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Why A Little Mammal Has So Much Sex That It Disintegrates

It’s August in Australia, and a small, mouse-like creature called an antechinus is busy killing himself through sex. He was a virgin until now, but for two to three weeks, this little lothario goes at it non-stop. He mates with as many females as he can, in violent, frenetic encounters that can each last up to 14 hours. He does little else.

A month ago, he irreversibly stopped making sperm, so he’s got all that he will ever have. This burst of speed-mating is his one chance to pass his genes on to the next generation, and he will die trying. He exhausts himself so thoroughly that his body starts to fall apart. His blood courses with testosterone and stress hormones. His fur falls off. He bleeds internally. His immune system fails to fight off incoming infections, and he becomes riddled with gangrene.

He’s a complete mess, but he’s still after sex. “By the end of the mating season, physically disintegrating males may run around frantically searching for last mating opportunities,” says Diana Fisher from the University of Queensland. “By that time, females are, not surprisingly, avoiding them.”

Soon, it’s all over. A few weeks shy of his first birthday, he is dead, along with every other male antechinus in the area.

The technical term for this is semelparity, from the Latin words for “to beget once”. For semelparous animals, from salmon to mayflies, sex is a once-in-a-lifetime affair, and usually a fatal one. This practice is common among many animal groups, but rare among mammals. You only see it in the 12 species of antechinuses and a few close relatives, all of which are small, insect-eating marsupials. (Although they look like rodents and are colloquially called marsupial mice, antechinuses are more closely related to kangaroos and koalas than to mice or rats.)

Why? Why do these marsupials practice suicidal reproduction, and why are they the only mammals that do so?

The question has vexed biologists for three decades, and many have offered answers. Some say that females don’t survive very well after breeding, so males are forced to hedge their bets by mating with as many as possible. Other suggest that it’s just a feature of the group, which have become locked into a weird breeding system through some unknown quirk of their evolutionary history. Yet others think the males are being altruistic, sacrificing themselves to leave more resources for the next generation.

But Fisher, who has been studying antechinuses for decades, favours a different idea. Her team gathered data on the lives and environments of a wide variety of 52 insect-eating marsupials, from the fully semelparous antechinuses, to relatives where a small number of males survive past their first sexual liaisons, to species that breed repeatedly.

It’s their diet that matters. These animals feed on insects, and some experience a glut of food once a year but very little at other times. This seasonality increases the further you get from the equator. The species with the most seasonal menus also had shorter breeding seasons, and their males were more likely to die after mating.

Fisher thinks that as the ancestors of antechinuses spread south through Australia and New Guinea, they encountered strong yearly fluctuations in their food supply. The females were better at raising their young if they gave birth just before the annual bonanza, and were well-fed enough to wean their joeys. Their mating seasons shortened and synchronised, collapsing into a tight window of time.

That probably wouldn’t have happened if they were placental mammals like shrews or mice, which could have produced several litters during the peak of food. But they were marsupials: their babies are born at an incredible early stage and rely on their mothers’ milk for a long time. A baby shrew suckles for days or weeks; a baby antechinus does so for four months. The females could only fit in one litter during the annual peak.

This had a huge impact on the males, which were forced to compete intensely with each other in a matter of weeks.  They didn’t fight. Rather than using claws or teeth, they competed with sperm. The more they had, the more females they impregnated, and the more likely they were to displace the sperm of earlier suitors. Indeed, Fisher found a clear relationship between suicidal reproduction and testes size. The biggest testes of all, relative to body size, belong to species whose males die en masse, followed by those where a minority survive to mate again, and then by those with several breeding seasons.

The males that put the greatest efforts into sperm competition fathered the most young. It didn’t matter if they burned themselves out in the process, if they metabolised their own muscles to fuel their marathon bouts. These animals are short-lived anyway, so putting all their energy into one frenzied, fatal mating season was the best strategy for them. Living fast and dying young was adaptive.

This idea was first proposed in 1979 but Fisher’s data, although mostly correlative, provides fresh support for it. She certainly finds it more plausible than the idea that the males are selflessly sacrificing themselves for the next generation. After all, the males usually live outside the females’ home ranges, so are unlikely to compete with their own young for resources.

