When people think about climate change and animals, they might picture melting sea ice, drought-stricken landscapes or species struggling in the wild. Far less attention is given to the animals living in our care. Yet as heatwaves become more frequent, intense and unpredictable, zoos may be facing one of the biggest animal welfare challenges of the coming decades.

The recent deaths of three lions at Tama Zoo, Japan during a heatwave that reached over 40°C have brought this issue into sharp focus. Investigations are ongoing, but the incident raises an important question. Do we understand how animals will – or won’t – cope with heat in a rapidly changing climate?

Many people are surprised to learn a lion can be vulnerable to high temperatures. After all, lions evolved in Africa and are often associated with hot, arid landscapes. But this assumption oversimplifies how animals experience heat. Temperature is only one part of the story.

Humidity can play an equally important role. Lions, like many mammals, do not sweat effectively like humans do. Instead, they seek shade, move less and pant to dissipate heat.

High humidity can make these cooling strategies less effective. Perhaps surprisingly, humans are often more heat tolerant than lions. Our ability to sweat over most of the body makes evaporation cooling remarkably effective, allowing us to remain active in conditions that may force lions to rest and seek shade.

In the wild, lions may have more choices available to them that help them to remain cooler. Wild lions can move across large landscapes, select favourable microclimates, rest in shaded areas and adjust their activity patterns – none of which may be feasible in a zoo. Wild lions are famously inactive for much of the day, often waiting until cooler periods to hunt and socialise.

Other vulnerable species are perhaps unsurprising, including polar bears, penguins and red pandas, all of which are adapted to cooler climates. However, climate change may also affect species that people typically associate with warm environments.

Great apes, tapirs, cheetahs and even elephants can struggle with dangerous levels of thermal stress when high temperatures are combined with humidity, which reduces the body’s ability to dissipate heat. For elephants in particular (due to their large body size), the risk is heightened when access to shade, water or other cooling opportunities is limited.

This highlights an important principle in animal welfare science: welfare is influenced not simply by the environment animals are exposed to, but by the degree of choice and control they have within it.

Climate change is changing welfare

Historically, many zoos, particularly in temperate regions such as the UK, were designed with cold weather in mind. Considerable effort has gone into protecting animals from low temperatures and rain. Extreme heat, by comparison, has often been viewed as a relatively minor concern. That assumption may no longer hold.

Climate change is increasing the frequency, duration and intensity of extreme heat events across the globe. As heatwaves become a more regular feature of British summers, zoos will need to think differently about how they manage animal welfare. Temperature control measures that were adequate ten or 20 years ago may not be sufficient for the conditions today and in the future.

Zoos have long managed environmental conditions for species with specialised needs. Tropical animals are often provided with heated indoor shelter, for example. The challenge is that climate change is shifting the environmental baseline upon which many of these systems were designed.

One strength of modern zoo management is its recognition that animals are individuals. Two animals of the same species living in the same enclosure may respond very differently to identical conditions. Older animals may have less physiological resilience. Animals with respiratory conditions or other underlying health issues may be particularly vulnerable.

The good news is zoos routinely maintain detailed individual records on behaviour, personality, social relationships, health history and life experience, which will help them monitor any changes.

Increasingly, extreme heat should be treated in the same way as severe storms, flooding or disease outbreaks: as a foreseeable animal welfare risk. This might involve expanding shaded areas, creating climate-controlled refuges, improving ventilation, installing misting systems and water features, and monitoring temperature and humidity in real time.

But infrastructure alone will not be enough. Zoos may also need to adapt their daily management practices, adjusting feeding and enrichment schedules and modifying routines to minimise heat exposure during the hottest parts of the day.

Tell-tale signs

One of the most valuable and commonly used indicators of animal welfare is behaviour. An animal that appears lethargic, withdrawn or less engaged with its surroundings may be signalling compromised welfare long before clinical symptoms emerge.

An animal experiencing thermal discomfort may alter its activity patterns, spend more time in certain locations, change its posture, eat less food or interact differently with other animals. Humans are no different.

During heatwaves we change our routines, seek cooler places, struggle to sleep and often reduce activity. In people, exposure to extreme heat also affects cognition and behaviour. Evidence suggests that attention, memory and decision-making can deteriorate under thermal stress, raising important questions about how heat may influence the behaviour and welfare of animals in our care.

