South Pacific sea snakes are changing color in response to pollution run off from cities and military sites. Rather than camouflage, as seen in a famous study of moths, their new hue reflects the fact that dark skin is better at soaking up toxic trace metals, which get shed along with the skin.

Renowned zoologist Professor Rick Shine of the University of Sydney has been studying sea snakes in New Caledonia. “Our study site is right beside the city. We walk out of our hotel, across a street, over the beach and swim to the reef," he told IFLScience. "There is water draining off roads so presumably quite substantial levels of pollution. In fact, the surprising thing is that we have diverse and abundant species of snakes and fish living there.”

Shine and his students noticed the turtle-headed sea snakes (Emydocephalus annulatus) at this site are completely black, as are those near a reef the Australian military has used as a bombing site and other locations with human activity. On pristine reefs, the snakes mix black markings with white bands or blotches. The sites are not polluted enough for the darker colors to represent camouflage, but Dr Claire Goiran of the University of New Caledonia remembered that a study of Parisian pigeons found higher zinc concentrations in dark rather than light feathers.

Goiran and Shine collected skins shed by sea kraits and had them tested for trace elements. In Current Biology, they report that dark colored patches of skin contained higher concentrations of zinc and manganese than white patches from the same snake. Other elements also appeared to concentrate in the dark patches, but the differences were not statistically significant.

Shine told IFLScience that melanin, the primary pigment in dark skinned snakes and humans, binds to a wide range of trace elements, concentrating them in the skin. When the snake sheds its skin, it dumps the (frequently toxic) elements with it. This reflects the pattern seen in the pigeons, but the effect is reinforced in the snakes because dark skin is also more attractive to algal spores. The algae causes darker skinned snakes to shed their skins more often, accelerating the rate at which they can get rid of toxic build-ups.

The benefit is apparently large enough to give the darker skinned snakes such a strong advantage in polluted areas that natural selection has allowed them to take over in the few decades since urban pollution started to affect the reefs.