Lekking and the evolution of sexual dimorphism in birds: comparative approaches

A recent comparative analysis based on out-group reconstructions of transitions in ancestral character states found no evolutionary association between lek mating and sexual dimorphism in birds. Reanalyzed by Felsenstein's method of matched pairwise independent comparisons, lekking birds are more sexually size dimorphic than their closely related nonlekking relatives, even when body size is controlled for. Felsenstein's pairwise method has the advantages that it can be used with continuous variables, it reduces the chance that confounding third variables are responsible for the relationship because only closely related taxonomic groups, which are likely to share similar ecologies and selective pressures, are compared, and it does not use ancestor and descendant character transitions, with the former being calculated from and therefore dependent on descendant states. Simulations show that, when the distribution of size dimorphism within the phylogeny is accounted for, size dimorphism evolves more often when birds lek. Using different comparative methods with real data will help us gauge the importance of the assumptions of these methods.