These frogs may be evolving because of road salt

When a winter storm lies on the horizon, the salt trucks spring into action. By spreading salt—regular old sodium chloride, the same kind we put on our food—over every inch of our roads, we’ve figured out how to make them safe for drivers even as slick snow and ice coat the asphalt, turning hazards into mere inconveniences.

But that salt doesn’t just disappear after a storm. Slowly, the salt trickles down off the road and into the water system while filling up rivers, lakes and wetlands—and massively boosting the salinity of these freshwater bodies. And for freshwater wildlife, living in a salty pond can lead to things like stunted growth, increased susceptibility to disease and even death, says Rick Relyea, a biologist at Rensselaer Polytechnic Institute. 

But Relyea and his colleagues have now found evidence that at least one freshwater species, the wood frog, might be able to adapt to saltier water within just a few generations. Exactly how this adaptation happens still isn’t clear, but this finding is a small bit of tentatively good news for efforts to protect wildlife from the dangers of road salt.

“It’s a great example of how evolution through selection can happen really quickly,” Relyea tells PopSci. “And on the upside, buy us some time until we fix some of these problems.”

Road salt can have a profound impact on the chemistry of some freshwater bodies of water. While a pristine lake would probably have less than five milligrams of chloride per liter of water, Relyea says, he’ll find some wetlands with hundreds of milligrams per liter. (For reference, that’s still not anywhere near salt levels in the ocean, where water has about 35 grams of chloride per liter, but still a big change for freshwater wildlife.)

Some freshwater bodies can end up saltier than others, depending on how shallow they are and how much inflow and outflow they have. In April 2022, Relyea and his colleagues collected wood frog eggs from nine different ponds in upstate New York. Each of the ponds was near a road but varied in salt content—ranging from one milligram of chloride per liter to a whopping 744 mg/liter. After collecting the eggs, the team raised each clutch into tadpoles at the lab in freshwater.

Once the tadpoles were swimming around, the researchers undertook what’s called a “time-to-death” experiment, which is exactly what it sounds like—they placed 15 tadpoles from each pond into a cup of water with a lethal dose of salt and tested to see how long they lived. They published their results on March 12 in the journal Ecology and Evolution.

Tadpoles from the eight ponds with the lowest salt concentration all died at about the same rate, with none still living after about two days. But tadpoles from the pond with the highest salt concentration survived much longer. By hour 30, when about half of the other tadpoles had died, around 90% of the saltiest-pond tadpoles were still swimming. By hour 50, when all of the other tadpoles were dead, more than 60% of the saltiest-pond tadpoles were still alive. Even on day three, long after the other tadpoles had passed on to the froggy afterlife, a few of the saltiest-pond tadpoles were still kickin’ it.

That saltiest pond is probably especially salty because it’s been surrounded by a large parking lot for around 25 years, Relyea says. And the remarkable survival of its frogs may not be coincidental. Relyea notes that wood frogs return to the same ponds and wetlands each year to lay eggs—meaning the eggs they collected may stem from a micro-population of frogs that has bred in that same parking lot-adjacent pond for more than two decades, or roughly 10 generations. With little genetic mixing between populations, this process could have driven the frogs to adapt to saltier conditions.

Theoretically, that would apply to the frogs from all the other ponds, too–and the second-saltiest pond in their study was still very salty, with more than 400 mg of chloride per liter. So was the third-saltiest pond, with around 300 mg/liter. Yet despite these still-very-high salt levels, tadpoles from the frog populations at these ponds performed no better than tadpoles from ponds with almost no salt in the time-to-death experiment.

The study authors said this might indicate that there is some threshold beyond which the frogs may start to adapt to high salt conditions, though Relyea also notes there might be some other factor at play here instead. “We don’t know how they did it,” he says, “we just know that they did it.”

For one, it’s also possible that the frog populations in the other ponds just haven’t had enough time to show any adaptations to salt, he says–if you came back and studied the frogs again in five or 10 years, maybe the frogs from the second-saltiest and third-saltiest ponds would also show some kind of adaptation.

Adaptations to new environmental conditions may often come with drawbacks, too. For example, Relyea has also found that zooplankton seem capable of adapting to high salt content—but when they do, they lose all sense of their circadian rhythm, or internal clock.

“They have no idea what time of day it is,” he says.

But these salt-tolerant frogs seem to be doing ok. In addition to seeing how long these tadpoles could survive in salty water, the team also tracked their growth, development and activity. They didn’t find any significant difference between the populations, though they did notice that saltier water seemed to make all of the tadpoles slightly less active.

This study adds to the growing collection of research on wood frogs and salt tolerance, which has yielded some contrasting results. For example, the new paper noted that while researchers in Connecticut have found that wood frogs from high-salt ponds were less likely to survive and slower to develop, researchers in Vermont have found that wood frogs from high-salt ponds might actually be larger and better at moving around.

Future research could tease out exactly how salt is affecting wood frogs and other wildlife. But Relyea notes that some of those other freshwater species, like plankton, can be way more sensitive to salt than the frogs seem to be. So, he adds, it’s important to work on limiting how much of that salt makes it to these ponds and wetlands in the first place–which not only helps to protect the environment, but also saves taxpayer money.