De-Icing Salt on Roadways May Harm Female Frogs

(CN) – Salt commonly used to de-ice paved surfaces can reduce the number and size of female frogs, a phenomenon that could potentially threaten the species’ survival.

The ratio of female tadpoles dropped 10 percent when exposed to road salt – suggesting that sodium chloride has a masculinizing effect, according to a study published Tuesday in the Canadian Journal of Fisheries and Aquatic Sciences.

Researchers from Yale University and Rensselaer Polytechnic Institute found that the effect is significant since the United States applies more than 22 million metric tons of road salt each year.

“The health and abundance of females is obviously critical for sustainability of any population because they’re the ones that make the babies,” said lead author Max Lambert. “So if you have a population that is becoming male-based, the population might be at risk.”

To test this phenomenon, the team conducted a series of water treatments in 500-liter tanks containing different levels of road salt and leaf litter from oak and maple trees. Several of the water treatments had tree-litter levels that matched amounts found in typical forest ponds.

Rearing tadpoles in oak litter without salt led to a female-dominant gender ratio of 63 percent, while the proportion of female frogs in water treatments with sodium chloride decreased by 10 percent, according to the study.

“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,” Lambert said.

Developing female tadpoles raised in oak litter were larger than male tadpoles, which is common among species that lay eggs. However, the female tadpoles exposed to road salt decreased in size.

“So you’re not only seeing fewer females but smaller females that may not be able to produce as many eggs,” Lambert said. “And the eggs are probably going to be lower quality.”

The team concluded that the reduced number of female frogs results from a process during development called “sex reversal,” occurring when simple elements – such as sodium – bind to a receptor in cells, mimicking estrogen or testosterone. This phenomenon can then trigger feminizing or masculinizing functions.

“The potential consequences to amphibian populations are interesting, including the continual masculinization of frog populations for many generations in habitats contaminated with high concentrations of road salt, which could potentially affect the abundances of frogs in these habitats,” said study co-author Rick Relyea.

Relyea, director of the Darrin Fresh Water Institute at Rensselaer Polytechnic Institute, suggested the findings could also be applicable to other species impacted by road salt.

“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,” he said.