The dual effect of stickleback nuptial coloration on rivals: manipulation of a graded signal using video playback

Abstract

Reproductive male three-spined stickleback, Gasterosteus aculeatus, were presented with pairs of videotaped images of another male played back at three different colour intensities: brightly coloured, moderately coloured and dull. Males attacked all three images but the proportion of attacks each received depended on the other image with which it was simultaneously presented. The moderately coloured image received the majority of attacks when presented with the brightly coloured or dull image, whereas the latter two images were attacked equally when presented together. Thus, nuptial coloration increased the capacity of an opponent to elicit attack from males but this increase was not open-ended. A dual-effect explanation for these colour effects suggests that dull males stimulated relatively low levels of aggression and little fear, whereas brightly coloured ones stimulated a high level of aggression but a level of fear sufficient to inhibit full expression of attack. Moderately coloured males received the highest levels of attack presumably because they stimulated high levels of aggression but relatively little fear. This interpretation emphasizes the importance of intensity of nuptial coloration in stimulating aggression and fear in an opponent and the role that these tendencies may play in determining a male's response to rivals.