These insights are relevant not only to zoos, but to all managed animal populations. Farms, wildlife parks, rescue centres and companion animal owners are all likely to confront similar welfare challenges in a future defined by more frequent and intense heatwaves.

Samantha Ward does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

Perhaps the most evocative windows into the minds of our ancient ancestors are the images they painted on the walls of caves – renderings known simply as rock art or cave paintings.

Rock art, which is found across the world, offers snapshots of human history, preserving representations of cultural traditions, symbolic communication and social identity. Common subjects include animals, hunting scenes and hand stencils – thought to have been made by blowing pigment over the artist’s hand.

Scientists have wrestled with basic questions about the origins of rock art such as: what was used to produce it, when was it made, and who made it?

Now, a recent study of cave art from Spain and Portugal has shown that ancient human DNA can be recovered directly from the cave walls. We may therefore be edging closer to answering “who made it?”

Using this technique to link a specific painting to an ancient artist may be a step too far. But there are still important things it can tell us about the past.

Previously, researchers had made considerable progress in answering the “what” and “when” questions. Scientists can analyse what pigments are preserved in rock art using techniques such as X-ray fluorescence spectroscopy, which allows them to identify materials such as red ochre and charcoal.

Scientists can also estimate the age of the art using techniques such as carbon-14 dating, carried out on minute quantities of organic material, to determine when rock art was created.

For example, in 2026, researchers reported that a stencil of a human hand had been found in Southeast Sulawesi, Indonesia. The researchers used a laser to bore a microscopic hole and collect samples from a fine layer of calcium carbonate that had formed over the painting.

The research team then measured the degradation of radioactive uranium contained in these mineral deposits to assign a minimum age to the painting of 67,800 years – making it the world’s oldest known minimum date for cave art.

The age of this art fits within a period when modern humans (Homo sapiens) are thought to have migrated through the area. But the wider Indonesian archipelago was previously home to other human species, such as Homo erectus and Homo floresiensis (nicknamed “the Hobbits”), which complicates matters significantly.

While the stylistic and technical complexity of the art suggests authorship by Homo sapiens, we have no way to be certain who made it.

Identifying a human linked to a prehistoric item has an established methodology. For example, in 2023, researchers recovered ancient DNA from a deer-tooth pendant found in Denisova cave, southern Siberia.

Analysis of the genetic sequence showed that the pendant was probably worn by a 25,000-year-old woman belonging to an ancient population known as Ancient North Eurasians, who lived across Siberia in Palaeolithic times.

Such discoveries transform otherwise anonymous objects into evidence of real people, giving us an intriguing glimpse into the lives of our prehistoric ancestors.

New genetic reservoirs

In the latest cave art study, scientists systematically sampled both painted and unpainted surfaces of cave walls at 11 sites across Spain and Portugal. Remarkably, they recovered ancient human DNA directly from the walls, demonstrating for the first time that these surfaces can act as reservoirs for genetic information.

Some of these cave wall samples were shown, through both genetic analysis and the archaeological context, to be at least 4,000 years old.

In two of the samples from Covarón cave in northern Spain, human DNA was sufficiently preserved to be identified as belonging to ancient western hunter-gatherers. These were populations that lived across much of Europe during Late Palaeolithic and Mesolithic times.

One of the samples from a pigmented part of a cave wall, in Escoural cave, Portugal, also yielded traces of ancient DNA, but it was not as well preserved as other samples.

Nevertheless, the research establishes a powerful new methodological framework and proof of concept. It raises the tantalising prospect of studying not only the images left behind, but also the communities of people who made them.

However, applying this technique successfully to identify the artist of specific ancient rock art is incredibly unlikely.

This research could, though, potentially resolve one of the most contentious debates in archaeology. This concerns a 66,700-year-old hand stencil discovered within Maltravieso cave in Extremadura, western Spain.

This controversial stencil appears to predate the arrival of modern humans in Europe. Some researchers have therefore argued that Neanderthals, not Homo sapiens, made the art.

Finding DNA from either species in this location, at similar times to the creation of the rock art, could offer a potential route for resolving the debate. But this too is much more difficult than it might seem.

Easy transfer

The presence of human DNA does not prove who created the rock art. DNA is easily transferred – actions as simple as breathing can transfer someone’s genetic material to a location or object. This is why DNA recovered from a knife cannot definitively conclude who committed a stabbing in a forensic case.

