: What’s that?
SD: Male dogs tilted more often than female dogs, and when they tilted, male dogs leaned further to the right.
AC: Why are boy dogs so dramatic?
SD: Aren’t all guys, really? Scientists don’t really know why this is happening yet, but one possibility is that male and female dogs may use their brain hemispheres a little differently when processing language. But only around 40 of the 103 dogs in the study actually tilted at all, so the researchers are very cautious about drawing any sort of broad conclusion.
AC: All right. But dogs don’t only tilt their head when people talk to them, right?
SD: Right. And this is where our very satisfying head tilt equals language explanation gets messier. Another study found dogs tilting their heads after being startled by a jack-in-the-box.
AC: Okay, that’s a reasonable response to a jack-in-the-box.
SD: Yeah, extremely reasonable. And that suggests, though, that head tilting might have something to do with novelty or surprise, not just language. There’s even this possibility called cognitive offloading.
AC: Which sounds like something I need at the end of every workday.
SD: Oh my gosh, same. Um, yeah, basically we make a physical gesture while our brain is working something out. You furrow your brow, you exhale, you stare into space for five seconds while trying to remember why you walked into the kitchen.
AC: Constantly.
SD: Same. The head tilt could be a dog version of that, a little physical reset while their brain processes something new.
But Courtney Sexton from that study we talked about doesn’t think that explains the whole thing because in the 2025 experiment, the dogs tilted overwhelmingly during the social language condition.
AC: Okay, I have another scientific hypothesis.
SD: Shoot.
AC: Dogs know that the head tilt makes us powerless to their adorableness.
SD: That is actually an important scientific question.
AC: Called it.
SD: You did, because humans do rate head tilting dogs as cuter. But Sexton doesn’t think dogs are sitting there plotting, you know, “If I rotate my head 15 degrees, perhaps she’ll give me cheese.”
AC: But you cannot prove they’re not.
SD: Okay, fair, but there is a reason it absolutely destroys us.
During domestication, dogs develop something called neoteny, meaning adult dogs retain juvenile features like large eyes and soft facial proportions, basically permanent baby face, and humans are very, very responsive to baby face. So the head tilt isn’t necessarily designed to be cute.
AC: We’re just helpless against it.
SD: Exactly. Which brings us back to the original question: why do dogs tilt their heads? And the slightly frustrating answer is: Scientists still don’t completely know.
It could involve language processing, attention, surprise, maybe even that little physical reset while the brain figures something out. But the strongest evidence we have suggests that at least sometimes the head tilt is a visible sign that your dog is actively processing and responding to you.
So when Tufo hears a word that he knows and tilts his head, that enormous Saint Bernard brain may actually be hard at work.
AC: Thinking important thoughts.
SD: Extremely important thoughts. Probably mostly about snacks.
AC: Yes. But now I have a question.
SD: Yes, go.
AC: If we can use something like a head tilt to get a tiny glimpse into what a dog is thinking, can we actually tell how smart a dog is?
SD: Funny you should ask, because the internet has come up with some pretty questionable ways to test your dog’s IQ.
AC: All right, I’m intrigued.
SD: And scientists have some much better ones.
AC: Unsurprising.
SD: Unsurprising. Now, we have to take a quick break, but when we come back, we’ll explain how you can actually test your dog’s IQ.
AC: I mean, Tufo genuinely sounds like a genius.
SD: I mean, he is. I won’t argue with, with that at all.
AC: Welcome back. Okay, Sarah, before the break, you promised that we could actually put Tufo’s brain to the test.
SD: I did. Because we spent this whole episode talking about what his head tilt might tell us about what’s happening inside his brain, naturally the next question is how smart is my adorable little Saint Bernard?
AC: Little?
SD: He’ll always be a little puppy to me.
AC: Debatable. Anyhow, uh, I’ve seen a pet IQ test online called the wall test where you hold up your dog and you slowly move them toward a wall to see if they put their paws out.
