Layered sexual selection: a comparative analysis of sexual behaviour within an assemblage of piophilid files

Although many theoretical and empirical studies have addressed the dynamics of sexual selection, little is known about the evolution of multiple sexual-selection mechanisms within the same system. I performed a qualitative comparative study of sexual behaviours in seven sympatric species of piophilid flies to identify and compare the apparent mechanisms of sexual selection operating in each system. In each of the seven species I observed several distinct types of male-male and male-female interactions, potentially representing multiple mechanisms of sexual selection. Male-male interactions included scramble competition and, in some species, bouts of intense combat. Male-female interactions exhibited two distinct patterns: (1) some form of premounting courtship followed by a simple copulatory sequence or (2) no premounting courtship but palpation behaviours during the copulatory sequence. Either pattern was combined with male-female struggle in some species. In two species, male mating success also appeared to depend on the ability to overcome a "revealing obstacle", a female adaptation that exposed variation in male performance without direct assessment or struggle. In each species these mechanisms operated in a rough sequence and thus could be viewed as "layers" of sexual selection, with each layer potentially reducing the subset of individuals that have opportunities to compete in the next layer. A brief review of the literature suggests that layered sexual selection is the typical pattern in many animal groups and thus may have important evolutionary consequences.