Dimorphic male brains and alternative reproductive tactics in a vocalizing fish.

Brains and types of behavior among sexually mature vertebrates are portrayed as having two phenotypic states: male and female. However, mating systems and the behavioral tactics employed by the two sexes are far more diverse than conveyed by this simple intersexual dichotomy. For example, in many species, sexually mature males may practice one of two alternative mating tactics. Recent studies of a sound-producing teleost fish now show that intrasexual dimorphism of vocal motor phenotypes among males accompanies the intrasexual divergence of types of reproductive behavior. Thus, one group of males, like females and juveniles, lacks the behavioral, neurophysiological and morphological traits typical of the sexually differentiated vocal motor pathway of the second group. The results are an explicit demonstration that for any one species: (1) alternative mating tactics can be paralleled by alternative phenotypes for the neurons that determine tactic-specific types of behavior, and (2) reproductive maturation is not obligatorily linked to the expression of neuronal secondary sex characteristics.