SD: Yes, yes. I’ve seen that one, too, but I don’t think anyone’s lifting Tufo, and that might actually be a good thing because experts say you should not do the wall test.
AC: Really?
SD: Yes. Don’t do it. Whether a dog sticks out their paw actually has nothing to do with intelligence. It’s a neurological reflex vets use to check that a dog’s nervous system is working properly, kind of like when a doctor taps your knee and your leg automatically kicks.
AC: Hmm. So it’s not really an IQ test at all.
SD: No.
AC: But how do I find out if my dog is a genius?
SD: Well, obviously they are. But scientists would say that’s actually the wrong question.
AC: Rude.
SD: Because there isn’t one thing called dog intelligence. Some dogs are amazing problem solvers, some are great navigators, and some are especially good at reading us.
AC: Which feels especially relevant after an entire episode about dogs listening to us.
SD: Exactly. And you can test this even at home. Just grab two identical cups and a treat. While your dog isn’t looking, hide the treat under one cup, then put both cups down and either point at the correct cup or just look at it, and then let your dog choose which cup they’re drawn to.
AC: So if they pick up the right cup…
SD: They successfully read your social cue. That’s a skill called social cognition.
AC: Hmm. Genius.
SD: Good at one particular type of cognition.
AC: Or genius.
SD: Fine. Genius. All dogs are geniuses. Yeah. But if they choose the wrong cup or, you know, just wander off down the hallway, that doesn’t mean they’re dumb. Dogs have different strengths, and just like us, they have to actually be motivated to participate.
AC: So if my dog completely ignores my carefully designed scientific experiment…
SD: You might just need better treats.
AC: Fair.
SD: And I think that brings us right back to the head tilt. One cute little behavior cannot tell us exactly what’s happening inside our dog’s brain, alas.
But it can give us a clue and remind us that there’s a whole lot going on behind those puppy dog eyes.
AC: Even when it looks like there absolutely is not.
SD: Watch it.
And that’s it for this episode. But don’t worry, we’ve got more episodes of Ask Us Anything live in our feed right now. Follow or subscribe to Ask Us Anything by Popular Science wherever you enjoy your podcasts. And if you like our show, leave a rating and review.
AC: We care what you think. We care what your dogs think, geniuses or not. Our producer is Alan Haburchak, and this week’s episode was based on stories written for Popular Science by Nidhi Sharma and Clarissa Brinkett.
SD: Thank you, team, thank you, dogs, and thanks everyone for listening.
AC: And one more time, if you want something you’ve always wondered about explained on a future episode, go to popsci.com/ask and click the “Ask Us” link. Keep the questions coming. Till next time, may your dog always listen when you call.
SD: May your pockets be full of sufficiently motivating treats.
AC: And may your dog’s head tilts always be ridiculously cute.
SD: Like, they can never not be. They’re always cute.
I consider myself to have a somewhat challenging personality. The popular Myers-Briggs test labels me an ‘INTJ,’ commonly referred to as an “Architect.” We’re supposedly independent, determined, curious, and original, but we’re also considered combative, overly critical, dismissive of emotions, and more concerned with “rationality and success over politeness.”
My family would generally agree with that description of me. Perhaps thankfully, we are a rare bunch: only two to four percent of people, and about one percent of women, are INTJs.
I’m expecting my first child, a daughter, in January. The prospect of meeting a mini “Architect” has me wondering: Are we born with our personalities?
The answer is complicated—and still evolving.
“All of it comes from genes, and all of it comes from [the] environment in a sense,” says Matthew Keller, a behavioral geneticist at the University of Colorado Boulder.
Dozens of twin studies have found that personality is partly heritable, but exactly how genes shape who we become is far from straightforward, and it’s even less clear how environmental factors play a role.
What is personality?
The very definition of personality is complex and somewhat “squishy,” Ted Schwaba, a personality psychologist at Michigan State University, concedes.
