The Gender Game: How Road Salt Changes Frog Population Sex Ratios

Gender is such a big issue that it isn't confined to just humans. Researchers from the Yale and Rensselaer Polytechnic Institute have discovered something extraordinary about frogs: chemicals in road salts used to de-ice paved surfaces alter the sex ratios in nearby frog populations.

Scientists have found that the proportion of females within tadpole populations was reduced by 10 percent when exposed to road salt, or sodium chloride, suggesting that the salt has a masculinizing effect. The study was published in the Canadian Journal of Fisheries and Aquatic Sciences.

"Many scientists have studied similar effects from exposure to pharmaceuticals and pesticides, but now we're seeing it from chemicals found in common road salt and leaf litter," said Maxime Lambert, the lead author of the paper. A doctoral student at the Yale School of Forestry & Environmental Studies, his work has revealed that this masculinization reduces the size and viability of species populations.

Given that more than 22 million metric tons of road salt are applied to roads in the United States each year, Lambert's findings have widespread effects. "The health and abundance of females is obviously critical for the sustainability of any population because they're the ones that make the babies," Lambert explained. "So if you have a population that is becoming male-based, the population might be at risk."

Previous research suggests that the observed "sex reversal" could be caused by a phenomenon in which simple elements, like sodium, can bind to a receptor in cells. Lambert and his team concluded that by mimicking the actions of testosterone or estrogen, road salts can trigger masculinizing or feminizing functions.

"There is a very small testosterone-like effect with one salt molecule," Lambert stated. "But if you're dumping lots and lots of pounds of salt on the roads every winter that washes into these ponds, it can have a large effect."

Rick Relyea, director of the Darrin Fresh Water Institute at Rensselaer Polytechnic Institute and co-author of the study, asserted their research could further inquiry on animals with a similar habitat. "The research raises the possibility that many other aquatic species could be affected by road salts in sub-lethal ways, not only in terms of altered sex ratios, but potentially in many other traits."