DNA establishes the association of someone with an item or location, not the actions they carried out during the deposition of their genetic material. Environmental activities such as rain can also transfer human DNA to these spaces, making it extremely difficult to identify a single person at a location.

Additional DNA can be deposited at very different times to when the painting was produced – by both modern and ancient humans. The potential for contamination is incredibly high, making it difficult to establish who may have been at a given location at a specific time.

Nonetheless, scientists can use this new research to explore important prehistoric questions such as the hominin species, population groups and sexes of those who entered specific cave systems, and their migration routes across landscapes.

In the general sense, we can also begin to connect prehistoric art with the people who created and lived with it.

Behind each rock-art hand stencil or hunting scene is evidence of a real person who, while separated from us by tens of thousands of years, felt the need to communicate their creativity – as we do today. Techniques such as this offer a vital opportunity to understand more thoroughly the human experience via our similarities with ancient people.

The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.

Struggling to sleep in the heat? The good news is that a handful of easy cooling tricks can help your body stay comfortable and make hot nights far more bearable. When temperatures rise, sleep often suffers. Hot nights can make it harder to fall asleep, increase waking during the night, and leave people feeling less [...]

You may be spotting bodies of dead bumblebees lying across pavements near you in the summer months.

There are several reasons for this, some to do with the weather and some very much due to humans.

Bumblebees are social, living in colonies that are maintained by very active workers with a relatively short lifespan of 4-6 weeks. This means, that in a relatively short space of time, older bees die off and to prevent disease making it’s way through the colony, the healthy young workers bring out their dead, moving them far away from their colony.

Another factor is the weather. It was the warmest June on record in England (and the second hottest for the whole UK) in 2026 and other high temperatures are expected this summer.

Thermal stress in bees is caused by this kind of extreme heat. Prolonged heat exposure alters development and threatens long-term population stability.

Heatwaves also severely disrupt reproduction, flight capacity and ability to forage. Social insects including bees rely on cooling behaviour for example fanning their wings when things get too hot to buffer the hive but there are limits as to how effective this is in extreme heat.

As it gets hotter bee foraging activity may increase, and they may cover greater distances. Bees can thermoregulate themselves by moving heat around their body while in flight, but extremes of temperature can affect their health.

Don’t kill your dandelions

Other elements play a part in bee deaths. Pesticide and herbicide use is commonplace and these chemicals affect the fitness of bees, causing them to die off. However, not only do these chemicals affect the bees directly, they can also remove important plants that they rely on for food causing them to starve. Dandelions for example, are a massively important nectar source for bees, but also a plant commonly controlled with herbicides. So don’t weed your garden and pull them out.

Pesticide use is one of the historical key causes for loss of bees. Pesticides applied to reduce the insects feeding on crops – is some what ironically – responsible for harming a group of animals responsible for their pollination.

The EU and the UK have banned the use of the most harmful neonictinoids, however pesticides are still routinely used, with harmful knock-on effects on pollinators. Insecticides such as organophosphates, synthetic pyrethroids and phenylpyrazoles cause paralysis and death of these insects, as well as disrupting the bees ability to navigate.

Habitat loss is another key issue for bees, with land use changing for housing developments, intensive agriculture and other human structures such as roads. Without these habitats to support the bees, their populations suffer.

Climate change is causing other problems.

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Read more: A common soil pesticide cut wild bee reproduction by 89% – here’s why scientists are worried

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Phenological mismatch is when two organisms that are dependent upon each other appear at the wrong time. With a changing climate this is happening. An example of this is that plants that the bees are dependent upon being in flower when the bees aren’t around. Essentially they are flowering at a time of year when the bees aren’t there. They could come out of hibernation too early and not have any nectar to feed on, or conversely the flowers could come out before the bees appear. This, of course, is a disaster for both flower and the bees.

Bees under threat

Insects are the most diverse and abundant group of animals on the planet, but in spite of this dominance they are the most threatened.

We are losing our bees at an alarming rate. In Europe, the number of wild bee species considered at risk of extinction has more than doubled from 77 in 2014 to at least 172 today (around 10% of assessed species).

But we really need them. They provide so much support for our planet, as decomposers, pest controllers, food for other animals and generally for keeping our systems clean and tidy. Scientists call these these roles ecosystem services. However, one such service they provide is directly very important for us, and disproportionately carried out by one group of insects, the bees, and that is pollination of our crops.

