Male mate choice selects for female coloration in a fish

Although sexual selection theory has proved successful in explaining a wide array of male ornaments, the function of ornaments occurring in females is largely unknown. Traditionally, female ornaments have been considered nonf unctional, being merely a genetically correlated response to selection for male ornamentation. However, this hypothesis is only relevant to species in which the ornament is basically the same in the two sexes. Alternatively, female ornaments may be influenced by selection acting directly on the fema les, either through female-female competition or male choice. We tested the latter hypothesis in mate-choice experiments with two-spotted gobies (Gobi usculus flavescens). In this small marine fish, females have bright yellow- orange bellies during the breeding season, a conspicuous trait that is not present in males. We conducted two aquarium experiments to test whether mal es preferred to mate with more colorful females. In the first experiment, m ales had a choice between two females that varied in natural coloration (an d belly roundness). In the second experiment, we manipulated belly colorati on and kept roundness constant. Males spent more time with colorful than wi th drab females in both experiments and also performed far more courtship d isplays toward colorful females. our study provides experimental evidence t hat males prefer ornamented females in a fish that is not sex-role reversed , supporting the hypothesis that female ornamentation is sexually selected.