“Antechinus mating habits have appeared in many documentaries, and the explanation of males selflessly sacrifing themselves to increase food supply for young is the one given in all the ones I have seen,” says Fisher. “I hope that documentaries and textbooks now start to give an evidence-based explanation of sexual selection.”

Reference: Fisher, Dickman, Jones & Blomberg. 2013. Sperm competition drives the evolution of suicidal

reproduction in mammals. PNAS http://dx.doi.org/10.1073/pnas.1310691110

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Life In A Rapidly Shrinking Puddle

What if you were born into a world that only blinks into existence once a year, and lasts for mere weeks or months before disappearing again? How would you live your life?

We can find the answer in East Africa, where a fish lives out its entire lives in rapidly shrinking pools of water.

During the rainy season, water fills small depressions in the savannah, creating temporary ponds.  It’s the cue that the turquoise killifish (Nothobranchius furzeri) has been waiting for. Its eggs, encased in mud and lying dormant within the soil, finally hatch. Right from the start, the baby fish are on borrowed time. They have a couple of months before their puddle dries out. Before that unpredictable deadline, they’ve got to grow to sexual maturity, mate, and lay the next generation.

The turquoise killifish has adapted to this precarious existence by evolving the shortest lifespan of any back-boned animal. In the wild, they live for a few months and they fare little better in captivity. Back in 2003, Italian scientists Stefano Valdesalici and Alessandro Cellerino showed that groups of captive killifish start dying after just six weeks. On average, they survive for nine weeks, and none of them make it past eleven. For comparison, other related killifish live for around a year, as do tiny mammals like shrews.

If they die young, they’ve got to live fast. By studying the turquoise killifish and a related species, Nothobranchius kadleci, Czech scientist Radim Blažek showed that their body length increases by up to a quarter every day in their first two weeks of life.

By days 11 to 13, the males are already wearing their bold red adult colours. By days 17 to 19, the females are sexually mature and start to release eggs, which the males lace with sperm. Again, these are record-breaking figures for vertebrates. Female laboratory mice take at least 23 days to become sexually mature, as do the tiny wild infantfishes. The killifishes beat that by almost a week.

At first, the females lay a few dozen eggs a day, but they start producing hundreds once they stop growing and start channelling all their energy into reproduction. One particularly enterprising female managed to lay 583 eggs in a day. The first of these fertilised eggs develop so quickly, that if there’s enough water left, they can hatch in just 15 days. In as little as a month, the next generation is born.

The killifishes show how animals can adapt to extreme environments by evolving extreme lifespans. Another example comes from Madagascar. In response to the country’s harsh and highly seasonal environment, Labord’s chameleon spends the majority of its life within its own egg. An entire generation hatches during rainy November, matures by January, breeds by February, and dies by April. Meanwhile, their eggs stay underground until the following November. This unusual cycle means that at any given time, there’s only one generation of Labord’s chameleon on the planet and they’re all the same age.

Lobards-chameleonReference: Blazek, Polacik & Reichard. 2013. Rapid growth, early maturation and short generation time in African annual fishes. EvoDevo, citation tbc.

Valdesalici & Cellerino. 2003. Extremely short lifespan in the annual fish Nothobranchius furzeri. Biology Letters http://dx.doi.org/10.1098/rsbl.2003.0048

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Red squirrels Give Birth to Fast-Growers If They Hear Crowds

As parents, we set our children up for life’s challenges by feeding them, caring for their health, and sending them to school. Many animals also provide for their young, and some do it in inadvertent and surprising ways.

Take the red squirrel. The first year of this cute creature’s life is marked by intense competition and imminent death. Its goal is simple: get a territory before winter sets in, or face death by starvation. That goal becomes tougher if there are lots of other competitors around, but a mother squirrel has ways of preparing her pups for these trials. If she hears the sounds of crowded forest, stress hormones surge through her body and begin affecting her pups even before they are born. When they finally pop into the world, they start growing faster. The hormones are a chemical message from mum: Live fast, so you don’t die young.

