For the boys!

Abstract

Sexual dimorphism occurs in many members of almost all animal groups, with males adopting sometimes spectacularly colourful appearances. Such differences are generally explained within the framework of sexual selection: male colouration is associated with heritable traits on which females base their choice of mate. Therefore, the almost universal explanation for gaudy male colouration is that it has evolved to attract females or, more generally, to increase mating opportunities. However, in a new paper, Tom Sherratt and Mark Forbes [1xSexual differences in coloration of coenagrionid damselflies (Odonata): a case of intraspecific aposematism?. Sherratt, T.N. and Forbes, M.R. Anim. Behav. 2001; 62: 653–660CrossRef | Scopus (20)See all References][1] present an interesting and illuminating counterexample to this paradigm.Coenagrionid damselflies exhibit sexual differences in colouration that are difficult to explain within the framework of female choosiness. Dorsally, the males are often much more brightly coloured than are the females, and sometimes have different patterning on their bodies. However, males neither defend territories nor engage in courtship displays; they simply rush at resting or slow-moving females in an attempt to mate. Because dorsal male colouration is never fully visible to females during copulation, it seems a very poor character on which female choosiness might be based.Unable to explain this colouration using sexual selection, Sherratt and Forbes suggest that sexual dimorphism here is driven by males seeking to avoid the costs of being mistaken for females by other males, and such signalling is probably beneficial to both parties. Male–male encounters are likely to cost the participants in risk of injury, energy expenditure and lost opportunity to pursue matings with females. The authors present a model that suggests that if males are selected to avoid harassment by other males, and females are selected to avoid excessive harassment by males, then males should evolve brighter colouration than the females. Crucially, in their paradigm, females want to avoid harassment, but males want to find females: thus, there is a sexual conflict. If males looked only slightly different from females, then there would be strong selection on females to mimic males. To achieve dimorphism, males have to adopt some phenotype that is less profitable for females to mimic – what better than an overtly conspicuous form?Antiharassment aposematism is an exciting and novel development that can be applied to a specific group. Further work is needed to test its generality and to examine its limits in species with slightly different mating systems. This is an entirely new avenue for considering the evolution of warning colouration, based on signalling unprofitability to conspecifics rather than to predators. No matter how general Sherratt and Forbes’ mechanism proves to be (and indeed the authors are cautious themselves), it is a timely reminder that not all signals that differ between genders are aimed at prospective mates. There is an essay to be written on why mechanisms based on attractiveness to the opposite gender are currently so popular in behavioural ecology, but this is not it!