Bees are one of the most diverse insect groups. They support an estimated 75% of global crop pollination, and nearly 90% of all flowering plants. The rest can be pollinated by other animals, or by the wind and weather.

If we lose that food security afforded by our pollinators we could have a global food crisis on our hands.

Anything that can be done?

When it comes to pesticide use, be a thoughtful and informed consumer. Support local, sustainably produced food where possible and, if feasible, buy from farmers who use nature-friendly practices. This does not necessarily mean choosing organic products, but rather considering how food is produced and the environmental impact of different farming systems. As a homeowner, explore non-chemical alternatives first, and apply any treatments sparingly and responsibly to minimise harm to beneficial insects and other wildlife.

One of the simplest ways to address habitat loss for bees is to make room for native species. Plant a diverse range of native flowering species, and leave some areas of lawn unmown. This can help create safe havens for bees.

In the short term there is one thing that people can do to help bees in distress. Providing sugary water to a bee you see struggling on the pavement on a hot summer’s day can help revive it. A mixture of sugar and water will do the trick at a two-to-one ratio of sugar to water..

Moving the bee, if safe to do so, to a flower or a shady spot too is also advised. But please do not do this if you have allergies, or are likely to get stung.

Alex Dittrich does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

The hunt for Planet Nine continues as new discoveries make the Solar System’s biggest mystery even harder to solve. Is there a massive undiscovered planet on the outer reaches of the Solar System? The idea has been around since before the discovery of Pluto in the 1930s. Labeled as planet X, prominent astronomers had put [...]

Nasa has named the crew of its next Artemis mission, which it promised would be an “extraordinary demonstration of what is possible”. Artemis is the ambitious American-led effort to return humans to the lunar surface by 2028.

However, the Artemis III mission will not travel to the Moon. Instead, the Orion spacecraft will stay in low Earth orbit and aim to dock with lunar landing vehicles launched separately by one or two space companies.

Nasa astronauts Randy Bresnik, Frank Rubio and Andre Douglas and Italian European Space Agency astronaut Luca Parmitano will carry out the mission in late 2027.

There are two potential landers that could fly on their separate mission to then rendezvous with the Orion spacecraft; one made by Elon Musk’s SpaceX and the other by Jeff Bezos’ firm Blue Origin. They will carry astronauts down to the Moon’s surface on subsequent missions.

However, the timetable for Artemis III and the first mission to return astronauts to the lunar surface (Artemis IV) has been thrown into doubt by the recent explosion of a New Glenn rocket. This rocket is intended to launch Blue Origin’s lander, called Blue Moon.

The blast could add significant delay to the Artemis schedule and there is speculation that it could even put the US out of contention to reach the Moon before China, which is planning its own Moon landing mission for 2030.

On May 28, 2026, Blue Origin’s New Glenn rocket, which is able to carry heavy payloads, was undergoing a standard engine fire test. For reasons which are still unclear, a huge explosion destroyed the rocket and damaged Blue Origin’s only launchpad, located at Cape Canaveral Air Force Station in Florida.

To reduce costs, Nasa’s Artemis programme relies on companies like Blue Origin to deliver key elements for lunar missions, like the landers.

Rockets capable of lifting heavy payloads into orbit will be needed for launching the building blocks of Nasa’s Moon base, which will be assembled over the next few years.

Under previous plans, SpaceX’s Starship vehicle would have been used as the lander for the first two Artemis missions to touch down on the Moon. However, Starship ran into problems during flight tests, so Nasa hatched a plan B.

This backup plan brought Blue Origin’s Blue Moon lander into the mix for Artemis IV, the first mission back to the lunar surface. Now, with the explosion, even their Plan B seems to have moved out of reach.

How bad is the situation though? Blue Origin only has one launch pad that, at the moment, cannot be used. But recent updates by the company’s CEO, Dave Limp, indicated that the situation may not be as dire as it might have been. He indicated that New Glenn might even return to launch before the end of the year.

The propellant farm (which stores the rocket fuel and oxidiser that are pumped into the rocket just before launch), as well as the water tower were undamaged. So were the booster and upper stages of another New Glenn rocket held in the pad’s integration facility. The launch tower will only need to be repaired and not rebuilt.

