The largest study of its kind reveals that wild turtles age slowly, live long lives, and uncovers several species that practically do not age
Jonathan the Seychelles giant tortoise, who is 190 years old, made headlines recently for being the “oldest living land animal in the world.” Although there is anecdotal evidence that certain species of turtles and other ectotherms, or “cold-blooded” creatures, live a long life, this evidence is spotty and mostly focuses on animals kept in zoos or a small number of individuals surviving in the wild. The largest study on aging and lifespan to date, conducted by an international team of 114 scientists and directed by Penn State and Northeastern Illinois University, has recently been published. It contains data gathered in the wild from 107 populations of 77 different species of reptiles and amphibians.
The researchers discovered several things, including for the first time, that salamanders, crocodilians, and turtles had extremely slow aging rates and prolonged lifespans for their sizes. They recently published their results in the journal Science. The research team also discovered that protective phenotypes, such as the hard shells of the majority of turtle species, lead to slower aging and, in certain circumstances, even to “negligible aging,” or the absence of biological aging.
“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,” said David Miller, senior author and associate professor of wildlife population ecology, Penn State. “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.”
In their study, the researchers used mark-recapture data, in which animals are taken, tagged, released back into the wild, and then watched, in conjunction with comparative phylogenetic approaches, which allow for investigation of organisms’ evolution. Their purpose was to compare ectotherm aging and lifespan in the wild to endotherms (warm-blooded animals) and investigate earlier assumptions about aging, such as manner of body temperature control and the presence or absence of protective physical features.
Miller explained that the ‘thermoregulatory mode hypothesis’ suggests that ectotherms — because they require external temperatures to regulate their body temperatures and, therefore, often 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 high metabolisms, whereas turtles age slowly because they have low metabolisms,” said Miller.
The team’s findings, however, reveal that ectotherms’ aging rates and lifespans range both well above and below the known aging rates for similar-sized endotherms, suggesting that the way an animal regulates its temperature — cold-blooded versus warm-blooded — is not necessarily indicative of its aging rate or lifespan.
“We didn’t find support for the idea that a lower metabolic rate means ectotherms are aging slower,” said Miller. “That relationship was only true for turtles, which suggests that turtles are unique among ectotherms.”
The protective phenotypes 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 these protective traits do, indeed, enable 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 protection and has contributed to the evolution of their life histories, including negligible aging – or lack of demographic aging – and exceptional longevity,” said Anne Bronikowski, co-senior author and professor of integrative biology, Michigan State.
Beth Reinke, first author and assistant professor of biology, at Northeastern Illinois University, further explained, “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.”
Interestingly, the team observed negligible aging in at least one species in each of the ectotherm groups, including 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,” said Reinke.
Miller added, “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, its chance of dying is still 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.”
Reinke noted that the team’s novel study was only possible because of the contributions of a large number of collaborators from across the world studying a wide variety of species.
“Being able to bring these authors together who have all done years and years of work studying their individual species is what made it possible for us to get these more reliable estimates of aging rate and longevity that are based on population data instead of just individual animals,” she said.
Bronikowski added, “Understanding the comparative landscape of aging across animals can reveal flexible traits that may prove worthy targets for biomedical study related to human aging.”
Reference: “Diverse aging rates in ectothermic tetrapods provide insights for the evolution of aging and longevity” by Beth A. Reinke, Hugo Cayuela, Fredric J. Janzen, Jean-François Lemaître, Jean-Michel Gaillard, A. Michelle Lawing, John B. Iverson, Ditte G. Christiansen, Iñigo Martínez-Solano, Gregorio Sánchez-Montes, Jorge Gutiérrez-Rodríguez, Francis L. Rose, Nicola Nelson, Susan Keall, Alain J. Crivelli, Theodoros Nazirides, Annegret Grimm-Seyfarth, Klaus Henle, Emiliano Mori, Gaëtan Guiller, Rebecca Homan, Anthony Olivier, Erin Muths, Blake R. Hossack, Xavier Bonnet, David S. Pilliod, Marieke Lettink, Tony Whitaker, Benedikt R. Schmidt, Michael G. Gardner, Marc Cheylan, Françoise Poitevin, Ana Golubovic, Ljiljana Tomovic, Dragan Arsovski, Richard A. Griffiths, Jan W. Arntzen, Jean-Pierre Baron, Jean-François Le Galliard, Thomas Tully, Luca Luiselli, Massimo Capula, Lorenzo Rugiero, Rebecca McCaffery, Lisa A. Eby, Venetia Briggs-Gonzalez, Frank Mazzotti, David Pearson, Brad A. Lambert, David M. Green, Nathalie Jreidini, Claudio Angelini, Graham Pyke, Jean-Marc Thirion, Pierre Joly, Jean-Paul Léna, Anton D. Tucker, Col Limpus, Pauline Priol, Aurélien Besnard, Pauline Bernard, Kristin Stanford, Richard King, Justin Garwood, Jaime Bosch, Franco L. Souza, Jaime Bertoluci, Shirley Famelli, Kurt Grossenbacher, Omar Lenzi, Kathleen Matthews, Sylvain Boitaud, Deanna H. Olson, Tim S. Jessop, Graeme R. Gillespie, Jean Clobert, Murielle Richard, Andrés Valenzuela-Sánchez, Gary M. Fellers, Patrick M. Kleeman, Brian J. Halstead, Evan H. Campbell Grant, Phillip G. Byrne, Thierry Frétey, Bernard Le Garff, Pauline Levionnois, John C. Maerz, Julian Pichenot, Kurtulus Olgun, Nazan Üzüm, Aziz Avci, Claude Miaud, Johan Elmberg, Gregory P. Brown, Richard Shine, Nathan F. Bendik, Lisa O’Donnell, Courtney L. Davis, Michael J. Lannoo, Rochelle M. Stiles, Robert M. Cox, Aaron M. Reedy, Daniel A. Warner, Eric Bonnaire, Kristine Grayson, Roberto Ramos-Targarona, Eyup Baskale, David Muñoz, John Measey, F. Andre de Villiers, Will Selman, Victor Ronget, Anne M. Bronikowski and David A. W. Miller, 23 June 2022, Science.
The study was funded by the National Institutes of Health.