Size-Assortative Courtship and Competition in Two Embiotocid Fishes

Abstract

Males of two embiotocids (kelp perch, Brachyistiusfrenatus, and shiner perch, Cymatogaster aggregata) differed strikingly in mating behavior (degree of homogamy or size-matched courtship). Differences in male morphology also were consistent with differing degrees of competition for mates in the two species. Male shiner perch only courted females of their own size group. Small male kelp perch, however, courted large as well as small females, and large males courted large females preferentially. Intermale aggression was strongly size-assortative (matched by size) only in shiner perch. Several morphological data were consistent with the behavioral observations. Gonopodium-to-body length ratios were relatively high for small kelp perch, but less strongly influenced by body size in male shiner perch. Testes-to-body weight ratios increased with male size in both species, but greatly so only in shiner perch. Sperm competition often results in greater degrees of assortative mating, intermale aggression, and sperm production. The behavioral and morphological evidence thus suggest that sperm competition is much stronger in shiner perch. In kelp perch both size-assortative mating and intermale competition are relaxed. These findings provide independent, comparative tests of predictions that support Warner and Harlan's (1982) argument for intense sperm competition in dwarf perch, Micrometrus minimus. Observations are also consistent with an alternative hypothesis that homogamy occurs when males of potentially competing age-groups differ too greatly in size for small males to successfully court large females (Ridley, 1983). Additional data on sperm competition and homogamy in these and other small embiotocids are needed to further test these two hypotheses.