Red on Black, Friend of Jack
Batesian Mimicry of Coral Snakes
Background
Animals mimic each other as a defense mechanism. This could mean mimicking another animal’s appearance, behavior, sound, etc. to ward away potential predators. Mimicry is a quality that is adapted over time — it takes natural selection to achieve the right appearance, or phenotype, to mimic. If two dangerous species mimic each other’s coloration, for example, then they would be an example of Mullerian mimicry. On the other hand, if a harmless species mimicked the coloration of a dangerous animal, the harmless animal would be displaying a form of Batesian mimicry. This is because the non-venomous animal is falsely signaling danger to its predators and reaping the benefit of less predation.
Coral snakes are highly venomous animals that possess what is known as aposematic coloration, or bright colors such as reds and oranges. This is seen in creatures like the poison dart frog. Any animal displaying aposematic colors are warning potential predators to stay away. Over time, predators that avoid animals with aposematic coloration will survive through natural selection, while other prey evolve to mimic the bright colors.
The old saying, “Red on black, friend of Jack; red on yellow kills a fellow,” explains Batesian mimicry perfectly. In North America, it helps people distinguish the venomous coral snake from a harmless mimic — the scarlet king snake. When the coral snake and the scarlet king snake inhabit the same geographic area, they are considered sympatric species. If the snakes live in different geographic areas, they are considered allopatric species. Allopatry and sympatry can be used to describe any species geographic relationship with one another.
Cusuco National Park, located in Honduras, is a tropical habitat for coral snakes. In the park’s perimeters, there are two species of coral snakes and nine different species of Batesian mimics. In Lauren Wilson’s study, they used clay models to figure out whether the sympatric or allopatric Batesian mimics were preyed on more. Specifically, the scientists were looking at mimics living in edge sympatry, or those right on the outskirts of the usual coral snake habitat.
Summary
The clay models were placed in areas of edge sympatry and allopatry, which were both at higher elevations. Coral snakes are not found above 1800 meters, forming a clear boundary that distinguishes edge sympatric from allopatric mimics. The latter can be found at elevations over 2000 meters.
There were tricolored clay models and white and black or red and black models that were not exact mimics. As a control, the scientists had a brown, “cryptic” clay snake, or one that did not possess aposematic coloring and relied more on camouflage. All 4 models were placed in each section of forest the scientists surveyed. The first experiment had 9 sections total, with 2 in the allopatric region and 7 in the sympatric region. The second year there were 18 sections, with 3 in the allopatric region and 15 in the sympatric. The models were checked for signs of attack every two weeks, like claw or bite marks.
The two tests were performed twice over the course of two years, and each year produced opposite results. One year the mimics in the edge sympatric habitat were preyed on more, and the next, the allopatric. The results also showed that the clay mimics that looked most similar to the corn snake were preyed on the most. This indicated that the further away from the central habitat of the coral snake, including mimics living in edge sympatry or allopatry, the less their mimicry helps them as a defense mechanism.
There was one problem with this conclusion — the abundance and diversity of coral snake mimics outside of the coral snake’s usual habitat range indicates the Batesian mimicry should help survival.
The scientists took these results to mean one of three things. One would be that the corn snake mimics living in edge sympatry and allopatry were maladaptive, meaning the mimicry is no longer a helpful adaption. An alternative explanation is that the mimics have a form of motion mimicry along with the coloration, that the clay models in the experiment could not perform. The last concerns negative frequency-dependent selection, meaning there were so many mimics present that their fitness, or likely-hood of survival, decreased.
