Sexual dimorphism is commonly explained as a consequence of selection on traits that increase male attractiveness to females, or simply allow males greater access to females. Here, we consider another explanation for sexual differences in coloration within species of the damselfly family Coenagrionidae (Odonata: Zygoptera). In many of these species, males are more brightly coloured than females and have different patterns. Yet they are nonterritorial and do not engage in displays: indeed, male competition for mates often resembles a scramble. We therefore argue that even if females show a degree of mate choice, then it is unlikely to be based on colour or pattern. Instead, we suggest that sexual dimorphism has evolved in this group primarily as a form of sex-related warning coloration. First, we argue that it is almost inevitable that male-male interactions will incur a small cost to both participants. We then provide some evidence that males are capable of using colour as a clue to sexual identity. Using a simple model, we show that if these conditions hold, then sexual dimorphism will readily evolve. Furthermore, the model shows that if females are selected to avoid excessive harassment by males as is often suggested, then males should evolve much brighter coloration than females. If the assumptions underlying our 'unprofitable mate' model are broadly correct, then not only does it offer a novel explanation for sexual dimorphism, but it also provides the first case example of the evolution of aposematism as a result of intraspecific interactions.

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Journal Animal Behaviour
Sherratt, T, & Forbes, M. (2001). Sexual differences in coloration of Coenagrionid damselflies (Odonata): A case of intraspecific aposematism?. Animal Behaviour, 62(4), 653–660. doi:10.1006/anbe.2001.1789