Outline and surface disruption in animal camouflage - PubMed (original) (raw)

Outline and surface disruption in animal camouflage

Martin Stevens et al. Proc Biol Sci. 2009.

Abstract

Camouflage is an important strategy in animals to prevent predation. This includes disruptive coloration, where high-contrast markings placed at an animal's edge break up the true body shape. Successful disruption may also involve non-marginal markings found away from the body outline that create 'false edges' more salient than the true body form ('surface disruption'). However, previous work has focused on breaking up the true body outline, not on surface disruption. Furthermore, while high contrast may enhance disruption, it is untested where on the body different contrasts should be placed for maximum effect. We used artificial prey presented to wild avian predators in the field, to determine the effectiveness of surface disruption, and of different luminance contrast placed in different prey locations. Disruptive coloration was no more effective when comprising high luminance contrast per se, but its effectiveness was dramatically increased with high-contrast markings placed away from the body outline, creating effective surface disruption. A model of avian visual edge processing showed that surface disruption does not make object detection more difficult simply by creating false edges away from the true body outline, but its effect may also be based on a different visual mechanism. Our study has implications for whether animals can combine disruptive coloration with other 'conspicuous' signalling strategies.

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Figures

Figure 1

Figure 1

Stimuli used in the experiment: (a) disruptive with all markings of high contrast (AH); (b) disruptive with all markings of low contrast (AL); (c) disruptive with high-contrast edge and low-contrast internal markings (EH); (d) disruptive with low-contrast edge and high-contrast inside markings (IH); (e) average of the markings on treatment AL (L); (f) average of the markings on treatments EH/IH (M); and (g) average of the markings on treatment AH (H). Within a single replicate of marked treatments (IH, EH, AH and AL) the pattern was the same, with only the contrast level and contrast placement differing. Across replicates, targets were based on different samples of bark, and so each replicate of targets had a unique pattern.

Figure 2

Figure 2

Non-parametric survival plot of the treatments with curves the probability of surviving bird predation over time. Survival top to bottom: IH>AH=AL=EH>L=M=H.

Figure 3

Figure 3

Example targets of the patterned treatments, (a) AL, (b) IH, (c) AH and (d) EH, pinned to trees, with the edge processed images from the visual model to the right of the images of each target.

References

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