A performance based approach to distinguish indices from handicaps in sexual selection studies

1. Central to theories of the evolution of conspicuous secondary sexual traits is that sexual traits honestly reflect at least one aspect of an individual's quality. In general, the reliability of sexual traits can be explained in two ways. Either sexual traits are reliable indicators of individual quality because they are costly and the costs in terms of survival are higher for an individual of low quality as compared to an individual of high quality (i.e. handicap principle) or sexual traits are reliable because of physical constraints and thus cost-free (reliability by design). 2. The distinction between indices and handicaps thus centres on whether the sexual signal under consideration entails costs in terms of survival for its bearer. Although measuring survival directly is still the most accurate way to determine whether this is indeed the case, it is often practically impossible. We propose to introduce performance measurements because one may detect potential costs at the whole-organismal level by quantifying performance traits relevant in terms of survival and reproductive success. 3. We present some examples of sexual signals that can be classified as indices or handicaps by using our performance-based approach. We also discuss some well-cited examples which could fit the model of a handicap and propose that measurements of performance could be a feasible test of this idea.