Scientists have long thought humans have a uniquely difficult birth compared to other primates.

And it’s true that from an evolutionary perspective, we face an obstetrical dilemma. As we evolved to walk on two feet, our pelvis changed shape and size and our birth canal became smaller and oddly twisted. Meanwhile, the evolution of our enormous brain meant that a large-headed baby needed to fit through it.

But our new study found that some primates have to give birth to babies that are squashed even more tightly in the birth canal.

Primate births usually happen at night which makes them difficult to study, especially in the wild. Because of this, we do not know much about them. Rare observations suggest a more solitary, shorter and less noisy labour compared to humans.

To get around this lack of direct observations, in 1949, the Swiss anthropologist Adolph H. Schultz looked for clues in anatomy instead. He measured the size of the pelvic birth canal and of the neonatal head in humans and seven other primates. He wanted to find out how tightly the baby’s head fitted through the birth canal. His results suggested that humans were unique in having a rounder birth canal, one in which the baby’s head only just fitted through. Other great apes seemed to have an oval canal with plenty of space for the baby’s head. Schultz’s influential study shaped how we think about human birth as unusual and harder than other primates.

However, the way he measured the birth canal was based on human anatomy. And the human standard measure Schultz used did not actually reflect how much space there was. He used the distance between the top of the sacrum (a large triangular bone at the base of the spine) and the top of the pubis bone to represent the diameter of the canal (a standard obstetric measurement in humans). Since the sacrum sits higher in the pelvis in other primates, only its lower part forms part of their birth canal. This is why their birth canals appeared so elongated in his diagram.

Our study instead used 3D scans of the pelvis and measurements of the newborn head from 29 primate species, including humans, to revisit Schultz’s study using more appropriate measurements. We found that, in reality, many primates experience a much tighter fit than humans do.

Some of the most striking examples were squirrel monkeys and tamarins, small primates from Central and South America that live high in the tree canopy. Their babies are born with heads that are about double the size of the maternal pelvic canal.

Birth for squirrel monkeys and tamarins might seem impossible with such a head-to-pelvis disproportion, but a 1995 study provides an explanation. After many sleepless nights spent observing squirrel monkeys giving birth in a primate centre, doctoral researcher Melissa Stoller managed to take X-ray images of the labour, revealing that the monkey’s pelvic bones temporarily dislocate, creating extra space for the baby to pass through. We think a similar strategy may help bushbabies, whose newborns also have unusually large heads relative to the mother’s birth canal. As we showed in a 2025 study, the pelvis of female bushbabies (but not males) is open at the front, where their pubic bones do not meet and elastic connective tissue between them can stretch and expand during labour.

Another adaptation concerns the way the baby moves through the birth canal. In these species, babies usually enter the canal face-first rather than with the top of the head (the orientation assumed by Schultz in his diagrams). This reduces the amount of space needed during birth. Stoller’s X-ray study showed this in squirrel monkeys and baboons, and it may be common in other monkey species too.

Neither of these adaptations could be easily adopted by humans. Because we walk upright, our pelvis must provide stable support to our body and cannot easily stretch during labour. In fact, when the pelvic ring dilates too much during birth, this can lead to severe pelvic pain, and difficulty in walking after the birth. Instead, our babies have evolved a highly plastic head that can mould to the shape of the maternal birth canal as it passes through it.

The dangers of giving birth

Giving birth is neither easy nor risk free. Despite huge medical advances, we still see three maternal deaths per 1,000 live births worldwide, and more in low-income or war-torn countries. Birth is even riskier for babies, with 17 deaths per 1,000 live births in sub-saharan Africa. Maternal mortality rates were much higher a hundred years ago, before antibiotics and advances in obstetric care.

While these adaptations make birth possible, they do not necessarily make it easy. Going back to squirrel monkeys, despite their flexible pelvis they have high maternal and foetal mortality.

Looking even more widely across mammals, there are many examples of unique adaptations for apparently impossible births; from female bats having evolved a pelvic girdle fully open at the front so the pelvis is more of a horseshoe structure to accommodate enormous babies, to spotted hyenas having to give birth through their extremely large clitoris.

Primates, and mammals at large, show a diversity of obstetrical dilemmas, each solved in different ways depending on their anatomy, ecology and movement. If we are unique, so are all other species.

Lia Betti received funding from the Leverhulme Trust and the Great Britain Sasakawa Foundation.

