Long-lived lizards: what causes longevity in cold-blooded creatures?

The largest ever study on ageing in reptiles and amphibians has unlocked some of the secrets of cold-blooded long life.

The 114-strong international research team has connected data from 107 wild populations of 77 different species of ectotherms (cold-blooded creatures, or reptiles and amphibians).

Published in Science, the study shows that ectotherms have much more variable longevity and ageing than birds and mammals.

“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, associate professor of wildlife population ecology at Penn State University, US.

“If we can understand what allows some animals to age more slowly, we can better understand ageing in humans, and we can also inform conservation strategies for reptiles and amphibians, many of which are threatened or endangered.”

The US researchers who led the study compiled data from over 100 long-term studies on ectotherms, conducted all over the world.

Tree frog in reeds
An Iberian tree frog (Hyla molleri). Credit: Iñigo Martínez-Solano

“It must be an enormous effort to manage that number of authors and comments from everyone,” says co-author Professor Mike Gardner, a researcher in biodiversity and ecology at Flinders University.

Gardner contributed information from the longest-running lizard survey in the Southern Hemisphere: a sleepy lizard (Tiliqua rugosa) survey that was started in 1983 by the late Professor Mike Bull. The study, now run by Gardner, is in its 40th year.

The compilation of information has produced a wealth of new discoveries, calling into question many previous assumptions about longevity in ectotherms.

One such assumption is that the way an animal regulates its temperature is linked to lifespan.

Painted turtle swimming in lake
A painted turtle (Chrysemys picta). Credit: Beth A. Reinke, Northeastern Illinois University

“We didn’t find support for the idea that a lower metabolic rate means ectotherms are ageing slower,” says Miller.

“That relationship was only true for turtles, which suggests that turtles are unique among ectotherms.”

They did, however, find evidence to support the protective phenotypes hypothesis, which suggests that animals with protective traits like armour or venom are going to live longer.

“It could be that their altered morphology with hard shells provides protection and has contributed to the evolution of their life histories, including negligible ageing – or lack of demographic ageing – and exceptional longevity,” says co-senior author Professor Anne Bronikowski, a researcher of integrative biology at Michigan State University, US.

Many animals in captivity have different lifespans to those living in the wild. Gardner says this is likely to be true of sleepy lizards – but their longevity makes this hard to tell.

Phylogenetic tree of animals
Supertree of all endothermic and ectothermic species included in this analysis. Branch lengths are not scaled. The red in the inner circle represents endotherms and blue represents ectotherms, as throughout the paper. Green bars are longevity estimates and orange bars are the aging rates. Silhouettes from Phylopic.org. Credit: Science journal

“I think lizards in captivity are going to last longer, but these things live so long that we probably don’t know of people who’ve had them for that long.”

The long-term study has shown sleepy lizards’ vulnerability to medium and long-term environmental changes – like droughts caused by El Niño years. Like other long-term studies in the paper, the ability to track individual animals over a long time has provided data that can’t be answered by shorter-term projects.

“One of the things this paper shows is that real value of those long-term studies,” says Gardner.

“It’s always difficult to keep funding these things because you’re kind of dependent upon these short term cycles.

“When I took over this study, it was at a point where it was like, ‘well, is it actually going to keep going?’.”

Gardner took over the sleepy lizard survey in 2017, following Bull’s untimely death. The current funding for the project expires at the end of this year, and Gardner is hoping for a grant that will extend it for a further three years.

Two sleepy lizards under a tree
Sleepy lizards. Credit: Mike Gardner

“But I’m beholden to these very short-term cycles.

“[These studies] do have value because you’ve got so much basic information about it that you can ask really in-depth questions, which you just can’t ask in other systems because you don’t have that fundamental baseline data.”

The sleepy lizard survey is also accepting public donations.

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