Photo by Ryan W Taylor
Photo by Ryan W Taylor

Red squirrels have a diverse diet, but the seeds of the white spruce are among the most important items on their menu. These trees are “mast seeders” meaning that they all produce tons of seeds every 2 to 6 years and very few seeds in others. In the bonanza years, the squirrels have plenty of spruce seeds to bury in the autumn and snack on through the harsh winter. This means that a bumper autumn for spruce is always followed by a crowded spring for squirrels.

That leads to conflict. Red squirrels defend large areas surrounding a central store of spruce cones. They’ll fight for this real estate, because if the youngsters don’t establish a territory before winter sets in, they’ll find themselves bereft of buried grains, and almost certainly die.

A team of Canadian scientists has been studying the fates of red squirrels in the Yukon, Canada for the last 22 years. Back in 2006, they showed that the squirrels can actually anticipate gluts of spruce seeds, and produce more young in anticipation. Now, the same team have found that females can also adjust how quickly their young grow up depending on how much competition they will face.

Photo by Ben Dantzer
Photo by Ben Dantzer

The team, led by Ben Dantzer, now at the University of Cambridge, found that in competitive years following a spruce bonanza, the fastest-growing squirrels fare best and are more likely to survive their first winter. In normal years, these fast-growers have no advantage.

This explains why the mothers don’t always give birth to fast-growing young. It comes at a price—squirrels tend to have shorter lifespans if they are born in crowded years, so their early spurt seems to cost them later on in life. It’s a cost that’s only worth paying if there’s some advantage to be gained.

To test this link between density and growth, Dantzer’s team carried out a wonderfully simple but beautiful experiment where they simulated the sounds of a crowd. They recorded the loud “rattles” that squirrels make to defend their territories, and played them back to the creatures at two levels—one representing six times more squirrels than the other. If the females heard what sounded like squirrel-infested woods, they gave birth to pups that grew much faster.

Photo by Ryan W Taylor
Photo by Ryan W Taylor

How? The answer lies in the mothers’ hormones. The more crowded an area is, the higher the levels of cortisol—a stress hormone—in the squirrels’ bodies. Indeed, if Dantzer’s team created more crowded forests—either genuinely so by feeding the squirrels and boosting their numbers, or dishonestly so by playing the rattle recordings—the females’ cortisol levels went up.

Mums with higher cortisol levels gave birth to pups that grew faster. This wasn’t just a correlation. The team also injected some females with extra cortisol, and saw that their offspring grew up 41 percent faster.

Why? This part of the story is less clear. We know from other studies that and early dose of stress hormones can steer the development of young animals, affecting their ability to learn or handle stressful situations. Think of the hormones as an fortune-telling system, plugged into the mother’s senses. It provides the young squirrel with omens of the conditions it will face in the outside world, and prepares its brain and body to deal with those challenges.

Reference:  Dantzer, Newman, Boonstra, Palme, Boutin, Humphries & McAdam. 2013. Density Cues Trigger Maternal Stress Hormones That Increase Adaptive Offspring Growth in a Wild Mammal. Science http://dx.doi.org/10.1126/science.1235765

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Why do killer whales go through menopause?

Here’s yet another reason why humans are weird: menopause. During our 40s, women permanently lose the ability to have children, but continue to live for decades. In doing this, we are virtually alone in the animal kingdom. From a cold evolutionary point of view, why would an animal continue to live past the point when it could pass on its genes to the next generation? Or put it another way: why don’t we keep on making babies till we die? Why does our reproductive lifespan cut out early?

One of the most popular explanations, first proposed in the 1966, involves helpful grandmothers. Even if older women are infertile, they can still ensure that their genes cascade through future generations by caring for their children, and helping to raise their grandchildren.* There’s evidence to support this “grandmother hypothesis” in humans: It seems that mothers can indeed boost their number of grandchildren by stepping out of the reproductive rat-race as soon as their daughters join it, becoming helpers rather than competitors.

Now, Emma Foster from the University of Exeter has found similar evidence among one of the only other animals that shows menopause: the killer whale.


