The cost of reproduction and sexual selection

When analysing how individuals allocate resources, sexual display should be regarded as is any other life-history trait: patterns of allocation are expected to be individually optimised. It thus follows that the costs of sexual selection cannot be studied by simply comparing unmanipulated individual effort and some measure of cost. This rather trivial consequence of life-history theory has received surprisingly little attention in studies of sexual selection despite the almost universal acceptance of the theory and the fact that several papers have addressed the question specifically in the context of sexual selection. We therefore again outline why sexual displays are: expected to be optimised at the level of the individual and why individuals with high access to resources should generally display at higher levels than individuals short of resources. We use some recent findings from studies of birds and spiders particularly relevant to the above questions that illustrate these principles. The examples we present show that the cost of sexual selection could be mediated in many ways and we thus suggest that future studies should focus on such mechanisms.