Time of death
Some animals are ephemeral. Others are almost eternal. Some age gracefully, while others self-destruct. Why are some creatures here for a good time, and others for a long time?
The adult lifespan of the Dolania americana mayfly is measured in minutes. Juveniles, called nymphs, burrow in the riverbeds of the southeastern US for a year before swimming up to the surface and splitting their skin. The mouthless adults emerge in flying clouds before dawn, but never see the sun rise. Males live for about half an hour. Females have just five minutes in which to feel the air on their wings, find a mate, and drop their eggs into the water.
Greenland sharks, on the other hand, can live for 400 years. In the deeps to the east of New Zealand, orange roughy can reach almost 250 years. A quahog clam found off Iceland in 2006 was dubbed Ming because its annual shell rings revealed it was born 507 years earlier, during the reign of the ancient Chinese dynasty. Meanwhile, a honey mushroom in Oregon nicknamed the Humongous Fungus extends over 10 square kilometres of forest and is at least 2500 years old. The so-called “eternal jellyfish”, however, can live practically forever.
Turritopsis dohrnii is also called the Benjamin Button jellyfish, because it has figured out how to age in reverse. If the adult form of one of these lentil-sized Mediterranean medusas is stressed or hurt—a small pinch with tweezers is enough—it falls to the sea floor and reverts to a previous stage of development, essentially beginning its life cycle again. It’s the equivalent of “a butterfly transforming back into a caterpillar”, according to the Italian researchers who discovered it, a cycle that could (in theory) continue indefinitely.
Scientists are still untangling what determines a species’ lifespan. In general, it seems like large animals live longer than small ones. Flying animals like birds and bats live longer than their earthbound relatives. In both cases, these creatures are less likely to become prey, so can perhaps afford to take their time to reach adulthood. Cold-blooded animals living in frigid waters, like the Greenland shark, are able to burn less energy and metabolise slowly. Some outliers, like the naked mole-rat—a mouse-sized rodent that can live more than 30 years—seem to have evolved a resistance to cancer, making them of intense interest to researchers hoping to extend human life.
Generally speaking, though, some animals live fast, reproduce early, and die young, while others mature slowly and die old. Each lifestyle represents a different evolutionary strategy, says Nic Rawlence, the director of the University of Otago’s Palaeogenetics Laboratory.
Growing quickly and breeding often can increase an individual animal’s chances of passing on its genes, and makes it easier for a species to adapt to environmental change—but it also comes at a cost. Within the nucleus of each cell, the ends of each chromosome are capped with repetitive sequences of DNA called telomeres. Telomeres act like the hem on your trousers, preventing the chromosome from fraying or tangling and protecting its genetic blueprint. Every time a cell divides, those telomeres shorten slightly. Over time, as they wither and eventually disappear, the chromosomes are left unprotected. With each division, they get increasingly battered, degrading the DNA, accumulating mutations, and leading to disease and death.
And here’s the thing: living fast means an animal’s cells divide more often. A bowhead whale (lifespan: 200 years) will therefore burn through its telomeres much more slowly than a giant Sunda rat (lifespan: six months). “There are always exceptions,” says Rawlence. “But that’s the general rule.”
Climate change, however, is beginning to break the rules, making some animals old before their time. In a study published in 2022, scientists tracked 22 populations of cold-adapted lizards in France’s Massif Central mountains over several generations. In the most heat-stressed, declining populations, newborn lizards had unnaturally short telomeres, making them essentially babies trapped in elderly bodies. The next generation’s telomeres were even shorter at birth.
Rapidly changing environments can cause a species to accelerate its life-history, Rawlence says, leading it to grow faster and reach adulthood sooner.
“But the problem is, if you’re born with really, really short telomeres, there’s no guarantee that you’re going to live to reach sexual maturity. Once you reach that tipping point, you’re in genomic meltdown,” he says.
The French lizards are similar to New Zealand’s cold-adapted lizards, Rawlence says—our geckos, skinks and tuatara. No-one has sequenced their telomeres yet, but it’s possible they might already be suffering from the same premature-ageing problem. “It’s disturbing,” he says, “and a really good example of climate-change impacts you wouldn’t necessarily think of.”
Lifespan is inextricably linked to reproduction. For a species to survive, sufficient numbers of individuals must live long enough to reach maturity and breed. But what happens next varies widely across the animal kingdom.