To gain some insight into the potential impact on the timescale for a next launch, we might be able to look at a similar situation faced by SpaceX in September 2016 where a Falcon 9 rocket exploded on one of their launchpads.

They were able to continue only four months after, but from one of their alternative launch sites, none of which were fully operational at that time. But Blue Origin has no alternative site at all. SpaceX needed more than a year to start up operations again on their damaged launch site.

Kiko Dontcev, vice president of launch at SpaceX, commented on the task ahead for Blue Origin. He described the rebuild as a high precision dissection of the damaged site to help better understand the incident. “In the initial days and weeks after you’re using a scalpel not a bulldozer,” he commented on X.

This might cast doubt on hopes that New Glenn could launch again this year. Indeed, Nasa’s chief Jared Isaacman told CNBC that the launch pad might not be restored until 2028. Currently, we just do not have the full picture of the damage and work needed. So, we can only hope that New Glenn launches can resume as soon as possible.

Nasa is already trying to limit the impact of the New Glenn explosion by urging Blue Origin to look at other rockets for launching its Blue Moon lunar landers. This would allow Nasa to still meet its deadline of landing astronauts on the Moon in 2028.

But New Glenn is still needed to land hardware such as the lunar terrain vehicles for Artemis missions and a robot rover called Viper, which will scout for water ice at the Moon’s south pole.

Exploring with pride

However, with 2030 approaching fast, the knock-on effects of the explosion could lead to China beating the US to the Moon. But the Artemis programme had already been hit by delays as a result of Nasa’s dependence on government whims and variable funding. Overall, the Artemis programme has had a relatively tight budget to work with.

Space agencies like Nasa must try (as much as possible) to decouple their efforts in space from ongoing turmoil in domestic politics. It could also do well to rely less on ventures led by billionaires, who often have their own private objectives.

One could view the Chinese lunar programme as being more stable, because of their one-party state as well as their strong sense of national pride. However, there is also a lack of transparency in their space programme and a focus on propaganda.

As an outside observer, it appears that Nasa’s current timeline for landing on the Moon and establishing a permanent lunar base is slipping away. But this may not be the disaster some in the space industry think it is. Perhaps we should embrace a pride in exploring space as a community of nations working together.

Daniel Brown does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

Each of us tells a story about who we are, often tracing our identity back through an imagined line of ancestors. Though identity is fundamentally cultural, we tend to anchor it in biology – in the idea of a stable genetic inheritance passed down through generations.

Population genomics has exposed a history far more complex, dynamic and intertwined than we might wish to imagine. Even in a place such as Britain, long imagined as an island of deep and uninterrupted heritage, genetic data suggest a history marked by intense migration, mixture and cultural reinvention.

Two new studies have reinforced this picture, by analysing DNA from the skeletal remains of British individuals who lived during Roman and medieval times.

Prehistoric Britain witnessed periodic major migrations interspersed with smaller and more regular movements of peoples across what was then a contiguous landscape.

After about 6100BC, rising sea levels isolated Britain from mainland Europe, helping to promote later historical narratives of a population relatively isolated.

Yet even early observers recognised otherwise. Writing in the first century AD, the Roman historian Tacitus noted the diversity of Britain’s tribes, suggesting their origins lay in Germany, Gaul and Iberia.

Such conclusions were drawn from physical, cultural and linguistic observations. Now it is testable, thanks to rapid advances in population genomics and ancient DNA sequencing, allowing direct ancestry reconstruction across demographic and political changes.

A major recent study by Marina Silva, from the Francis Crick Institute in London, and colleagues analysed more than 1,000 ancient genomes from across Britain during the first millennium AD.

The pre-print, which has not yet been published in a journal, asks one simple question: could the main historical events of Britain – the Roman occupation, Anglo-Saxon migration, the Viking Age and the Norman conquest – be detected in the genetic data of the populations that lived through these eras?

The answer was complicated. The Roman period, for all its political and cultural upheaval, left surprisingly little mark on the genetic structure of the wider population. About 80% of the individuals who lived during Roman times in Britain cluster almost exactly with those of the immediately preceding Iron Age, arguing for genetic continuity and no replacement. Even in urban centres where occupying Roman elites were most prevalent, the broader population retained overwhelmingly local ancestry.

