sea snake
Claire Goiran

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Dark skin lets sea snakes slough off pollution

In the industrial United Kingdom, dark-colored peppered moths (Biston betularia) famously flourished, as they were better camouflaged against the soot-coated trees than their lighter counterparts. But a new study shows another reason why species might darken in response to pollution: not to blend in, but to detox. Animals living in polluted areas can pick up toxic metals like lead and zinc from their diet. But these toxins bind to melanin, a pigment that many animals produce to create brown and black colors. So in theory, a dark-colored animal could use melanin to sequester these toxins, and even slough them off with its skin. Researchers tested this idea in turtle-headed sea snakes (Emydocephalus annulatus, pictured), in which some individuals are striped, and others are jet-black. Sure enough, when studying museum specimens and surveying areas in the South Pacific, reefs near cities and other polluted sites had far more black snakes: On average, four out of five snakes in these sites were black, compared with fewer than one in seven in pristine reefs, the team reports today in Current Biology. And the discarded skins of urban sea snakes—particularly the dark colored bands—were chock-full of heavy metals, further supporting the idea that dark snakes might shed pollutants along with their skin. Dark-colored pigeons (Columba livia) appear to get a similar advantage in cities, so this process may be widespread: One more for the list of ways in which animal species have rapidly evolved to cope with humanity.