Is countershading camouflage robust to lighting change due to weather?

Olivier Penacchio,Julie M Harris,P George Lovell

doi:10.1098/rsos.170801

Olivier Penacchio, Julie M Harris + Show 1 more

Open Access

https://doi.org/10.1098/rsos.170801

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Abstract

Countershading is a pattern of coloration thought to have evolved in order to implement camouflage. By adopting a pattern of coloration that makes the surface facing towards the sun darker and the surface facing away from the sun lighter, the overall amount of light reflected off an animal can be made more uniformly bright. Countershading could hence contribute to visual camouflage by increasing background matching or reducing cues to shape. However, the usefulness of countershading is constrained by a particular pattern delivering ‘optimal’ camouflage only for very specific lighting conditions. In this study, we test the robustness of countershading camouflage to lighting change due to weather, using human participants as a ‘generic’ predator. In a simulated three-dimensional environment, we constructed an array of simple leaf-shaped items and a single ellipsoidal target ‘prey’. We set these items in two light environments: strongly directional ‘sunny’ and more diffuse ‘cloudy’. The target object was given the optimal pattern of countershading for one of these two environment types or displayed a uniform pattern. By measuring detection time and accuracy, we explored whether and how target detection depended on the match between the pattern of coloration on the target object and scene lighting. Detection times were longest when the countershading was appropriate to the illumination; incorrectly camouflaged targets were detected with a similar pattern of speed and accuracy to uniformly coloured targets. We conclude that structural changes in light environment, such as caused by differences in weather, do change the effectiveness of countershading camouflage.

Highlights

Countershading is a pattern of coloration thought to have evolved in order to implement camouflage
Understanding how countershading can provide useful camouflage is complicated by the fact that the best pattern of shading is critically affected by animal shape and posture, and by lighting effects that vary with time of day and weather [6]
We report experimental work on human observers, showing that detection of countershaded targets is affected by weather conditions that result in different optimal patterns of countershading

Summary

Introduction

Countershading is a pattern of coloration thought to have evolved in order to implement camouflage. Understanding how countershading can provide useful camouflage is complicated by the fact that the best pattern of shading is critically affected by animal shape and posture, and by lighting effects that vary with time of day and weather [6]. The degree to which brightness varies between the top surface and elsewhere on the body is dependent on the specific lighting conditions [6] This is illustrated, which shows how a uniformly grey ellipsoid (figure 1a) would appear under a cloudy sky (figure 1b) or a sunny sky (figure 1c), displaying a noticeably different pattern for each illumination. A quantitative model describing how best to choose coloration for a specific light distribution and animal shape has been missing These earlier studies could not, answer the question of whether and how tightly a countershading pattern has to be related to the actual illumination of a scene to be effective.

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