The function of red and banded patterns in snakes: the eyes of the beholders—also, the function of bands in fish

Abstract

Abstract The functions of bright bands in coral and other snakes have long been a puzzle. A common opinion is that coral snakes display aposematic marks and that other red and black banded snakes in the Americas are mimics of coral snakes. In two previous reviews, I challenged this prevailing opinion and showed that red and bands are probably defensive against snake predators. Here, I offer a plausible mechanisms through which red and bands could be protective against ophidian predators: the function of bands is to interfere with the focusing ability of snakes, which eyes, as in fish, rely on a ball lens that moves back and forth, a peculiarity among terrestrial vertebrates. Simulations show how the spherical aberrations created by ball lenses produce local maxima in contrast when out of focus, which would feel in focus. I conjecture that red targets specific visual receptors in snakes active in low light: these receptors would have a slow and saturating response. This conjecture is justified by the lability of vision in snakes and the overabundance of red light in dim light. Those conjectures allow precise and well-respected predictions and credibly explain a wide range of observations, such as the geographical repartition of tricolour snakes, the evolution of triadal patterns, the evolution of specific triadal-like patterns in some Erythrolamprus, and the repeated evolution of black and white banded snakes with a red dorsal line. Fish eyes have similar optics, and the function of the ubiquitous fish bands could likewise be to interfere with the vision of predators.