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The two-genome waltz: how the threat of mismatched partners shapes complex life

Two people are dancing a waltz, and it is not going well. One is tall and the other short; one is graceful, the other flat-footed; and both are stepping to completely different rhythms. The result is chaos, and the dance falls apart. Their situation mirrors a problem faced by all complex life on Earth. Whether we’re animal or plant, fungus or alga, we all need two very different partners to dance in step with one another. A mismatch can be disastrous.

Virtually all complex cells – better known as eukaryotes – have at least two separate genomes. The main one sits in the central nucleus. There’s also a smaller one in tiny bean-shaped structures called mitochondria,  little batteries that provide the cell with energy. Both sets of genes must work together.  Neither functions properly without the other.

Mitochondria came from a free-living bacterium that was engulfed by a larger cell a few billion years ago. The two eventually became one. Their fateful partnership revolutionised life on this planet, giving it a surge of power that allowed it to become complex and big (see here for the full story). But the alliance between mitochondria and their host cells is a delicate one.

Both genomes evolve in very different ways. Mitochondrial genes are only passed down from mother to child, whereas the nuclear genome is a fusion of both mum’s and dad’s genes. This means that mitochondria genes evolve much faster than nuclear ones – around 10 to 30 times faster in animals and up to a hundred thousand times faster in some fungi. These dance partners are naturally drawn to different rhythms.

This is a big and underappreciated problem because the nuclear and mitochondrial genomes cannot afford to clash. In a new paper, Nick Lane, a biochemist at University College London, argues that some of the most fundamental aspects of eukaryotic life are driven by the need to keep these two genomes dancing in time. The pressure to maintain this “mitonuclear match” influences why species stay separate, why we typically have two sexes, how many offspring we produce, and how we age.


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Dinosaurs grew fast, had teen pregnancies and died young

TyrannosaurusThis is an old article, reposted from the original WordPress incarnation of Not Exactly Rocket Science. I’m travelling around at the moment so the next few weeks will have some classic pieces and a few new ones I prepared earlier.

For some dinosaurs, the best strategy was to grow fast and breed early. New fossil evidence suggests that at least three species, including celebrities like Tyrannosaurus and Allosaurus, were having sex in their teens. In this way, their pace of growth and maturity was closer to that of modern birds and mammals than it would be to a reptile scaled-up to the same size.


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Orangutans are masters of conserving energy


Between office jobs, motorised transport, the Internet and television, it’s never been easier to be inactive. Many humans in Western countries are masters at conserving energy but in the rainforests of Borneo and Sumatra, there is an animal that would put hardened couch potatoes to shame – the orang-utan. These apes are no slackers – they lead active lives in the jungle canopy. But relative to their size, they still use up less energy than any other mammal except for sloths.

Herman Pontzer from Washington University, who made the discovery, thinks that orangutans have evolved to live life in the slow lane because they can’t be sure of a steady food supply. They mostly eat fresh fruit and, being large animals, they need lots of it. But rainforests are chaotic places where ripe fruit can disappear quickly, unpredictably and for a long time. If orangutans aren’t getting any fuel, they have to minimise the amount of energy they spend, so that they don’t starve to death. And they’re very good at it.


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Short lives, short size – why are pygmies small?

This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.

Batwa pygmies in UgandaFor decades, anthropologists have debated over why pygmies have evolved to be short. Amid theories about their jungle homes and lack of food, new research suggests that we have been looking at the problem from the wrong angle. The diminutive stature of pygmies is not a direct adaptation to their environment, but the side-effect of an evolutionary push to start having children earlier.

Andrea Migliano at the University of Cambridge suggests that pygmies have opted for a ‘live fast, die short’ strategy. Their short lives gives them very limited time as potential parents, and they have adapted by becoming sexually mature at a young age. That puts a brake on their pubescent growth spurts, leaving them with shorter adult heights.

Pygmies are technically defined as groups of people whose men are, on average, shorter than 155cm (or 5 feet and an inch for the Imperial-minded). Strictly speaking, the word is restricted to several ethnic groups of African hunter-gatherers, like the Aka, Efe and Mbuti. But the world is surprisingly replete with shorter-than-average groups who also bear the colloquial moniker of pygmies, including some from Brazil, Bolivia, South-East Asia and Papua New Guinea.