Most people think of personality as traits. The adjectives we use to describe a person: creative, hard-working, honest, kind. But personality also encompasses identity, life story, reputation, and how other people see you, says Schwaba.
Brent Roberts, a personality psychologist at the University of Illinois Urbana-Champaign, adds that skill, ability, interests, and motivations also play a role. Someone can have remarkable talent in, say, soccer or music, but without the interest or motivation to pursue those areas, one’s ability may never become central to what they do.
“Personality is really any kind of stable characteristic of a person that makes them who they are,” says Schwaba.
The Myers-Briggs test is a popular self-report questionnaire that categorizes people into 16 different personality types. Video: Myers-Briggs Explained in Less than 5 Minutes – 16 Personalities, Frank James
While personality itself remains somewhat difficult to define, most psychologists use the standardized “Big Five” model to measure it. The framework scores people along five broad traits: openness, conscientiousness, extraversion, agreeableness, and neuroticism, the tendency to experience negative emotions.
Though imperfect, the model lets researchers compare personality across people and over time. The Big Five and other measures of personality have proven consistent predictors of school and work performance, relationships, health, and even longevity.
Heritability tells us how much of the variation in personality, or other traits, can be linked to genetics. If something is 100 percent heritable, all the differences we see among people would be linked to genetics, Keller explains. If it is 0 percent heritable, then genetics would explain none of those differences.
So, when researchers say personality is about 40 percent heritable, they don’t mean 40 percent of a person’s personality comes from their genes. An outgoing mother may pass along genetic variants that make her child more likely to be outgoing too, but her daughter is not destined to share the trait.
And just because a trait is heritable doesn’t mean it is immutable or set at birth. Environmental factors still matter. Height, for example, is roughly 80 to 90 percent heritable, yet genes don’t determine exactly how tall someone will become. A child with tall parents may have the genetic potential to be tall, but without proper nutrition may never reach that potential.
Most psychologists use the standardized “Big Five” model to measure personality. Video: The Big 5 OCEAN Traits Explained – Personality Quizzes, Practical Psychology
Both height and personality are genetically complex. A 2022 study of 5.4 million people identified more than 12,000 genetic variants associated with height, while a 2024 study of the Big Five identified 254 genes associated with at least one trait. There is no single gene for height nor is there a single gene for being extroverted, agreeable, or conscientious. Instead, these traits appear to arise from the combined effects of many genes, each with a very small influence.
Schwaba likens individual genetic variants to grains of sand on a beach. One grain tells you nothing about what the beach will look like, but collectively thousands of them help shape the landscape.
What else shapes our personality?
If genes explain only part of personality differences, what explains the rest? Scientists are still trying to figure that out.
Over the past few decades, researchers have investigated everything from parenting and birth order to socioeconomic status and major life events. Except for extreme experiences such as abuse, trauma, or severe deprivation, most non-genetic aspects have turned out to have surprisingly small effects.
Roberts calls it “death by a thousand cuts.” No single experience appears to explain much, but thousands of small environmental influences may accumulate over a lifetime to shape personality.
Studying environmental factors is much harder than studying DNA. Scientists can sequence a genome, but understanding environmental impacts requires psychologists to follow thousands of people for decades.
Researchers do know that personality isn’t fixed; it can change over the course of one’s life. People can even deliberately alter some traits. Therapy can change personality, Schwaba says, and adults can even become more outgoing, for example, if they put intensive effort into socializing and pushing themselves outside their comfort zone.
Scientists aren’t certain when personality first emerges. Schwaba says individuality is obvious by preschool, but recently he and a colleague found themselves wondering whether future extroverts might kick more in the womb.
“We don’t actually know of a study on that,” he says.
Will my daughter be a mini-Architect? Maybe in some ways. The genes she inherits may nudge her personality in certain directions, but they won’t lock her into them. Who she becomes will depend on a lifetime of experiences that scientists are still trying to untangle.
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
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