Sexual differences in coloration of Coenagrionid damselflies (Odonata): a case of intraspecific aposematism?

Sexual dimorphism is commonly explained as a consequence of selection on tr aits that increase male attractiveness to females, or simply allow males gr eater access to females. Here, we consider another explanation for sexual d ifferences in coloration within species of the damselfly family Coenagrioni dae (Odonata: Zygoptera). In many of these species, males are more brightly coloured than females and have different patterns. Yet they are nonterrito rial and do not engage in displays: indeed, male competition for mates ofte n resembles a scramble. We therefore argue that even if females show a degr ee of mate choice, then it is unlikely to be based on colour or pattern. In stead, we suggest that sexual dimorphism has evolved in this group primaril y as a form of sex-related warning coloration. First, we argue that it is a lmost inevitable that male-male interactions will incur a small cost to bot h participants. We then provide some evidence that males are capable of usi ng colour as a clue to sexual identity. Using a simple model, we show that if these conditions hold, then sexual dimorphism will readily evolve. Furth ermore, the model shows that if females are selected to avoid excessive har assment by males as is often suggested, then males should evolve much brigh ter coloration than females. If the assumptions underlying our 'unprofitabl e mate' model are broadly correct, then not only does it offer a novel expl anation for sexual dimorphism, but it also provides the first case example of the evolution of aposematism as a result of intraspecific interactions. (C) 2001 The Association for the Study of Animal Behaviour.