Camouflage that is spot on! Optimization of spot size in prey-background matching

Abstract

Camouflage is a key predation prevention mechanism. The most intuitive camouflage strategy is background matching, where a prey resembles the colour and pattern of its background. Spots (e.g. geometric ‘elements’ that constitute a pattern) are a common feature on the body of prey, but the effect of their size in relation to elements in the background is rarely examined in camouflage studies. Here, we test the survivorship of computer generated prey possessing patterns of elements grouped into eight size classes on backgrounds composed of elements of the same eight size classes. All 64 possible combinations were presented to human volunteers (n = 10) acting as ‘predators’ in a computer simulation. As hypothesised, prey survivorship was high when the prey-background element size difference was low. We found significant asymmetry, however, in survivorship pattern: prey morphs with elements larger than those of the backgrounds were harder to detect than morphs with elements smaller than those of the backgrounds. When testing for potential trade-offs we determined that, in a habitat consisting of an equal proportion of two backgrounds with a large element size difference between them, net survivorship of specialist prey morphs (same element size as one of the backgrounds) was similar to that of generalist morphs (intermediate element size). When the two backgrounds were less distinctive, i.e. reduced element size difference between the backgrounds, generalist prey morphs achieved significantly better net survivorship than specialists.