In contrast, the early medieval period, from around 410AD (when Roman rule collapsed) to 1066AD, saw a substantial influx of new ancestry from across the North Sea. The researchers were able to detect this influx by comparing the British samples with genetic data from populations in other parts of north-west Europe. Continental ancestry associated with Anglo-Saxon migration appears in more than 70% of of the burials in southern “Anglo-Saxon” Britain.

Thus, migration was not just cultural but demographic on a scale sufficient to leave its imprint on the shape of population structure.

Yet even this transformation cannot be generalised. From about 700AD to 1000AD, further waves of continental influence appear in Britain, with the arrival of settlers from central Europe (seemingly from France and the Rhineland) and, to a lesser extent, the south of Europe. However, the Viking Age leaves a more uneven and regionally variable genetic signal than its historical prominence might suggest.

While a Scandinavian component is clearly present in northern and eastern regions, it is rarely of a magnitude comparable to that found in early medieval migrations. Most surprisingly, the Norman conquest of 1066 appears to have been largely an elite process, leaving little detectable trace in the genomes of the common population.

Genome-wide ancestry profiles straddle the date of the conquest, with no hint of abrupt population replacement. Despite all its drama, the conquest seems, at the level of population genetics, to have involved elite replacement by relatively few individuals.

A second pre-print study provides a closer view of what this looked like on the ground. Focusing on a rural cemetery at Priory Orchard in Surrey, Flavio De Angelis, from Arizona State University in Tempe, and colleagues examined individuals buried across the centuries before and after the Norman conquest.

Again, the results are surprising: rather than any clear genetic break after 1066, both pre- and post-conquest burials fall within the same cluster, showing shared ancestry and no evidence for demographic turnover. The continuity is not just qualitative, but visible in the statistical similarity of ancestry components across generations.

Instead, the community reflects a much longer history of interaction across the North Sea world. Its ancestry includes Anglo-Saxon-associated components, significant Scandinavian input dating to the Viking period, and smaller continental contributions.

Crucially, these elements are already present before the Norman arrival and persist afterward. The Norman conquest, in genetic terms, is barely visible. What looks, on historical timelines, like a moment of dramatic rupture appears, at the level of the common individual, as a continuation. Genes tell the story of populations and detect localised impacts of migration, but they do not map neatly onto geopolitics.

Taken together, these studies point to a crucial distinction. Cultural and political change does not necessarily equate to demographic change. Britain’s history is neither one of uninterrupted continuity nor of repeated population replacement, but something more complex: long-term mixture punctuated by events that reshape institutions more than populations.

Some migrations – such as those of the early medieval period – left deep and measurable genetic legacies. Others, despite their prominence in historical narratives, left only faint traces. The discrepancy is striking: the scale of genetic change does not map neatly onto the scale of historical attention.

Modern genetic data reinforce this picture. Contemporary populations across the British Isles do not form a single, uniform group. Instead, they cluster into overlapping but distinct lineages reflecting different regional histories and varying degrees of past migration.

These patterns echo the ancient record, but they did not affect all regions equally. Wales and Ireland retain stronger continuity with earlier populations, while England shows clearer evidence of ancestry linked to early medieval migration from northern Europe. Scotland occupies an intermediate position, reflecting both long-term continuity and later Scandinavian influence.

Importantly, these differences are matters of degree, not kind. All populations of the British Isles share deep common ancestry overlaid by layers of migration whose effects vary regionally. The structure we see today is the product of these layered histories, not the survival of isolated or “pure” populations.

What emerges is not a story of rooted, bounded identities, but of continual connection. British identity – like all identities – has been assembled over millennia through movement, interaction and adaptation.

Modern genomes do not simply tell us who we are; they preserve how we got here. History does not make migration exceptional – it reveals it as the norm.

Jay Silverstein does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

It may sound too bizarre to be true but the Amazon molly (Poecilia formosa), a fish that inhabits rivers, lakes and swamps in Mexico and Texas, exists over much of its range in populations that are 100% female. In 1932, the Amazon molly became the first known vertebrate to reproduce by cloning itself, producing all-female populations. A new genetic study has given scientists insights into the longstanding mystery about how and why this happens.

The proportion of females in the human population is roughly 50%. A few countries such as Maldives (38% female) and Moldova (54% female) diverge from this, but these differences can largely be explained due to male immigration and emigration. However, much more dramatic sex ratios are found in the animal kingdom. Kentish plover bird populations, where males care for offspring, comprise only 14% female, and sea turtle populations, where sex is determined by temperature often exceed 75% female.

