When analysing how individuals allocate resources, sexual display should be
regarded as is any other life-history trait: patterns of allocation are ex
pected to be individually optimised. It thus follows that the costs of sexu
al selection cannot be studied by simply comparing unmanipulated individual
effort and some measure of cost. This rather trivial consequence of life-h
istory theory has received surprisingly little attention in studies of sexu
al selection despite the almost universal acceptance of the theory and the
fact that several papers have addressed the question specifically in the co
ntext of sexual selection. We therefore again outline why sexual displays a
re expected to be optimised at the level of the individual and why individu
als with high access to resources should generally display at higher levels
than individuals short of resources, We use some recent findings from stud
ies 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 fu
ture studies should focus on such mechanisms.