Some cold-blooded animals have famously long life spans—at 190 years old, Jonathan the Seychelles giant tortoise (Aldabrachelys gigantea) holds the current record for oldest land animal—yet compared to mammals and birds, there has been relatively little research into how these creatures age.
Now, two research papers published today (June 23) in Science delve into how cold-blooded creatures grow old, both in the wild and in captivity. Together, the two studies found highly variable rates of aging among these animals, with some species aging very quickly and others—especially turtles and tortoises—aging at almost imperceptibly slow rates.
“It’s really important to study aging from a lot of different perspectives,” says Peter Sudmant, an assistant professor of integrative biology at the University of California, Berkeley, who was not involved in either of the studies. “A lot of what we’ve learned about aging comes from comparing and contrasting different species.”
Galapagos giant tortoises (Chelonoidis niger) can weigh more than 500 pounds and can live for more than 100 years.
One study examined aging in wild animals. An international team involving roughly 100 collaborators and spearheaded by biologist Beth Reinke of Northeastern Illinois University compiled and analyzed data from local population surveys conducted all over the world for 77 species of reptiles and amphibians to test hypotheses about how characteristics of a species and its environment are related to the pace of aging. One of the questions the group wanted to answer was whether ectotherms (cold-blooded animals) aged more slowly than endotherms (warm-blooded animals).
Reinke says that she would have expected the answer to be yes. “Ectotherms, on average, have a lower metabolism. So it would make sense for them to age slower,” she says. “We know that there’s an energy cost to metabolism and endotherms have to put so much energy into just maintaining body temperature, which ectotherms don’t have to do.”
But when body size was controlled for, it turned out that endotherms and ectotherms aged at about the same rate on average, but ectotherms had much more variable rates of aging, with some aging substantially faster than endotherms and others aging substantially slower. “To me, this was surprising and fascinating and raised so many more questions,” says Reinke.
Some of these questions have to do with what exactly is causing mortality in both groups—that is, are animals dying because of their inherent biology or because of related environmental factors such as predation or food scarcity?
Having data on aging in wild animals is important for future studies on evolutionary pressures that might have contributed to different aging rates, but captive populations are important too, as they can help researchers begin to disentangle the effects of internal biology and the effects of environment on mortality. In zoos, says Steven Austad, a biologist at the University of Alabama at Birmingham who was not involved in either study, “the environment is safe enough that—presumably—the biological potential of a species will emerge.”
The second study, led by biologists at the University of Southern Denmark, examined this biological potential in 52 species of turtles and tortoises in zoos and aquariums in multiple countries. They found that increasing body weight was correlated with increasing life expectancy and that, unlike in mammals, the males of a given species generally live longer. By comparing their data on three species of turtles and tortoises in captivity with published data on the same species in the wild, “we found that the populations in captivity have lower aging rates,” says lead author Rita da Silva, a biologist at the University of Southern Denmark. “So there is an effect of the environment” on aging rates.
A tuatara (Sphenodon punctatus), a species identified as slow to age
Both Reinke’s and da Silva’s studies found negligible evidence of aging in some of their cold-blooded subjects. In the wild, Reinke’s study found little aging in some types of turtles, tortoises, and the tuatara, which looks lizardlike but belongs to its own order. Da Silva’s study also found this in some captive animals: about 75 percent of the turtle and tortoise species analyzed had slow or negligible aging rates.
But what exactly is negligible aging? Have turtles unlocked the secret of immortality?
Probably not, authors of both studies say. In these studies, the aging rate is defined as the speed at which the probability of mortality increases. Humans, for example, are a lot more likely to die at 60 than they are at 20. A turtle or tortoise with a very slow aging rate might be only slightly more likely to die at 60 than at 20. But that doesn’t mean that they live forever—or even, necessarily, longer than we do. Some of the species in da Silva’s study, for example, have aging rates that are slower than humans but still have shorter lifespans.
This is possible because, if a creature has a 20 percent chance of dying at age 20 and a 20 percent chance of dying at age 40, its aging rate (using the groups’ metric) is technically slower than a creature that has a 2 percent chance of dying at age 20 and a 5 percent chance of dying at age 40. Even though the first creature is technically aging slower, that doesn’t mean that its average life span will necessarily be longer than the second creature, since it has a higher chance of dying in a given year. In other words, aging rate “does not directly translate into years lived,” says da Silva.
Furthermore, the aging rate only takes mortality into account, not any of the infirmities that come with old age. “What the studies do not say is that turtles and tortoises do not age,” says Austad. Similar to humans, “older turtles get cancer and cataracts and heart disease.” The tortoise Jonathan, for example, is blind from cataracts and no longer seems to be able to smell, Austad notes.
But very slow aging rates are still interesting, and Sudmant and Austad agree that studies like these are essential to improving our understanding of aging across species.
“Everything we have learned about the genetics and molecular biology [of aging] has really shown that there’s a number of key, highly conserved, ‘everyone’s-got-them’ genes and processes and pathways that are the major drivers,” says Sudmant. “It’s really important to study different species because while a lot of the mechanisms of aging are universal, at the same time there’s this very curious thing in the world: among organisms on this planet, there’s a 150,000-fold variation in lifespan.” Comparisons among species could help researchers identify the reasons behind such massive differences in lifespan.
“Since some of these species age very slowly,” says Austad, “potentially there’s something that we might be able to learn from them about slowing the aging process.”