“If red touches yellow, you're a dead fellow; if red touches black, you're all right, Jack." To avoid being eaten by predators like black bears, bobcats, and hawks, the scarlet kingsnake mimics the vivid stripes of the deadly, venomous coral snake.
And in the decades since the local extinction of the eastern coral snake (Micrurus fulvius) in parts of the southeastern U.S., the scarlet kingsnake (Lampropeltis elapsoides) has evolved even more convincing mimicry. They’re now better imitators than they ever were, and that’s because the trick still works, for now.
Batesian mimicry, in particular, is when a toxic species is mimicked by a more palatable one to deceive predators, conferring some survival advantage. The most precise mimics are favored by natural selection when their model becomes increasingly rare.
The coral snake used to live as far north as the North Carolina Sandhills, but they went extinct in this region back in 1960. (Local extinct is called extirpation.) Christopher Akcali and David Pfennig from the University of North Carolina, Chapel Hill, wanted to see if and how mimicry in this snake lookalike case evolved after the extinction of the model.
In this figure, you have the non-venomous scarlet kingsnake (a) on the left and the highly venomous eastern coral snake (b) on the right. Below, the green is where only kingsnakes are present; the red is where they both still exist.
The duo compared 5 pre-extirpation coral snakes to 27 post-extirpation kingsnakes collected between the 1970s to the 2000s from the Sandhills. They measured the width of each of their rings and calculated the proportions of red and black. They also compared 23 coral snakes to 23 kingsnakes collected from the Florida panhandle, where the two still live side by side.
Surprisingly, they found that the red and black bands of the recently collected Sandhill kingsnakes more closely resemble coral snakes, compared with kingsnakes collected in the 1970s, who have larger black bands. In the 50 years following coral snake extirpation, the kingsnakes underwent rapid evolution of mimicry. By contrast, no such change occurred in the Florida panhandle where the venomous snakes are still abundant; those mimics were still imprecise.
Turns out, the best mimics live along the border between areas that have only kingsnakes (allopatric) and areas that still have both (sympatric). Where the model is rare, the chances of mistakenly attacking it is low, and predators are more willing to risk attacking imprecise mimics. As a result, only precise mimics are favored in edge regions like the Sandhills. "If you are a predator, and you’re in an area like Florida, where coral snakes are everywhere, then you should avoid anything that looks like a coral snake,” Akcali explains Nature. “If you are in North Carolina where coral snakes are really, really rare, predators can benefit from attacking [mimics] sometimes.”
Deadly models can fuel an “evolutionary momentum,” the study says, that drives the further evolution of precise mimicry, even after model extinction. Few predator generations have passed, and historically, the costs incurred by mistaking a noxious coral snake for a tasty kingsnake were much greater than the other way around. Though eventually, as predators catch on, scarlet kingsnakes in the Sandhills will probably stop resembling coral snakes.
The work was published in Biology Letters this week.
Images: David Pfennig (top) & C.K. Akcali, D.W. Pfennig, Royal Society 2014 (middle)