Typically, your phenotype—or your outward appearance—matches your genotype when it comes to sex. But, in a little-studied phenomenon dubbed “sex reversal,” the outward characteristics of animals change enough that they no longer match their genetic sex. Such reversal has been recorded in amphibians, reptiles, and fishes.
A new study published last week in Science Advances explored sex reversal in wild “agile frogs” (Rana dalmatina) sampled from breeding ponds in Hungary. In prior studies by this group of researchers from the Centre for Agricultural Research in Budapest, about 20 percent of phenotypically male adults had the female XX (instead of XY) genotype, indicating transitions from female-to-male.
Read more: “The Queer Lives of Frogs”
Sex reversal was higher in habitats that were altered by human activities, aligning with laboratory findings that environmental changes were the cause of it. As such, the study authors wondered whether increasing environmental stress from climate change would endanger agile frog populations and ultimately lead to their extinction.
“Barely any information is available about the transgenerational effects of sex reversal on other aspects of stress tolerance, such as the ability to survive heat stress or pathogen infection, even though these tolerances are highly relevant in the era of climate change and pandemic diseases,” they wrote.
To test the impacts of sex reversal on fertility and offspring viability, the researchers compared the reproductive success of sex-reversed and sex-concordant males. They did so by conducting mating trials in an outdoor container adjacent to breeding ponds. They caught pairs of similar-sized males of the two types as they made a run for the ponds, put each male in a container with an egg-bearing female, and recorded the reproductive output. Then, they staged “intrasexual” competition trials in which pairs competed to mate with a single female.
The results showed that the original, genotypic sex of the male had no significant effects on reproduction. Females dropped eggs for 12 sex-concordant and 11 sex-reversed males, and all yielded viable offspring. The survival rates of embryos to the tadpole stage, as well as tadpole growth rates, also were the same. Nor did the intrasexual competition trials produce any differences; indeed, the three egg masses that were laid occurred with sex-reversed males.
All of which is to say, sex reversal isn’t an “evolutionary dead-end” as feared. Rather, concluded the study authors, “female-to-male sex-reversed agile frogs aren’t excluded from reproduction either by male-male competition or by female choice.”
Of course, all of their offspring are born XX—that is, female—and the implications of that twist are still TBD. 
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Lead Image: Bókony et al., Sci. Adv. 12, eaeg3488 (2026)
