It may be a distant memory as the height of summer approaches, but all that roadside salt laid down in the winter in the icy, snowy parts of the U.S. is changing the bodies and brains of butterflies in strange ways, says a team of scientists at the University of Minnesota.
The discovery, described in the Proceedings of the National Academy of Sciences, shows that this common practice to improve safe driving could be seriously altering the physiology of animals that live near the road.
By pouring chemical fertilizers onto crops and letting the runoff drain into the ocean, humans have long been altering the makeup of macronutrients such as nitrogen and phosphorous – nutrients that living things often need in relatively large amounts. This has led to the harmful algal blooms that crop up in the ocean and in lakes, sucking all the oxygen out of the water, essentially causing fish and other living things in the area to suffocate.
Relatively less research has been done on micronutrients, the authors say – nutrients, such as salt, that are needed in very tiny amounts but are crucial for an animal's development. That's probably why animals (including humans) can get serious salt cravings, the authors point out.
But unless you have access to a vending machine full of potato chips, salt is usually pretty hard to come by in freshwater environments. That's why butterflies are often seen 'puddling,' or congregating around muddy, mineral-rich water – or even drinking the salty tears of crocodiles, as this orange Julia butterfly was caught doing on video.
But any good thing in excess can quickly turn to a bad thing. So the study authors wondered if all that extra salt along the roads was having an impact on the plants and animals near it. Even though many states have switched to other de-icing chemicals or use sand, some 17 million tons of de-icing salt per year is still applied to roads in the United States, according to the Salt Institute in Alexandria, Va.
Since the butterflies would be getting much of their salt from the plants they ate, the researchers gathered plants from Cedar Creek Ecosystem Science Reserve, which lies just north of Minneapolis and St. Paul. They compared salt levels in four plant species within 5 meters of the road, and plants that were at least 100 meters away. Two of the four species (oak and milkweed) had 1.5 to 30 times as much sodium if they were by the roadside, compared to the same species growing at a safe distance.
In the lab, the researchers fed some monarch butterflies milkweed that was collected near the road and fed other monarchs the milkweed that was collected far away, in more pristine prairie grasses. They found that the monarchs that ate the roadside milkweed had far lower survival rates (40.5%) than the prairie-fed ones (58.2%).
The salty roadside plants also had another effect, depending on sex: The males bulked up on thoracic muscle (responsible for moving wings and legs), while the females developed larger eyes.
But there could be something else about the roadside milkweed — say, that it was covered in pollution from the exhaust of cars — causing these effects. To make sure, the researchers then fed an artificial diet to a different butterfly species, the cabbage white butterfly. Since the researchers prepared this food rather than collecting it from the wild, they could specifically alter the salt content while keeping all other aspects of the food the same.
Sure enough, the cabbage whites' mortality rates on the high-sodium diet were also worse than those on the low- or medium-sodium diets. As with the monarchs, the cabbage whites raised on extra salty food ended up more muscular. Strangely enough, the females seemed to have the opposite relationship to dietary salt – the more sodium, the less muscle they had. Instead, they seemed to get brainier, devoting more volume (relative to their bodies) to neural tissue.
The scientists aren't sure exactly why these sex-differentiated changes occurred, but they think the effects of road salt could be even more noticeable in areas with more traffic, or with different kinds of soil. And it could have significant long-term evolutionary impacts on a wide range of species that are affected by the excess salt, from insects to birds and moose.
"We already see evidence of altered foraging behavior in some species — ants that live closer to roads forage less actively for sodium than those farther from the road," the authors wrote, "and moose also show some preference for roadside ponds because of salt runoff."