Some seemingly dangerous animals are really just sheep in wolves’ clothing. They’re harmless, but by imitating the appearance and warning signals that dangerous animals use to advertise their defenses (like toxins or painful stings), they fool predators into thinking they’re tough guys, too.
Take the robber flies, for example. Some members of the family imitate the black and yellow striping of bumblebees and wasps, while others sport orange wings to look like tarantula hawks. Meanwhile, the non-venomous scarlet kingsnake (above) copies the pattern of black, red, and yellow bands worn by its neighbor, the coral snake, one of the most potently venomous snakes in North America.
How closely a mimic imitates its model often depends on how many of the models there are. Think of it like this, says evolutionary ecologist David Pfennig, who’s been studying mimicry in snakes for the last 15 years at the University of North Carolina: Say there’s a population of mimics that’s surrounded by lots of deadly models. The predators in the area are under strong selection to avoid the model (which they’re not doing actively—it’s a preference that’s innate, and not learned, with natural selection favoring traits and genes that help predators detect and avoid the prey’s warning signals) and its lookalikes because the chances of encountering the model are very high. Here, even poor mimics can get by with a less than perfect resemblance.
If the models are rare compared to the mimics, though, and predators are less likely to encounter them, then the selection to avoid both model and mimic is more relaxed. In this case, trying to eat a crude mimic is less risky, which drives precision in the pretenders.
But what happens to a mimic when its model disappears completely? Pfennig had the perfect opportunity to find out. In the North Carolina Sandhills, a thousand square miles or so of sandy hills and pine tree-dotted savanna, kingsnakes are pretty common, but coral snakes have always been considered rare. Today, they might not be there at all—researchers haven’t found any in the area since 1960. They’re locally extinct, leaving the kingsnake with a disguise that wouldn’t seem to do it much good.
“When we embarked on this study, I thought that we would most likely find no change,” Pfennig said in an email. “After all, only about 50 years had transpired since coral snakes went extinct in the populations (that’s about 15 to 20 snake generations).”
If there was going to be a change at all, Pfennig figured the mimics would become less accurate. In an earlier study, he’d found that kingsnakes’ patterns were closer to the corals’ in areas where they lived alongside each other, but not as good in places where there weren’t any coral snakes.
The local predators avoided the mimics in the former areas, but not the latter. If mimicry breaks down in locations where the model is gone, Pfennig says, he expected something similar during times when it's absent, like after extinction.
But that’s not what he and his grad student Chris Akcali found in the Sandhills. When they compared kingsnake specimens that had been collected between the 1970s and 2010s with preserved specimens of pre-extinction coral snakes and coral snakes still living in Florida, Pfennig said, “we witnessed the evolution of more refined mimicry.” Contrary to the scientists’ expectations, the Sandhill kingsnakes actually looked more and more like coral snakes as half a century passed without the models around.
Since coral snakes were rare in the Sandhills before they went extinct there, there was already strong selection for precise mimicry in the kingsnakes. Pfennig and Akcali think that things kept moving in that direction because too few generations of predators have passed to reverse their avoidance of the deadly snakes and anything that looks a lot like them.
“Somewhat paradoxically, selection imposed on mimics by predators can generate an evolutionary momentum that continues to favor more precise mimicry,” Pfennig said. “Even after the dangerous model has gone extinct.”
That momentum won’t last though, and the researchers expect the kingsnakes’ mimicry will eventually get less accurate. The biggest driver of that will probably be how desperate predators are to find food. If times get tough and animals becomes more willing to attack mimics, then there’s less pressure on the snakes to keep up the charade. On the other hand, if the snakes or their predators move back and forth between the Sandhills and areas where coral snakes are still present, that could bring in genes that have to do with avoiding mimics in the predators and/or genes for good mimicry in the snakes, which might let the mimicry linger.
For now, the kingsnakes do a very good impression of the long-gone corals. Good enough that Pfennig says it caught him a little off guard. “Keep in mind that what makes a scarlet kingsnake look like a coral snake is a complex array of pattern elements: width of rings and the amount of red, black and yellow in each ring,” he said. “That you could get noticeable refinement of such a complex trait evolving in only a few dozen generations was surprising to me. It’s always exciting in science when you get results that you did not expect.”