The relative importance of local and global visual contrast in mate choice

Colour signals are often made up of a number of colour patches. The position of these patches within a pattern has the potential to influence signal conspicuousness, especially if viewers attend to local rather than global aspects of the colour pattern. If local contrast is important, then signal efficacy should be highest when highly contrasting patches are adjacent; if local contrast is not important, contrasting colours can be anywhere in the pattern. We tested whether global or local colour patch contrast influenced the attractiveness of male guppies, Poecilia reticulata. We assessed contrast on both global and local scales using two methods. The first, overall pattern contrast, estimates the global pattern contrast without accounting for which colours are adjacent. The second, boundary strength analysis, accounts for local contrast by estimating the contrast intensity (ΔS) and contact lengths between adjacent patches. We tested behaviour under three different light environments: blue, broad spectrum (control) and red to modify visual contrast. The light environment had a significant effect on all measures of colour pattern contrast. The male overall pattern contrast measures did not affect female preferences under the three light environments. The coefficient of variation (CV) of chromatic ΔS was the only measure to show a significant effect on female preferences. Females favoured males that displayed high variation in local contrast (high CV of ΔS, independently of the light environment). Therefore, variation in local visual contrast appears to be more important than overall pattern contrast in guppies, regardless of the light environment.