Compensatory traits and the evolution of male ornaments

Abstract

How ornaments that are used during mate choice and rival assessment remain reliable has been a source of contention for many years. Such signals are hypothesized to be costly (i.e., 'handicaps'), but empirical studies testing for costs of sexually selected ornaments are equivocal at best, contradictory at worst. We review recent studies finding compensation for sexually selected ornaments, in both intra- and inter-specific studies, suggesting that other traits evolve to mitigate costs of ornaments. We synthesize these studies to elucidate the role of compensatory traits in the evolution of reliable ornaments and explain how selection to reduce ornament costs may influence aspects of the phenotype that are not subjected to direct sexual selection and may obscure our ability to directly measure ornament costs. Both intraspecific studies and comparative studies in a phylogenetic framework are important for our understanding of how the costs of signals may be reduced by compensation, but each approach answers different questions about ornament evolution. We also elaborate on a general theoretical model that can be useful when testing for costs of ornaments in correlational and experimental studies. We recommend that future investigators should consider compensatory traits when testing for ornament costs, especially when manipulating ornamentation.