Optimizing countershading camouflage

Abstract

Countershading, the widespread tendency of animals to be darker on the side that receives strongest illumination, has classically been explained as an adaptation for camouflage: obliterating cues to 3D shape and enhancing background matching. However, there have only been two quantitative tests of whether the patterns observed in different species match the optimal shading to obliterate 3D cues, and no tests of whether optimal countershading actually improves concealment or survival. We use a mathematical model of the light field to predict the optimal countershading for concealment that is specific to the light environment and then test this prediction with correspondingly patterned model "caterpillars" exposed to avian predation in the field. We show that the optimal countershading is strongly illumination-dependent. A relatively sharp transition in surface patterning from dark to light is only optimal under direct solar illumination; if there is diffuse illumination from cloudy skies or shade, the pattern provides no advantage over homogeneous background-matching coloration. Conversely, a smoother gradation between dark and light is optimal under cloudy skies or shade. The demonstration of these illumination-dependent effects of different countershading patterns on predation risk strongly supports the comparative evidence showing that the type of countershading varies with light environment.

Keywords: animal coloration; camouflage; defensive coloration; shape perception; shape-from-shading.

Fig. 1.

The top row shows the plan view of the experimental treatments (surface reflectance of model cylindrical “prey”): countershading optimized for cloudy weather (CloudyCS) (A); optimized for sunny conditions (SunnyCS) (B); dark green (Dark) (C); two-tone (sharp dark/light transition; TwoTone) (D); mean color of cloudy optimized treatment, CloudyMean; (F) mean color of sunny optimized treatment (SunnyMean) (E); light green (Light) (G). Below these are the prey types photographed when the illumination is that of a sunny (middle row) or cloudy (bottom row) day.

Fig. 2.

Proportion surviving (mean ± SEM) in each treatment and illumination condition (Left, diffuse; Right, direct sun). Total frequencies across all blocks are given inside the bars as numbers alive:dead. The treatments have been ordered by mean survival across illumination conditions for ease of comparison.