What we can learn from turtles so as not to age

An international team of 114 scientists, led by Pennsylvania State University (Penn State) and Northeastern Illinois University, in the United States, has presented the most comprehensive study to date on the aging and longevity of reptiles and amphibians around the worldpublished in the magazine Science.

Among their numerous findings, they document for the first time that turtles, crocodiles and salamanders have especially low aging rates and long lifespans for their size. The team has also found that protective phenotypes, such as the hard shell of most turtle species, contribute to slower aging and, in some cases, even ‘insignificant aging’, i.e. to the absence of biological aging.

At 190 years old, Jonathan the Seychelles giant tortoise has recently made headlines for being the “world’s oldest living land animal.” Although there is anecdotal evidence such as this that some species of tortoises and other ectotherms – or “cold-blooded” animals – live very long, the evidence is sparse, focusing mostly on animals that live in zoos or a few individuals that live in zoos. In nature. Now, the team presents this study that includes data collected in nature from 107 populations of 77 species of reptiles and amphibians from around the world.

“There is anecdotal evidence that some reptiles and amphibians are slow-aging and long-lived, but until now no one had really studied this on a large scale across numerous species in nature,” said David Miller, lead author and associate professor of population ecology. Wildlife, Penn State.

“If we can understand what allows some animals to age more slowly, we will be able to better understand aging in humansand we will also be able to report on conservation strategies for reptiles and amphibians, many of which are threatened or in danger of extinction”, he adds.

In their study, the researchers applied comparative phylogenetic methods – which allow investigating the evolution of organisms – to mark-recapture data, in which animals are captured, tagged, released into the wild, and observed.

Their goal was to analyze the variation in aging and longevity of ectotherms in nature compared to endotherms (warm-blooded animals) and to explore previous hypotheses related to aging, including the mode of regulation of body temperature and the presence or absence of protective physical features.

Thermoregulation mode hypothesis

Miller explains that the “thermoregulation mode hypothesis” suggests that ectotherms—because they require external temperatures to regulate their body temperature and thus tend to have lower metabolisms– age more slowly than endotherms, which internally generate their own heat and have higher metabolisms.

“People tend to think, for example, that mice age quickly because they have a high metabolism, while turtles age slowly because they have a low metabolism,” he says.

However, the team’s findings reveal that the aging rates and lifespans of ectotherms are both above and below known aging rates for endotherms of similar size, suggesting that the way how an animal regulates its temperature—cold-blooded versus warm-blooded— not necessarily indicative of their rate of aging or length of life.

“We found no support for the idea that a lower metabolic rate means that ectotherms age more slowly,” Miller says. That relationship was only true for tortoises, suggesting that tortoises are unique among ectotherms.”

Artist's impression of the surface of the exoplanet Gliese 486b

The advantages of protection

The protective phenotype hypothesis suggests that animals with physical or chemical traits that confer protection—such as armor, spines, shells, or venom— have slower aging and greater longevity. The team documented that, indeed, these protective traits allow the animals to age more slowly and, in the case of physical protection, live much longer for their size than those without protective phenotypes.

“It could be that their altered morphology with hard shells provides them with protection and has contributed to their evolving life histories, including insignificant aging – or no demographic aging – and exceptional longevity,” he says. Anne Bronikowskico-author and professor of integrative biology at Michigan State University.

Beth Reinke, first author and adjunct professor of biology at Northeastern Illinois University, further explains that “these various protective mechanisms can reduce animal mortality rates because they are not eaten by other animals. Thus, they are more likely to live longer, and that puts pressure on them to age more slowly, he says. The greatest support for the protective phenotype hypothesis is found in turtles. Again, this shows that turtles, as a group, are unique.”

A giant wave hangs over a coastal area, in a file image

Interestingly, the team observed negligible aging in at least one species from each of the ectotherm groups, including frogs and toads, crocodiles, and turtles.

“It sounds drastic to say that they don’t age at all, but basically its probability of dying does not change with age once it has outgrown», emphasizes Reinke.

“Negligible aging means that if an animal’s chance of dying in a year is 1% at 10 years, if it’s alive at 100 years, its chance of dying is still 1%,” he explains. In contrast, in adult females in the United States, the risk of dying in a year is about 1 in 2,500 at 10 years and 1 in 24 at 80. When a species shows negligible senescence (deterioration), the aging just doesn’t happen.”

Image of the Galapagos Islands

international contribution

Reinke notes that the team’s novel study was only made possible by contributions from a large number of collaborators around the world. studying a wide variety of species.

“The possibility of bringing together these authors, who have worked for years on the study of their own species, is what has allowed us to obtain these more reliable estimates of the rate of aging and longevity, based on population data rather than on individual animals,” he adds.

Bronikowski adds that “understanding the comparative landscape of aging between animals can reveal flexible traits that may be targets worthy of biomedical study related to human aging.”