Interactions between background matching and disruptive colouration: Experiments using human predators and virtual crabs

Abstract

Abstract Interactions between disruptive colouration and the match between prey and background spot size were manipulated in two experiments that used time taken by human ‘predators’ to find artificial prey (virtual crab morphs) against heterogeneous backgrounds as a measure of the their camouflage. Experiment 1, in which the spots and their placement imitated the arrangement on the crab Carcinus maenas, tested whether high and low contrast spots touching the body outline (disruptive ‘edge’ morphs) made the artificial prey more difficult to detect than when the spots did not touch the outline (non-disruptive ‘inner’ morphs) against three different backgrounds: ‘small’, ‘middle’ and ‘large’ spot size. In Experiment 2, the range of spot sizes and their positions (‘edge’ and ‘inner’) on the crab morphs were varied to determine the most effective combination against the ‘middle’ background. Altogether, 640 volunteers participated in these computer trials, representing a high degree of independent replication. All patterned morphs were significantly harder to detect than plain morphs, indicating that possessing at least some degree of background matching can provide camouflage. Both experiments demonstrated that various morphs, though not having the same spot sizes as the background, had similar or better survivorship as those with matching spot sizes—indicating that optimal camouflage did not come from background matching alone. In Experiment 2, edge-disrupted morphs consistently took longer to find than their non-disruptive counterparts. The relative effects of edge disruption, contrast, and background/prey spot size are clearly context-dependent, highlighting the complexity of prey concealment.