What Turtles Can Teach Humans About the Science of Slow Aging

New data shows that several types of the shelled reptiles can slow—and even stop—aging if the environmental conditions are right.

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.

To quantify the role senescence played in mortality, the researchers compared their data for each species to theoretical survival curves that predicted the risk of death in a population of animals to grow exponentially each year after sexual maturity. They focused on what happened at a specific point in the curve: the age at which 80 percent of the turtles in each species had died. They considered this to be a point in time well after the onset of senescence.

Then, they determined the aging rate of each species by calculating the direction and steepness of the curve at that point. A positive rate (or an upward curve) indicated that the species was experiencing some level of senescence—that their risk of death was rising with age. A rate of zero meant that the risk of death was constant, and a negative value (or a downward curve) implied that it was declining. The steepness of the curve gave insight to how quickly the senescence was increasing or decreasing.

About 75 percent of the species in their sample displayed little to no senescence, and 80 percent of them had aging rates that were lower than those of modern humans. Two species, the Greek tortoise and black marsh turtle, even displayed what da Silva calls negative senescence, in which the risk of dying actually decreased with age.

It’s important to note that all of the turtles were captive, living in ideal conditions for long life: enclosed habitats with controlled environmental and reproductive conditions, as well as easy access to sustenance and care. “They don’t have to spend all of their energy on finding food or avoiding predators,” da Silva says. “So they can just allocate all of that energy to surviving.”

Studying captive animals helps eliminate external factors like predators, human encroachment, and difficulty finding food or shelter, allowing the researchers to focus just on demographic trends. But it doesn’t exactly reflect how the turtles fare in the wild. So for three of the species, the scientists were able to compare their own results with previous data collected from wild populations. Two of them, the pond slider and Home’s hinge-back tortoise, showed much steeper aging rates in their natural habitats than in captivity, while the painted turtle showed slightly less senescence in the wild.

Steven Austad, a biogerontologist at the University of Alabama Birmingham who was not involved in the work, says that the results are compelling, but that more in-depth studies in the wild are needed to determine if turtles have the capacity to stop aging altogether. That would mean that traditional evolutionary theories—which consider senescence to be universal across all species—are wrong. “To really test evolutionary ideas, you test them in the environment that the evolution occurred in,” he says.

Colchero and da Silva’s paper was accompanied by a broader study, published in Science on the same day by an independent group of scientists, reporting aging rates across 107 populations of turtles, crocodilians, amphibians, and scaled reptiles in the wild. By compiling and comparing data sets from over 100 scientists, evolutionary biologist Beth Reinke of Northeastern Illinois University found at least one species in each group of animals that displayed negligible senescence. Across the board, all types of turtles showed high longevity and extremely low aging rates.

That doesn’t necessarily clash with the high aging rates that Colchero and da Silva found in two specific wild populations. “There’s a lot of variation even within species that we have no explanation for,” says Reinke, who contributed 13 years’ worth of her own data on painted turtles for the comparative study. (Of the five populations of yellow-bellied toads, for example, one showed nearly no senescence, while another population had a very steep aging rate.) Reinke also discovered a dependence on temperature: senescence increased for reptiles and decreased for amphibians that lived in warmer areas, supporting Colchero and da Silva’s conclusion that the surrounding environment influences how animals age. 

Neither study could probe which biological mechanisms are driving this effect, since the researchers weren’t able to analyze tissue samples. And, notably, both research teams lacked the data to say much about how the shelled reptiles ultimately did die, or what their physiology was like in later stages of life. “The results need to be validated in wild populations, and by looking at animal’s physical and reproductive statuses—not just mortality patterns,” says Austad, who penned a perspective in Science about the two papers. He also suggests a future focus on a sample of longer-lived turtles, since the species in Colchero and da Silva’s work had average life expectancies of only up to 60 years, with many well below 30.

Colchero agrees. “This is not conclusive at all,” he says. “It’s something that we hope will open new avenues of research, that people who are really interested in the physiology of aging will start looking at species of turtles and tortoises.”

Until then, the jury is still out on whether turtles, or other organisms, will ever be able to cheat death. This new data suggests, at least, some biological potential to stave it off for a very long time—if the conditions are right. “No aging is basically eternal youth, and the distinction about whether they’re aging really slowly or not at all is an important one, conceptually,” Austad says. Finding a turtle that truly never ages would overhaul how scientists understand aging, and evolution, altogether. But barring that, he continues, “If they simply age slower than people, there could still be lessons.”

Solving the mystery of how turtles avoid senescence may eventually lend insight into human aging. “We can for sure say that we know more about mortality in general, how species age and how species die, than we knew before,” da Silva says. “And if we start actually seeing all of these trajectories of mortality across the tree of life, we’ll learn a lot that we can probably translate to human mortality in the future.”

What Turtles Can Teach Humans About the Science of Slow Aging

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