Sexual-Selection Models for Exaggerated Traits are Useful but Constraining

Abstract

SYNOPSIS. Science is driven by productive hypotheses and technology, but these may sometimes limit the questions posed. For instance, Fisherian runaway sexual selection and related hypotheses have helped us understand the evolution of exaggerated visual sexual dimorphism. Species with indistinguishable sexes, however, may use different behavioral mechanisms when pairing and thus possess different adaptations. In the monomorphic Midas cichlid ( Amphilophus citrinellum ), females chose large aggressive males in a restrained situation, as sexual selection predicts, but males did not choose. The nuchal hump of males swells coincidently with pair formation. However, overly large humps were shunned by females while the normal— size hump facilitated sex recognition. This species is polychromatic, and pairs mate assortatively by color in Nicaragua. Some have suggested the Midas cichlid might therefore show how sexual selection produces explosive speciation of cichlids in Africa. All females, however, are biased toward normal—color males. The color of gold morphs modulates aggressive responses of the other fish. All else equal, the benefit to gold in a fight equals 15% more weight than the opponent. Pair formation succeeds best when the typically smaller female of a pair is relatively more aggressive than the male. The pair combination, gold male with normal female, is difficult to produce; making the female the same size as the male removes the disability. Pair formation is a negotiated process in which the male tests the aggressiveness of the female relative to self. That puts the behavioral mechanisms of the male and female in conflict.