Of my many vices, the one for which I am most unapologetic is my craving for salty snacks. It turns out I am not alone. Sodium is both limited and prized in nature, and animals from insects to giraffes will go far out of their way to get a taste. For some butterflies, however, human influence has made the salt search as easy as my trip to the chip isle in the supermarket. Millions of tons of salt are dumped each winter onto American roads to melt snow and ice. But where does the salt go after it has served this purpose? New research by a team of scientists lead by Emilie Snell-Rood at the University of Minnesota in the USA shows that some of it can wind up in butterflies, often to dramatic effect.

Butterflies and moths are notorious salt freaks. But because the salt they need isn't found in their preferred diet, they resort to some extreme behaviours to satisfy their cravings. Some species drink tears while others drink blood, or secretions from carrion or faeces. Less peculiar, other species congregate at the edge of water, where they engage in ‘puddling’, a behaviour where mainly males bioaccumulate salt by imbibing absurd quantities of water, which they eject like a lawn sprinkler. To what end? Salt, it seems, is good for these insects. It helps with flight and digestion, and can directly enhance fitness because salt from puddling males is transferred to females during mating and then from females to her eggs.

But even for these salt junkies, there can be too much of a good thing. When Snell-Rood and her colleagues reared cabbage white butterflies on high salt diets, their survival was significantly reduced. More worrying, similar reductions in survival were observed when the team reared monarch butterflies on milkweed plants isolated from the roadside versus low-sodium plants collected from a nearby meadow. In comparison to these meadow plants, roadside milkweed exposed to wintertime salting contained around 10 times the amount of sodium. And much of this excess sodium wound up in the feeding butterflies, where, for those butterflies that did survive, it caused some unexpected changes.

If my wife and I sat on the couch gorging on chips, the excess salt would potentially increase our risks of hypertension and cardiovascular disease. But what doesn't kill salty butterflies seems to make them stronger – at least for males. Surviving males of both tested butterfly species reared with higher sodium intake buffed up, investing significantly more in thoracic muscle mass than did low-sodium butterflies. Surviving females, by contrast, invested less in muscle and instead redirected their developmental energy to neural tissue. Cabbage white females reared with high salt made bigger brains, while monarch females made bigger eyes.

As yet, there is little idea of why some butterflies succumbed to the toxic effects of salt and others did not, nor of the evolutionary consequences of these salt-induced responses in butterflies. Beefcake males may be better flyers, more able to migrate or locate mates. Brainy females may be more efficient foragers or make more discerning partners, perhaps hunting down the salty males that would pass on more sodium to their eggs. And looking further, perhaps these salty eggs fare better under cold stress, the salt helping to melt the ice, just as man intended! What is clear, however, is that our footprint reaches the natural world in ways we can hardly imagine. In this case, our impact may conceivably be positive. Alternatively, because too much salt lowers survival, we may be turning our roadsides into butterfly no-fly zones.

C. J.
W. A.
Anthropogenic changes in sodium affect neural and muscle development in butterflies
Proc. Natl. Acad. Sci. USA