The earlier explanations for a short stature worked for some of these groups, but they could never account for all of them. Some scientists suggested that smaller people move more easily through dense jungles, but some pygmies live outside forests. Other theorised that they could maintain their body temperature more easily, but many live in cool and dry climes.

One of the more popular theories put forward by Jared Diamond suggested that small people are more resilient to starvation and malnourishment when food becomes scarce. But this can’t be the whole story for Africa groups like the Turkana and Massai manage to be some of the tallest people on Earth despite facing similarly unstable food supplies!


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A life in the trees is a longer one

An assortment of tree-living mammals

In The Descent of Man, Darwin talked about the benefits of life among the treetops, citing the “power of quickly climbing trees, so as to escape from enemies”. Around 140 years later, these benefits have been confirmed by Milena Shattuck and Scott Williams from the University of Illinois.

By looking at 776 species of mammals, they have found that on average, tree-dwellers live longer than their similarly sized land-lubbing counterparts. Animals that spend only part of their time in trees have lifespans that either lie somewhere between the two extremes or cluster at one end. The pattern holds even when you focus on one group of mammals – the squirrels. At a given body size, squirrels that scamper across branches, like the familiar greys, tend to live longer than those that burrow underground, like prairie dogs.

These results are a good fit for what we already know about the lives of fliers and gliders. If living in the trees delays the arrival of death, taking to the air should really allow lifespans to really take flight. And so it does. Flight gives bats and birds an effective way of escaping danger, and they have notably longer lives than other warm-blooded animals of the same size. Even gliding mammals too tend to live longer than their grounded peers.


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The evolution of animal personalities – they’re a fact of life


Animals have distinct personalities and temperaments, but why would evolution favour these over more flexible and adaptible mindsets? New game theory models show that animal personalities are a natural progression from the choices they make over how to live and reproduce.

Any pet owner, wildlife photographer or zookeeper will tell you that animals have distinct personalities. Some are aggressive, others are docile; some are bold, others are timid.

An animal's reproductive decisions can determine if it is a hawk...In some circles, ascribing personalities to animals is still a cardinal sin of biology and warrants being branded with a scarlet A (for anthropomorphism). Nonetheless, scientists have consistently found evidence of personality traits in species as closely related to us as chimpanzees, and as distant as squid, ants and spiders.

These traits may exist, but they pose an evolutionary puzzle because consistent behaviour is not always a good thing. The consistently bold animal could well become a meal if it stands up to the wrong predator, or seriously injured if it confronts a stronger rival. The ideal animal is a flexible one that can continuously adjust its behaviour in the face of new situations.

And yet, not only do personality types exist but certain traits are related across the entire animal kingdom. Aggression and boldness toward predators are part of a general ‘risk-taking’ personality that scientists have found in fish, birds and mammals.

Max Wolf and colleagues from The University of Groningen, Netherlands, have found a way to explain this discrepancy. Using game theory models, they have shown that personalities arise because of the way animals live their lives and decide when to reproduce.


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The heavy cost of having children


Blogging on Peer-Reviewed ResearchWhile philosophers and poets muse on the meaning of life, natural selection casts a dispassionate eye on the whole affair. From the viewpoint of evolution, there is only one thing that matters – that we survive long enough to pass our genes on to the next generation, as many times as possible. And from the viewpoint of evolution, we are not doing a very good job.

415px-expecting_mother.jpgBirth rates in several countries around the world – the UK, Japan, China – are falling dramatically. Women are having fewer children and they are having them later, close to the end of their fertile period. But the fact that women undergo menopause at all seems strange, and the reasons for this reproductive expiry date has long puzzled biologists. There doesn’t seem to be any obvious benefit to ending a woman’s child-bearing potential with many years or decades to spare. Nor is menopause a symptom of our healthy modern lives – even in traditional societies, women often survived long past this point.

