Wildlife & Biodiversity

Road salt can change sex ratios in frog populations, finds study

The researchers found that the proportion of females reduced by 10 per cent when exposed to road salt

By Rajeshwari Ganesan
Published: Wednesday 23 November 2016

The scientists concluded that the masculinising effect of salt was likely the result of a mechanism known as “sex reversal” during development (Credit: iStock)

Naturally-occurring chemicals found in road salts, commonly used to de-ice paved surfaces, can alter the sex ratios in nearby frog populations according to a new study co-led by scientists at Yale and Rensselaer Polytechnic Institute (RPI). Changing sex ratios could reduce the size and viability of species populations.

The results were published in the Canadian Journal of Fisheries and Aquatic Sciences. Authors included Max Lambert, doctoral student at the Yale School of Forestry & Environmental Studies, who is the lead author and David Skelly, the Frank R Oastler Professor of Ecology at Yale; Meredith Smylie, a research associate at F&ES, and scientists from Rensselaer Polytechnic Institute.

The researchers found that the proportion of females within tadpole populations reduced by 10 per cent when exposed to road salt, or sodium chloride, suggesting that the salt has a masculinising effect.

They also found that exposure to fallen oak leaves also significantly altered the sex ratios in the frog populations, as well as the size of individual females in some cases. However, the maple leaf litter, on the other hand, was seen to have no effect.

More than 22 million metric tonnes of road salt is applied to roads in the United States each year. Maple and oak trees are dominant trees throughout temperate North America.

“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,” says Lambert.

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

For the experiment, conducted at the Rensselaer Aquatic Lab in New York, the researchers created a series of water treatments in 500-litre-tanks containing varying levels of road salt and leaf litter from maple and oak trees. Many of the water treatments contained tree litter levels that mimicked natural leaf litter found in typical forest ponds.

The research built upon work done at RPI as part of the Jefferson Project at Lake George on the potential impacts of road salt and leaf litter on the health of aquatic ecosystems. “The potential consequences to amphibian populations are interesting, including the continual masculinisation 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,” says Rick Relyea, director of the Darrin Fresh Water Institute at RPI and co-author of the study. “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,” she adds.

According to the study, frogs reared in oak leaf litter in the absence of salt exhibited a female-biased sex ratio (63 per cent). But when salt was added it decreased the proportion of female frogs to 53 per cent.

Meanwhile, within populations exposed to oak litter, developing female tadpoles were always larger than male tadpoles, which is fairly common among species that lay eggs. But when salt was added the female tadpoles decreased in size.

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

The scientists concluded that the masculinising effect of salt was likely the result of a mechanism known as “sex reversal” during development. Previous research suggests that such outcomes could be caused by a phenomenon in which simple elements such as sodium can bind to a receptor in cells, mimicking the actions of testosterone or estrogen. This, in turn, can trigger masculinising or feminising functions.

“There is a very small testosterone-like effect with one salt molecule. However, 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,” says the report.

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