Almost all animals breed on multiple occasions throughout their life—like Jonathan, a Seychelles giant tortoise living at the governor’s residence on the South Atlantic island of St Helena, who is still getting laid at the ripe old age of 190. A few—like mayflies, and salmon—breed for just one season and then die.
This adaptation is called semelparity, or “big-bang reproduction”. It is very rare in mammals, with the extraordinary exception of Australia’s 12 species of antechinus. These carnivorous marsupials live in tree hollows and look just like mice, but are only distantly related to them (the name means “not a hedgehog” in Latin.) Males live for precisely 11 and a half months, and spend the final fortnight of their lives destroying themselves in an orgy of sex and violence.
As the days lengthen in late winter, all the females become fertile at once. The strategy ensures their offspring will be born when insect numbers peak in spring. In response, testosterone and steroid hormones flood the males’ bodies, collapsing their immune systems and riddling them with stomach ulcers, but allowing them to cannibalise every last ounce of their energy reserves for reproduction.
Over two frantic weeks, each male antechinus mates as many times as he can—the violent bouts can last 14 hours—even as his fur falls off, his internal organs disintegrate, he develops gangrene, and eventually dies. Females carry their babies around for 100 days, by which time each litter weighs four times as much as its exhausted mother. After weaning, most females die, too, although a few survive to breed again for one more year.
When it comes to suicidal reproduction, octopuses are even more extreme.
Despite their huge brains and superlative intelligence, even the largest kinds of octopus live for just four or five years, and most fail to reach their second birthday. Of the roughly 300 octopus species, just two breed more than once. The rest sacrifice themselves in spectacular fashion before they ever meet their young, says Z Yan Wang, a neurobiologist at the University of Washington in the US. There is no peaceful decline, no gradual failure of the systems that sustain life. “They are actively hurtling towards death.”
Wang studies Octopus bimaculoides, the California two-spot octopus, an abundant species which copes well in captivity. In her lab’s aquarium, she watched as the females carefully tended their eggs. But before their offspring hatched, the octopuses’ behaviour turned macabre. They stopped eating, bit off their own arms, tore strips from their skin, and flung themselves onto rocks. (Males are also thought to self-destruct at some point after mating, but are harder to study than den-bound females.)
In the lab, Wang examined female octopuses’ brains at different life stages. An organ called the optic gland, which is similar to the pituitary gland in humans, seems to be responsible for the octopus death spiral, Wang says: when it is removed, the animals resume eating and live longer.
Her team found that shortly after mating, a female octopus’s optic gland produces three different steroid hormones that together seem to trigger death. “It’s like the opposite of life insurance,” says Wang. “It makes sure that the octopus dies.”
These behaviours can be troubling to watch, she acknowledges. “It’s weird to watch an octopus eat its own arm… but I don’t despair when I study this, because it is so innate. It’s as natural for them as hunting—and it has such an important purpose.”
How could such an early, violent death be adaptive for octopuses? The cephalopods’ fast, infinite growth—they keep growing even after they reach adulthood—and penchant for cannibalism could be part of the answer, Wang says. When two-spot octopuses hatch from the egg, they’re the size of your pinkie fingernail—and they must immediately disperse or be swallowed by their siblings. A female will sometimes eat a male after mating. She might not necessarily consume her own offspring, Wang says, but she’d certainly eat a neighbour’s. “You need some mechanism to make sure that this highly intelligent animal will allow the next generation to proliferate and grow.”
The octopus mother’s self-annihilation might not be the perfect mechanism, she says, but it does the job. “We were always taught that [evolution] is about survival of the fittest, but I think there’s another way of looking at it, which is survival of the good enough. If you come up with a good enough strategy—no matter how MacGyvered or over-the-top it is—if it confers some evolutionary benefit, it’ll stick around.”
Suicidal reproduction is uncommon, but menopause is even rarer. In just a few mammal species that we know of—humans, orcas, false killer whales, short-finned pilot whales, belugas, narwhals—females stop breeding well before the end of life. There’s no evidence these animals experience the sorts of symptoms human females often do, like hot flushes or insomnia. (Over in the insect kingdom, meanwhile, there’s a species of Japanese aphid where females begin life as soldiers, mature into mothers, then spend their final fortnight as living glue bombs, suicidally defending their colony against predators with an explosion of sticky wax.)
Unlike solitary octopuses, these are all highly social animals. But there are plenty of long-lived, gregarious animals, like elephants and gorillas, that don’t experience menopause. So why did it evolve just a handful of times?