Nicole Torres Tamayo is currently working as a researcher at the Institut Català de Paleontologia Miquel Crusafont (ICP-CERCA). She is affiliated with the Department of Anthropology of University College London and the Institute of Evolutionary Medicine od the University of Zurich.

What happens when a leader suddenly disappears? In politics, business and other human organisations, leadership transitions can trigger intense power struggles. Rivals compete for control, alliances shift and institutions can become unstable.

Similar dynamics occur throughout the animal kingdom. Our new research on tropical paper wasps, published in the journal Animal Behaviour, shows just how chaotic leadership struggles can be – but also how societies can remain stable even while conflict rages.

Many animal societies revolve around a single dominant breeder. In cooperative paper wasps, as in many other social wasp species, dozens or even hundreds of females live together in a colony. But reproduction is usually controlled by one dominant individual: the queen. The other females help raise her offspring by foraging for food, feeding larvae and defending the nest.

Unlike in honeybee, yellowjacket wasp or ant colonies, however, these helpers are not sterile. If the queen disappears, any of them could potentially take over and become the next breeder. Often there is a queue – ladies-in-waiting, hoping to be queen. But succession isn’t always predetermined, and in some cases, it can become a contest.

How is the contest settled?

It was in a derelict building in Panama that we found the answer to this question. We experimentally removed queens from 19 wild colonies of the tropical paper wasp, Polistes canadensis, and watched to see what happened next.

The effects were immediate. Aggressive interactions between females increased sharply as several of them competed for dominance. The colony’s usual patterns of behaviour broke down and its dominance hierarchies rapidly became unstable. Rather than a smooth transfer of power, succession turned into a period of widespread conflict involving many members of the colony.

At first glance, this kind of turmoil looks risky. Fighting takes time and energy, and wasps distracted by conflict might neglect essential tasks such as foraging for prey, feeding larvae and maintaining nest structure and hygiene. Violent fights over leadership have been reported in other paper wasps, resulting in societal collapse.

But that wasn’t what we observed.

Despite the chaos, the colonies continued functioning. While some wasps fought to be queen, others avoided the conflict and instead stepped up their investment in foraging and brood care. They ignored the conflict and kept the colony running.

Cooperation didn’t disappear – it was redistributed.

One surprising finding was that the peaceful wasps didn’t appear biologically different from the fighters. In many animal societies, traits such as body size, age or position in the hierarchy help predict who will compete for leadership. For example, in meerkats the largest and oldest females are most likely to inherit the dominant breeding role. In naked mole rats, females already high in the hierarchy fight hardest when the queen dies.

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Read more: Of mice and matriarchs: the female-led societies of the animal kingdom

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But in our wasp colonies we found no clear differences in body size, age or previous status between individuals that fought and those that stepped back.

This suggests the behaviour may reflect strategic decisions rather than fixed roles. Some wasps may judge that competing for dominance offers them a good chance of producing their own young, while others may gain more by assuring the survival of the brood, which are typically the wasps’ siblings. Investing in the survival of your close relatives is an alternative reproductive strategy and explains the evolution of helping behaviour in animal societies.

Cooperation during conflict

Social insects are often portrayed as perfectly organised societies with rigid rules. Honeybee and ant colonies, for example, typically have sterile workers, leaving little competition over who becomes the next queen. But paper wasps are different. Workers retain the ability to reproduce, and in the tropical species we studied there appears to be no “next in line” when the queen disappears.

Aggression-driven succession might seem too costly for a society to tolerate. Yet our results show it can work, so long as some animals compensate for the disruption by maintaining essential tasks. Even during intense leadership battles, cooperation can persist if some members adjust their behaviour to keep the system functioning.

The balance the wasps in our study maintained may be a common feature of social systems more broadly. When power struggles intensify, stability depends upon those who keep crucial work going in the background.

It’s easy to get distracted when political rivals are fighting it out, but, perhaps we should be shining a light on the unsung heroes who quietly keep things ticking over.

Seirian Sumner receives funding from UK government's Natural Environment Research Council (NERC) and the Biotechnology and Biological Sciences Research Council (BBSRC). She is a Trustee and Fellow of the Royal Entomological Society, and author of the book 'Endless Forms: Why We Should Love Wasps'.

Owen Corbett receives funding from the UK government's Natural Environment Research Council (NERC).