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Road Salt Sex Change: How Deicing Messes with Tadpole Biology

Road salt can cause deformities, impact survival rates, and even switch female tadpoles into males.

Virginia Dept. of Transportation

Winter is coming, and so is truckload after truckload of rock salt. By the time daffodils start popping up in the spring, Americans will have dumped more than 24 million tons of the stuff on our roads, parking lots, and driveways. Dissolved in melting snow and ice, the salt will enter streams, rivers, and ponds—with some serious consequences for wildlife, especially frogs, which may spend their formative weeks immersed in this slurry.

Studies have found that rock salt can cause lethargy, weight loss, and deformities in tadpoles and that increased salinity hurts survival rates for the eggs and the tadpoles of wood frogs and spotted salamanders. By messing with how quickly gray treefrog tadpoles break down leaf litter, road salt can even affect an ecosystem’s nutrient cycle. We’re just beginning to comprehend the ecological impact of those crunchy crystals, but the latest discovery might be the most bizarre.

Road salt may be turning little girl tadpoles into little boy tadpoles. According to a new paper published in November in the Canadian Journal of Fisheries and Aquatic Sciences, salinity has the power to alter sex ratios in wood frog tadpoles, reducing the proportion of females by around 10 percent.

RayMorris1/Flickr

This finding is important, says lead author Max Lambert, a doctoral candidate at Yale University, because we don’t tend to think of salt as a chemical that affects sex. The herbicide atrazine and contraceptives made of synthetic estrogen have long been known to feminize male frogs, but salt? Good ol’ NaCl?

“These other chemicals are certainly in the environment and having an effect,” Lambert says, “but we may have focused on them to the exclusion of other things out there.”

It’s pretty obvious that removing females from a species’ natural sex ratio could lead to population declines and even collapse. Which is, you know, just lovely considering the numbers: Seventy percent of the world’s amphibian species are dropping fast, and while those in the United States are doing a little better, they’re still contending with habitat destruction, chytrid fungus, and other types of hormone-disrupting pollution.

Chemicals found naturally in the environment can affect an amphibian’s development, too. As tadpoles, some frog species eat leaves that are chockful of phytochemicals, and when those leaves get good and wet, they leach those substances into the surrounding water. In a 2010 study, researchers found that when they soaked oak leaves in their tadpole pools, the little guys grew up to have more endocrine system abnormalities than usual, particularly the males.

To follow up on their research, Lambert and his coauthors also looked at how different kinds of leaf litter can influence whether tadpoles end up male or female. Specifically, they compared tadpoles raised in pools with only submerged oak or maple leaves, as well as tadpoles growing up in pools with oak or maple leaves in addition to road salt. Both tree species are common across the northeastern United States, but maples are quickly outpacing oaks thanks to an overabundance of deer (which gobble up oak seedlings and acorns), fire suppression, and logging. Overall, the scientists wanted to see if this shift in plant life might be having an effect on tadpoles, too, and if road salt is making these impacts better or worse.

Interestingly, the experiment showed that oak leaves tended to have a slight feminizing effect. Female tads are usually bigger than males, but the oak leaf pool produced females that were more than 3 percent bigger than normal. When the researchers added salt, however, that sexual dimorphism got turned on its head, with the males becoming 0.7 percent larger than the females.

That oak leaves might supercharge females is something that hasn’t really been identified before, Lambert says. “This could be pretty ecologically important, particularly if salt contamination completely undoes the size advantage females get from the oak litter.”

Next, Lambert and his colleagues want to test these findings in other frog species, as well as incorporate other kinds of rock salt. The recent experiments used pure sodium chloride, but various other salts and mixtures are available on the market. There’s really no way to know what any of them might do to tadpoles until we run the tests.

If nothing else, Lambert’s research adds to a growing body of knowledge that suggests the effect of road salt on the environment is not neutral. This winter we’ll sprinkle enough salt to fill up Epcot Center 296 times. As with the salt on our tables, perhaps we should learn some moderation. 


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