Scientists see a strange — and worrying — climate change effect in frogs

The role human-induced climate change plays in the decline of frog species just took a strange turn.

Scientists are becoming keenly aware that animals’ bodies are being affected in peculiar ways due to climate change. Amphibians are no exception. A study published Monday in the journal Proceedings of the National Academy of Sciences examines the link between rising global temperatures and senescence (mortality related to aging) in frogs and toads in Europe and North America.

Hugo Cayuela is a postdoctoral researcher at the Université Claude Bernard in France. He says the research ties changes in frog aging to climate change.

“Accelerated aging is a potential threat for amphibians because it removes from the populations old/large individuals that strongly contributed to breeding,” Cayuela tells Inverse.

In other words: amphibians are aging faster than ever before in warmer environments, it’s disrupting their numbers, and humans are to blame.

How they did it — The scientists measured the impact of temperature and precipitation on senescence and aging in four toad and frog species residing in North America and Europe.

The researchers gathered data, monitoring some of the amphibians for a nearly 30-year period between 1990 and 2019. They would search for toads using headlamps at night and seek out frogs by day, studying several different sites of frogs and toads.

Then, using climate data, the researchers calculated the average annual temperature and precipitation at each site.

Finally, the researchers used statistical models to calculate the relationship between climate and aging in amphibians. Frogs are “excellent biological models” to study the impacts of the climate crisis on animal aging since their metabolic processes are known to be vulnerable to warmer or cooler temperatures.

“We collected demographic data in natural populations of frogs and toads in North America, and we used mathematical models to show that mean temperature affects senescence measures,” Cayuela says.

What they found — The scientists found three key findings from their research:

1. There was a strong link between increased annual temperature and senescence. Researchers found higher senescence — aging — rates in frogs in warmer temperatures.
2. These increased rates of aging correlated with shorter lifespans in all but one of the amphibian species. These species diverged more than 100 million years ago, suggesting other amphibians not studied may also be at risk of accelerated aging.
3. Female amphibians aged faster than males at lower temperatures, whereas male amphibians experienced more significant aging at higher temperatures. The sex difference may be linked to reproduction in females.

Based on their findings, the researchers concluded, “Climatic conditions could be a primary driver of aging” in ectotherms — animals like frogs that require sunlight and other external sources to maintain their body temperatures.

Although researchers have previously established a link between senescence and laboratory animals — such as the common fruit fly — this is the first time scientists have observed such a connection in the wild. But it’s hardly good news for the longevity of the frog.

While some amphibian species may adapt, Cayuela says others with “low genetic variation” will have a harder time evolving to avoid climate-change-related senescence.

Why it happens — Typically, amphibians in the sites studied face long seasons of lower temperatures, causing them to wait out the cold weather for eight to nine months — a period known as “overwintering.”

During this cold season, amphibians’ bodies enter into a hypermetabolic state that interrupts aging. But warmer temperatures may be interfering with this metabolic process, potentially speeding up the aging — and the extinction — of this key animal group.

Why it matters — In recent years, mounting evidence has revealed the ways the climate crisis threatens not only the long-term survival of animal species, but the development of their actual bodies.

Birds in the Amazon are evolving smaller bodies and longer wingspans to cope with drier and hotter conditions. Rabbits are “shapeshifting” and increasing their ear size in response to warmer temperatures.

Now, we can add amphibians to the list of creatures whose bodies are being upended by the climate crisis. The study’s findings come at a critical turning point for amphibians — one that could determine their survival or extinction.

“Amphibians are the most threatened vertebrate class on earth, with the IUCN Red List of Threatened Species listing 40 percent of species threatened with extinction,” Cayuela says.

Increased aging resulting from the climate crisis may impact not just amphibians’ metabolic processes but could also shorten their lifespans.

“Improving our understanding of the influence of climatic conditions on lifespan and actuarial senescence patterns is critical to predicting how climate change will impact population dynamics of ectotherms in the next decades,” the researchers write.

But why should we care about the death of amphibians? Cayuela says it’s because these animals serve critical functions in ecosystems, acting as a food source for some humans, reducing mosquito populations, and serving as test subjects in medical research.

Amphibians also “belong to ancient vertebrate lineages that colonized emerged lands more than 250 million years ago” and serve as an example of Earth’s ancient biodiversity, Cayuela explains.

What’s next — Global temperatures are on track to increase by at least 2.4 degrees Celsius— even taking into account green pledges made by world leaders at the recent UN COP26 summit — according to Climate Action Tracker.

That forecast likely spells bad news for not just humans, but amphibians as well.

“Increasing temperature caused by climate change could thus exacerbate the decline of amphibian populations that already face multiple anthropogenic stresses —e.g., habitat loss and degradation, landscape fragmentation,” Cayuela says.

Abstract: Variation in temperature is known to influence mortality patterns in ectotherms. Even though a few experimental studies on model organisms have reported a positive relationship between temperature and actuarial senescence (i.e., the increase in mortality risk with age), how variation in climate influences the senescence rate across the range of a species is still poorly understood in freeranging animals. We filled this knowledge gap by investigating the relationships linking senescence rate, adult lifespan, and climatic conditions using long-term capture–recapture data from multiple amphibian populations. We considered two pairs of related anuran species from the Ranidae (Rana luteiventris and Rana temporaria) and Bufonidae (Anaxyrus boreas and Bufo bufo) families, which diverged more than 100 Mya and are broadly distributed in North America and Europe. Senescence rates were positively associated with mean annual temperature in all species. In addition, lifespan was negatively correlated with mean annual temperature in all species except A. boreas. In both R. luteiventris and A. boreas, mean annual precipitation and human environmental footprint both had negligible effects on senescence rates or lifespans. Overall, our findings demonstrate the critical influence of thermal conditions on mortality patterns across anuran species from temperate regions. In the current context of further global temperature increases predicted by Intergovernmental Panel on Climate Change scenarios, a widespread acceleration of aging in amphibians is expected to occur in the decades to come, which might threaten even more seriously the viability of populations and exacerbate global decline.