Jonathan the Seychelles giant tortoise, who is 190 years old, made headlines lately for being the "oldest living land mammal in the world." Although there is some anecdotal evidence that some species of turtles and other ectotherms, sometimes known as "cold-blooded" creatures, live a long period, it is sparse and mostly focuses on animals kept in zoos or a small number of individuals living in the wild. The largest research on aging and longevity to date, conducted by an international team of 114 scientists and directed by Penn State and Northeastern Illinois University, has just been published. It includes data gathered in the field from 107 populations of 77 different species of reptiles and amphibians.
They first demonstrated that salamanders, crocodilians, and turtles had extremely low aging rates and prolonged lifespans for their sizes among their many other results, which they present today (June 23) in the journal Science. The research team also discovered that protective phenotypes, like the hard shells of most turtle species, help to delay aging and, in some circumstances, even prevent biological aging.
There is anecdotal evidence that some reptiles and amphibians age slowly and live a long time, but no one has examined this issue extensively across a wide range of species in the wild up until now, according to senior author and associate professor of wildlife population ecology David Miller from Penn State. We can better understand aging in humans and develop conservation plans for reptiles and amphibians, many of which are vulnerable or endangered, if we can figure out what causes some creatures to age more slowly.
In their study, the researchers used mark-recapture data, in which animals are taken, tagged, released back into the wild, and then watched, in combination with comparative phylogenetic approaches, which allow exploration of organisms' evolution. Their objective was to examine the differences in ectotherm lifespan in the wild compared to endotherms (warm-blooded animals) and study earlier aging-related concepts, such as the way in which the body regulates temperature and the presence or absence of protective physical features.
According to Miller, the "thermoregulatory mode hypothesis" contends that endotherms, who internally produce their own heat and have greater metabolisms, age more rapidly than ectotherms, who depend on external temperatures to maintain their body temperatures.
For instance, Miller noted, "people frequently believe that mice age rapidly due to their high metabolisms, but turtles age slowly due to their low metabolisms.
However, the team's findings show that ectotherms' aging rates and lifespans differ significantly from those of known endotherms of comparable size, indicating that an animal's ability to regulate its body temperature — cold-blooded versus warm-blooded — is not always a reliable indicator of that animal's aging rate or lifespan.
According to Miller, there is little evidence to support the notion that ectotherms age more slowly because they have lower metabolic rates. "That association was exclusively present in turtles, suggesting that they are distinct from other ectotherms."
According to the protective phenotypes theory, animals having physical or chemical characteristics that provide protection, such as armor, spines, shells, or venom, age more slowly and live longer. The researchers found that animals with these protective characteristics do in fact age more slowly and, in the case of physical protection, live far longer for their size than those without them.
According to Anne Bronikowski, co-senior author and professor of integrative biology at Michigan State, "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."
Beth Reinke, the study's first author and an associate biology professor at Northeastern Illinois University, added, "Because they are not being eaten by other animals, these multiple defensive systems can lower animal death rates. They have a higher likelihood of living longer, which puts pressure on them to age more slowly. Turtles provided the strongest evidence for the protective phenotype hypothesis that we could find. Once more, this indicates how distinctive turtles are as a group."
Interestingly, the scientists found that at least one species in each of the ectotherm categories had minimal aging, including turtles, crocodilians, and frogs and toads.
"It seems dramatic to claim that they don't age at all, but basically, after they're past reproduction, their chance of dying does not alter with age," said Reinke.
Miller also said "Negative aging implies that if an animal lives to be 100 years old but has a 1% probability of dying in a year at age 10, it still has a 1% chance of dying in a year (1). In comparison, the chance of death in a year for adult females in the U.S. is around 1 in 2,500 at age 10 and 1 in 24 at age 80. Aging simply doesn't occur when a species shows minimal senescence (deterioration)."
Reinke pointed out that the team's innovative work was only made possible thanks to the participation of several international partners who were researching a wide range of species.
We were able to obtain these more trustworthy estimates of aging rate and longevity that are based on population data rather than simply individual animals because we were able to gather these writers together who have all spent years and years studying their own species.
Understanding the comparative landscape of aging in many animal species might identify adaptable features that can represent worthwhile targets for biological research into human aging, according to Bronikowski.