Aposematism and gregariousness: the combined effect of group size and coloration on signal repellence

Many aposematic species have evolved an aggregated lifestyle, and one possible advantage of grouping in warningly coloured prey is that it makes the aposematic signal more e¡ective by generating a greater aversion in predators. Here we investigate the e¡ect of prey group size on predator behaviour, both when prey are aposematic and when they are not aposematic, to separate the e¡ects of warning coloration and prey novelty. Naive domestic chicks (Gallus gallus domesticus) were presented with either solitary or groups of 3, 9 or 27 live larvae of the aposematic bugTropidothorax leucopterus. Other naive chicks were presented with larvae of the non-aposematic bug Graptostethus servus either solitary or in groups of 27. Attack probability decreased with increasing group size of aposematic prey, both when birds were naive and when they had prior experience, whereas prey gregariousness did not a¡ect the initial attack probability on the G. servus larvae. In a separate experiment, groups of mealworms were shown to be even more attractive than solitary mealworms to naive chicks. We conclude that the aversiveness of prey grouping in this study can be explained as increased signal repellence of speci¢c prey coloration, in this case a classical warning coloration. These experiments thus support the idea of gregariousness increasing the signalling e¡ect of warning coloration.