The favoured idea is that women retire early from child-bearing for the same reasons that athletes retire from their sports at a young age – their bodies cannot handle the strain. Childbirth is a taxing process for a woman and at some point, it becomes too risky for mother and child. Scientists have suggested that menopause is an evolutionary respite from the burdens of having children. Now, Dustin Penn at the Austrian Academy of Science and Ken Smith from the University of Utah have found compelling evidence to support this idea.


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Going strong at 100 – extreme lifespans don’t mean extreme disability

Blogging on Peer-Reviewed Research
People in most parts of the world are living longer and longer, thanks to great leaps in medicine and sanitation over the last century. But these growing life expectancies bring with them a sense of unease. The biggest worry is the possibility that medical advances are artificially prolonging life with little regard for its quality. Old age, after all, brings with it an increased risk of chronic diseases such as cancer, as well as both physical and mental decline.

Old-man.jpg This is not just a moral question, but an economic one too. The “oldest-old” are the fastest growing demographic in Western countries. If this expanding part of the population is indeed becoming more and more dependent on care from relatives or the state, the costs to society will start to skyrocket. But new research from Denmark suggests that this grim vision of the future is a fictitious one.

Kaare Christensen and colleagues from the University of Southern Denmark found that the proportion of elderly Danes who manage to remain independent holds steady at about 30-35 percent between the ages of 90 to 100. Individual people certainly risk losing their independence as they get older but the unhealthiest ones tend to pass away earlier despite improvements in medicine. This means that from society’s point of view, exceptional long-life won’t lead to exceptional levels of disability.

The scale of Christensen’s study is unprecedented. It exploited the fact that Denmark has kept a record of everyone living in it since 1968. Each person is assigned with a 10-digit identification number that links all their information across official registries. In 1998, Christensen’s team used this resource to contact every one of the 3,600 people who were born in Denmark in 1905 and were still alive.


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Duck-billed dinosaur defended itself by outgrowing predators

A good defence was a vital part of life in the Cretaceous. Plant-eaters needed effective ways of warding off the crushing jaws of Tyrannosaurus and its kin. Some species like Triceratops and Ankylosaurus had fairly obvious protective equipment, including horns, frills and armoured plates. But others lacked defensive armaments, and had to fend off predators through subtler means.

Take Hypacrosaurus. It was one of the duck-billed dinosaurs known as hadrosaurs, and like most other members of the group, its soft body lacked any obvious protection. Its main advantage was size; a fully-grown adult was an immense animal that almost rivalled T.rex in height. Its name even means “near the highest lizard”. And if a large size is your only defence, it’s a good idea to grow quickly.

That’s exactly what Hypacrosaurus did. Lisa Noelle Cooper from Kent State University has shown that the dinosaur reached its towering size in record time and grew much faster than the predators that hunted it. Other dinosaurs sought refuge behind shields and armour, but this otherwise defenceless species hid in plain sight, behind a large bulk attained at an extraordinary rate.



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Disease-ravaged devils have started living fast and dying young

Blogging on Peer-Reviewed ResearchEvolution dances to the tune of death. Killers – be they predators, diseases or competitive peers – can radically shape a species’ life cycles by striking down individuals of a certain age. The survivors respond by changing their “life histories” – a collection of traits that defines their reproductive cycles, including how often they breed, when they start to do so and how many young they have.

TDFT.jpgIf an animal’s adult life is short and brutal, they tend to grow quickly and become sexually mature at a young age – a strategy that maximises their chances of siring the next generation. The Tasmanian devil may be the latest species to switch to this live-fast, die-young tactic, for their adult population is slowly being wiped off by a contagious cancer.

I’ve blogged about the disease before. Known as devil facial tumour disease (DFTD), it was first reported in 1996, when devils first started appearing with horrendous facial tumours. Since then, it has spread across half of the devil’s home range and has cut a swathe through its populations. Hamish McCallum at the University of Tasmania calculated that the disease, if left unchecked, could drive the Tasmanian devil to extinction within 20-25 years.

But amazingly enough, the devils have started to adapt. So fatal is the disease to adults that the devil population is getting younger and younger and Menna Jones, a colleague of McCallum’s, has found that they are starting to reproduce at a much earlier age too. The surviving devils are in a race against time to reproduce before the cancer kills them off.