What determines whether an animal becomes male or female? For frogs, sex is much more complicated than we thought.

For some creatures, like reptiles and fish, sex can be heavily influenced by the environment. Sea turtles that grow up in warmer sand are more likely to become female, for example. Mammals, however, are much more bound to genetics: If you’re genotypically male in the womb, you’re likely to develop outwardly as such.

Amphibians such as frogs lay somewhere in the middle. They’re mainly influenced by genetics, but the environment also plays a role. In the laboratory, certain pollutants like synthetic estrogens and herbicides have been shown to induce genetically male frogs to develop outwardly as females.

Research has also begun to suggest this happens in the wild. In 2014, a paper in the Proceedings of the National Academy of Sciences found that pollution-laden runoff into suburban ponds in the U.S. might be turning larval male amphibians into females. (Related: 99 percent of these sea turtles are turning female—here’s why.)

However, continuing work by the same research group shows that sex reversal is also taking place in more pristine forest ponds—suggesting it’s also a natural phenomenon, at least in this species.

A study published in February in the journal PeerJ found sex-reversed frogs in the majority of water bodies studied. It also found no relationship between the degree to which the area around ponds was developed by humans and the proportion of sex-reversed animals.

“This isn’t just a story about pollution–instead, it suggests that frogs can adjust their sexual destiny to local circumstances,” which may include variations in temperature or some other environmental variable, says Rick Shine, a researcher at Macquarie University and the University of Sydney, both in Australia.

“That sounds ridiculously sophisticated for a simple frog, but recent studies have documented exactly the same sophistication in a few lizard species” and other animals, says Shine, who wasn’t involved in the paper.

Pond inspections

In the study, the authors studied green frogs (Rana clamitans) at 18 Connecticut ponds whose landscapes varied in the degree of suburban development; four were located in 100 percent forested areas.

The team analyzed the genotypic sex of the frogs, using a new genetic technique and compared it with the animal’s outward, or phenotypic, sex. They also looked for egg-like cells in the frogs’ testes.

Female green frogs have two X chromosomes, whereas males have an X and Y. The researchers found that males outnumbered females in all but one of the 18 ponds they studied. In seven of the ponds, they found genotypically female frogs that had developed as males (XX males), and in eight, they noted genetically male frogs that developed as females (XY females). The proportion of sex-reversed animals was generally below five percent, but peaked at 10 percent in one pond.

Eleven of the ponds had significant quantities of male frogs with egg-like cells in their testes. In one, in a mostly forested area, 44 percent of the frogs had these so-called “intersex” characteristics, though in most the proportion was lower.

The researchers can’t say why some ponds had higher levels of sex-reversed or intersex frogs than others. It doesn’t appear directly related to temperature, synthetic chemical makeup, or another variable that the team measured, says Max Lambert, a postdoctoral researcher at the University of California, Berkeley, and an author on both of the studies.

Healthier genes?

Lambert explains that male frogs can sometimes naturally develop female cells. Imagine an XX individual begins developing ovaries, but then, early in its larval life, switches toward becoming a male—but some of the egg-like cells remain. But it could also happen that as-yet-unknown environmental conditions might cause some female cells to develop in the normal course of testicular development.

“We think both are probably true, and right now what causes either to occur remains a sexual mystery,” says Lambert, who published the research while a Ph.D. student at Yale.

In frogs, sexual development (and reversal) happens when the animals are still larvae, or tadpoles. Once frogs reach adulthood, they cannot switch sexes so far as we know, Lambert adds.

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While it might seem unnatural, sex reversal in frogs might have at least one useful function—boosting genetic diversity, says Jodi Rowley, an amphibian biologist at Australia’s University of New South Wales and a National Geographic explorer.

“Usually males just pass the Y chromosome on to their offspring, and it doesn't get a chance to recombine like the X chromosomes,” Rowley explains.

“So having a female with a Y chromosome breed, it gives a change for the Y chromosome to create new ‘healthier’ versions, and possibly get rid of some bad mutations.”

Pollution still harms frogs

The findings in no way exonerate pollutants like the widely used herbicide atrazine, scientists caution.

Studies have shown, for example, that in the lab, exposing frog eggs to even 0.1 parts per billion of atrazine leads to a larger proportion of females than a control group of frog eggs in which no artificial chemicals are added, says Tyler Hoskins, a researcher at Purdue University.

“There's no doubt that we are releasing pollutants into the waterways, and that these chemicals can cause sex reversal,” Rowley says.

“While it now appears from this study that sex reversal happens relatively frequently for one species, it's too early to know how widespread this is across the landscape... and in the roughly 7,000 known frog species.”