Mia Lybkær Kronborg Nielsen, a marine biologist at the University of Exeter, studies orcas off the west coast of North America, where researchers have been monitoring the same populations for almost 50 years. Female killer whales stop having calves in their late 30s or early 40s, but regularly live to 80, possibly even reaching 100. Male orcas, however, tend to die around the age of 50—suggesting females have extended their lifespan, rather than ending fertility early.
The presence of these matriarchs has many benefits for the pod, Nielsen says. They share food with their children and grandchildren, teach young orcas how to hunt, and use their prodigious memories to lead the way to the best fishing grounds. They especially coddle their sons, and middle-aged male orcas whose mothers are still around have significantly better survival rates. When a grandmother orca passes away, her grandchildren are 4.5 times more likely to die during the ensuing two years.
“Killer whales are easy to feel connected to,” says Nielsen. “They live in family groups with grandmothers that make sure everyone’s eating, and teaching younger ones valuable life lessons. The fact that their life history is so similar to ours is just another thing making them feel so relatable.”
But couldn’t female orcas—and humans—do all this while also continuing to breed, as elephants do? “There’s something in the life of menopausal species that has tipped the balance and makes it advantageous to stop reproducing and stay alive for a long time,” Nielsen says.
That something is probably the orcas’ unique social structure. Young orcas of both sexes stay with their family group as they mature, but males mate outside of the pod, meaning their children are raised by others. The more babies born to the pod’s females, the fewer resources there are to go around—and, in fact, the calves of older mothers die at higher rates than those of younger mothers. By instead helping keep her sons alive and well fed, an older female orca can increase the success of her lineage without having any extra mouths to feed.
This kind of thinking can also shed light on the evolutionary role of menopause in humans. According to American historian Susan Mattern, it has been key to our success as a species. In contemporary hunter-gatherer groups, menopausal women are expert gatherers, producing a surplus of food. Studies of 18th- and 19th-century French Canadians and Finns have also shown that the presence of a grandmother increases a toddler’s chance of survival.
Social structure likely plays a role, too. Among ancestral humans, it was common for sons to stay with their mothers while daughters left to join their partners in other family groups. The Finnish study suggests that competition between these women and their mothers-in-law may have helped promote menopause. The researchers found that children were at least 50 per cent more likely to die when mothers-in-law and daughters-in-law were giving birth around the same time.
Menopause, then, probably helped to reduce competition between unrelated women, and allowed more children to survive to adulthood and beyond.
We have a pretty good idea of human longevity, even if we don’t yet know how to extend it—the oldest recorded person was Frenchwoman Jeanne Calment, who lived to 122 years and 164 days.
In some mammals, teeth can provide some indication of age, and to age a fish, scientists can count the annual growth lines in its otoliths, tiny structures inside its ear. But studying other species’ lifespan is much trickier, particularly for those that live for decades or centuries, like New Zealand’s tuatara.
To estimate the spiky reptiles’ longevity, researchers including Nicola Nelson from Victoria University of Wellington have been stalking them by night on Cook Strait’s North Brother Island. The study started in the 1980s, and every few years the scientists capture as many of the 500-strong population as they can between dusk and dawn. (The cold-blooded creatures move more slowly in the dark, so can easily be picked up and marked or identified.)
Tuatara stop growing at around 30 years old. After that, this mark-recapture method is the only way to determine their age in the wild. If they’re not found for several years, researchers assume they’re dead—on its own, not a particularly accurate way of measuring lifespan. But scientists can also use modelling to convert the cumulative recapture data into a lifespan estimate. In 2022, Nelson contributed to a global study of 77 reptile and amphibian species, which found that tuatara lived the longest—137 years!—and had the second-slowest rate of ageing.
That’s not to say most tuatara will be around that long, even in the absence of predators, Nelson explains. It’s more akin to the 100-year milestone for humans. For tuatara, 137 is the age by which 95 per cent have died. The researchers have not yet confirmed that any individuals have actually gotten that old, but on other islands, adult tuatara first marked in the 1940s as adults were caught again recently, suggesting they were at least 100. Some may even live longer. In 1935, Robert Falla, the director of the Dominion Museum in Wellington, reported Māori stories of a tuatara that lived in a shell pit on Mōtītī Island, off Tauranga, for 300 years.
“This is going to be something we continue to learn about for decades, as we collect more data on the same individuals—our old friends,” says Nelson. “And it’ll be other researchers that do it, not me.
“It’s beyond my lifespan, this work.”