A Fifth of the Males, 4% of the Clutches: How Heat Rewrites Frog Populations from Within

CLIMATE CHANGE CAN EMPTY A POND WITHOUT DRAINING IT

Start with the scale of the problem. The Second Global Amphibian Assessment, published in 2023, found that 41% of amphibian species are threatened with extinction, more than any other group of vertebrates. Under the Kunming-Montreal Global Biodiversity Framework, the world has committed to halting human-induced extinction of known threatened species.

The usual picture of how climate change drives extinction involves lost habitat: dried wetlands and shifting ranges. This study shows a quieter route, one that works inside a population that still looks healthy from the outside.

How heat rewires a population In agile frogs, sex is set by chromosomes, as in mammals: XX is female and XY is male.

But excess heat during the tadpole stage can turn some genetically female tadpoles into normal-looking, fertile males. In theory, this need not be a disaster. Lead author Veronika Bókony noted that models suggest populations can partly absorb it, because a sex-reversed male fathers offspring that are all genetically female.

That buffer depends on one condition: the sex-reversed males must actually breed. The study, as reported, tested that condition four ways:

• In paired trials, about a third of females mated with sex-reversed males laid only part of their eggs, holding the rest back for a better mate.
• In the wild, across 16 populations, sex-reversed males made up about 20% of males but fathered only 4% of clutches.
• In the lab, 61% of offspring of sex-reversed males underwent sex reversal themselves, against 17% for offspring of normal males.
• Under a simulated heatwave, offspring of sex-reversed males carried higher loads of ranavirus, a pathogen that has sometimes killed more than 90% of tadpoles in a single outbreak. Each effect alone is manageable. Together, they compound: heat produces males that rarely breed, whose offspring are more likely to be reversed again and more likely to fall sick. The models suggest some small populations could vanish within 60 to 70 years. Adding ranavirus deaths brought extinction about 9.5 years sooner.

Where the

chain breaks: monitoring What makes this hard to manage is that a population under this pressure can look normal. Conservation surveys typically count adults and judge their sex by appearance. A sex-reversed male looks like any other male, so a population could appear to have a healthy sex ratio while its breeding capacity quietly erodes.

Dominique Potvin of the University of the Sunshine Coast said scientists underestimate how many sex-reversed animals exist. The gap is therefore in monitoring, not in conservation intent. India should pay attention. It has 458 recorded amphibian species, many found only in the Western Ghats and comprehensive surveillance of ranavirus in its wild amphibians is scarce.

A threat that combines heat stress and disease would go largely unseen here under current monitoring. For the GS3 answer on climate change and biodiversity, take this insight into the exam: warming threatens species not only by destroying habitats but by distorting reproduction and immunity in ways that standard population counts cannot see.