Most animal species reproduce sexually. This involves the fusion of two gametes, the sperm and egg, that develops into an embryo. A process, known as recombination, randomly shuffles the genetic material from the mother and father. This produces increased variability in the offspring, and new combinations of traits. The genetic diversity improves the chances of survival for the species if its environment changes.

But the Amazon molly reproduces asexually, where there is no mixing of genetic material. This reduces genetic diversity, making populations vulnerable to extinction – if one Amazon molly is susceptible to a disease, they all are.

And there is another problem to being identical. Asexual species are more likely to accumulate harmful mutations. This phenomenon, known as Muller’s ratchet, predicts that clones should go extinct within 10,000 years. Yet, the Amazon molly, – a hybrid that arose through sexual reproduction between a female Atlantic molly (P. mexicana) and a male sailfin molly (P. latipinna) – has survived for over 100,000 years.

So, what is the secret to their sustained existence?

Gene conversion is a process where one version of a gene is replaced by another. In most species, such as humans, it is used to repair damaged DNA. However, in the Amazon molly, gene conversion has slowed Muller’s ratchet. The new study found that gene conversion appears to play the same role as recombination. This essentially enables the fish to purge harmful mutations and preserve beneficial ones. Indeed, despite reproducing asexually, the Amazon molly shows differences in body shape between populations, demonstrating evolution in response to its local environment.

The Amazon molly reproduces via a process called parthenogenesis, also known as “virgin birth”, where young are produced from an unfertilised gamete. This allows rapid growth of successful genotypes, the genetic blueprints of organisms, as all of the Amazon mollies can reproduce without finding a mate. As such, animals created via virgin births can colonise habitats quickly.

Parthenogenesis can be obligative, like in the Amazon molly, where it is the only means of reproduction. But, it can also be facultative, where species can switch between sexual and asexual reproduction. For example, the marbled crayfish, reproduce sexually in their native range but rapidly establish themselves in new habitats asexually, often from a single female.

The Amazon molly has a type of parthenogenesis known as gynogenesis where sperm is required to stimulate development of the unfertilised egg. So, the Amazon molly still needs to “mate” each time she reproduces, but the sperm is not incorporated into the offspring.

The Amazon molly mates with males from species closely related to them, which reproduce sexually. Although the genes of these males are not passed on to the next generation, it is still advantageous for them. That’s because female animals often follow trends when it comes to selecting a mate. So when the female fish of their own species see the males with an Amazon molly, they are more likely to mate with them.

Parthenogenesis is common in invertebrate animals, including ants, bees and wasps. It is less common in vertebrates but has been found in other fish, amphibians, reptiles including the Komodo dragon, birds such as Californian condors and sharks for example hammerheads.

Other all-female parthenogenic vertebrates include the whiptail lizards, where almost a third of species are comprised solely of females. The New Mexico whiptail lizard has even become a queer icon. Unlike the Amazon molly, these “lesbian lizards” do not need sperm from a male to stimulate egg development. They just need to engage in mating behaviour to stimulate ovulation, bypassing males completely.

Some blue-spotted salamanders have reproduced asexually for several million years. Although the all-female populations of the salamanders reproduce in a similar way to the molly fish, requiring sperm to stimulate development, they are kleptogenic. This means that they replace a portion of the mother’s DNA with a portion of DNA from the male’s sperm, incorporating a small amount of his genetic material into the offspring. This facilitates the genetic diversity that has enabled the salamanders to survive for so long.

Like the Amazon molly, the Brahminy blind snake, also known as the flowerpot snake due to its habit of burrowing in potted plants, is the only other known vertebrate that breeds exclusively via parthenogenesis.

The snakes have three copies of each chromosome, rather than the usual two, probably due to an error in cell division at some point in the evolutionary history of the species. Increased numbers of chromosomes have been found in many species, including salmon with four copies, and sturgeon fish with eight copies.

Increased numbers of chromosomes generates increased genetic diversity, which probably explains how the blind snake clones have survived for so long.

There could be more all-female animals out there yet to be discovered. After all, up until a few years ago we didn’t know that female snakes have two clitorises.

Louise Gentle works for Nottingham Trent University.