‘It sounds dramatic to say that they don’t age at all, but basically their likelihood of dying does not change with age once they’re past reproduction.’
UNIVERSITY PARK, Pa. — In the largest study of its kind, scientists have discovered what keeps certain species from aging — possibly opening the door to longer lifespans for humans.
An international team, led by researchers from Penn State and Northeastern Illinois University, says they’ve unlocked the secrets of aging among reptiles and amphibians worldwide. Across 77 different species from around the globe, researchers found some with extremely long lifespans, lasting for more than a century. While some might think these animals are just sitting at the top of their respective food chains, the team found there’s more to it than that. Some of the creatures living on Earth simply aren’t aging at all!
Researchers say one of the prime examples is Jonathan the Seychelles giant tortoise. The ancient turtle recently became the “oldest living land animal in the world” after reaching the age of 190! Previous studies on ectotherms — or “cold-blooded” animals — have focused on animals living in closely monitored environments like a zoo. The new study collected data from 107 wild populations worldwide.
Those findings reveal that turtles, crocodilians, and salamanders all age particularly slowly and have exceptionally long lifespans for creatures of their size. Moreover, protective characteristics, like the hard shell of a turtle, actually contribute to biological aging almost stopping completely.
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“Anecdotal evidence exists that some reptiles and amphibians age slowly and have long lifespans, but until now no one has actually studied this on a large scale across numerous species in the wild,” says senior author David Miller, an associate professor of wildlife population ecology at Penn State, in a media release. “If we can understand what allows some animals to age more slowly, we can better understand aging in humans, and we can also inform conservation strategies for reptiles and amphibians, many of which are threatened or endangered.”
Does metabolism really have a connection to aging?
The team investigated the evolution of these organisms using mark-recapture data. This process captures, tags, and then releases animals back into the wild. Their goal was to compare the variations in ectotherm aging and longevity with that of endotherms — warm-blooded animals like humans.
Miller explains that one hypothesis researchers wanted to examine was the “thermoregulatory mode hypothesis.” This theory suggests that since cold-blooded animals use external temperatures to regulate their body temperatures, they have lower metabolisms and therefore age more slowly. Meanwhile, since warm-blooded creatures generate their own heat, they have higher metabolisms and age faster.
“People tend to think, for example, that mice age quickly because they have high metabolisms, whereas turtles age slowly because they have low metabolisms,” Miller says.
However, the study debunked that hypothesis — finding that ectotherms have aging rates and lifespans ranging well above and well below the lifespans of endotherms. Study authors say this suggests that how an animal regulates their body temperature doesn’t necessarily affect their longevity.
“We didn’t find support for the idea that a lower metabolic rate means ectotherms are aging slower,” Miller reports. “That relationship was only true for turtles, which suggests that turtles are unique among ectotherms.”
The secret is hiding in the shell!
It turns out that another theory may explain the incredible longevity of these animals. Study authors say the “protective phenotypes hypothesis” suggests that animals with particular physical or chemical traits that give them protection in the wild live much longer than their more vulnerable peers. These traits include armored skin, spines, shells, and even poisonous venom.
“It could be that their altered morphology with hard shells provides protection and has contributed to the evolution of their life histories, including negligible aging – or lack of demographic aging – and exceptional longevity,” explains Anne Bronikowski, co-senior author and professor of integrative biology at Michigan State.
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“These various protective mechanisms can reduce animals’ mortality rates because they’re not getting eaten by other animals. Thus, they’re more likely to live longer, and that exerts pressure to age more slowly. We found the biggest support for the protective phenotype hypothesis in turtles. Again, this demonstrates that turtles, as a group, are unique,” adds Beth Reinke, first author and assistant professor of biology at Northeastern Illinois University.
With that in mind, the study found that at least one species in each ectotherm group displayed little to no signs of aging throughout their lives. These groups included frogs and toads, crocodilians, and turtles.
“It sounds dramatic to say that they don’t age at all, but basically their likelihood of dying does not change with age once they’re past reproduction,” Reinke says.
“Negligible aging means that if an animal’s chance of dying in a year is 1% at age 10, if it is alive at 100 years, it’s chance of dying is still 1% (1). By contrast, in adult females in the U.S., the risk of dying in a year is about 1 in 2,500 at age 10 and 1 in 24 at age 80. When a species exhibits negligible senescence (deterioration), aging just doesn’t happen,” Miller adds.
“Understanding the comparative landscape of aging across animals can reveal flexible traits that may prove worthy targets for biomedical study related to human aging,” Bronikowski concludes.
The study is published in the journal Science.