There are three ways to die: of injury, disease, or old age. Over time, humans have gotten better at avoiding the first two, but as we get older, senescence—the gradual deterioration of bodily functions with age—is inevitable. Some species seem to do better than others, though: Take the hydra, a tiny freshwater creature that some scientists have deemed potentially immortal. Last year, a naked mole rat made headlines for turning 39, five times the typical lifespan for similarly sized rodents. And just a few months ago, a giant Aldabra tortoise named Jonathan celebrated what was believed to be his 190th birthday, making him the world’s oldest living land animal.
Cases like these beg the question: Is it possible to escape aging?
The authors of a study published in Science last month say yes. Well, if you’re a turtle. With an extensive analysis of 52 species of turtles (a designation that includes both water dwellers and land-lodging tortoises), the team of four scientists found that the majority of them showed exceptionally slow—and in some cases, negligible—senescence while in captivity. That doesn’t make them immortal; turtles can still die from illness or injury. But unlike birds and mammals, their overall risk of death doesn’t increase with age. “We confirmed something that was suspected a long time ago, but never proven,” says Fernando Colchero, a biodemographer at the University of Southern Denmark.
Aging rate is a measure of how the risk of death increases among a population of organisms as they get older. For birds and mammals, that risk is thought to grow exponentially with age. But for most of the turtle species in the study, that rate was nearly flat, no matter how old they got.
Colchero and his colleagues also found that the environment the animals lived in plays a role. “Turtles and tortoises, based on comparing our results to those from animals in the wild, can actually change their aging rates dramatically when conditions improve,” he says, referring to factors like protection from predators, a controlled climate, and unlimited access to food and shelter. That’s distinct from previous work using primate data that reported increases in longevity due to better living conditions, but no significant reduction in mortality due to slowed aging.
What gives? Some evolutionary theories propose that senescence is the result of an energy trade-off. Most mammals and birds stop growing once they reach sexual maturity, Colchero says, at which point their energy gets prioritized for procreation, rather than cellular repair. Without sufficient upkeep to counter wear and tear, bodies become more vulnerable to chronic age-related conditions, as well as injuries or infections. “But many reptiles don’t. They keep growing, which means that they seem to be really efficient at repairing damages and keeping bodily functions working well,” he says.
According to Rita da Silva, a biologist who led the study with Colchero, animals with this quality are prime candidates for evading senescence. It’s an idea that’s been around since the 1990s, and to prove it, the researchers collected demographic information from the Zoological Information Management System, a database of records from zoos and aquariums maintained by the nonprofit organization Species360. They selected species that had data for at least 110 animals, and focused only on turtles living in fresh water or on land.