Intensity Of Sperm Competition Research Articles

Size-Assortative Courtship and Competition in Two Embiotocid Fishes

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.

Testes size, ejaculate quality and sperm competition in birds

The relationship between testes size, ejaculate quality (volume, sperm concentration, number of sperm per ejaculate) and sperm competition in birds was analysed using data collected in artificial insemination studies. I hypothesized that ejaculate quality, because of natural selection, should be superior in species with intense sperm competition compared with other species. In regression analyses, testes weight increased with body weight, with an exponent less than one, and ejaculate volume increased with testes weight with an exponent not significantly different from one, whereas sperm number per ejaculate increased with testes weight with an exponent larger than one. Species with relatively large testes also produced ejaculates with a high sperm concentration. Monogamous species with a relatively low intensity of sperm competition copulate rarely, but deliver ejaculates with a relatively large number of sperm. Monogamous species with a high intensity of sperm competition copulate frequently, but produce ejaculates with a relatively small number of sperm. Males of polygynous species, which also experience intense sperm competition, copulate rarely with specific females, but produce many ejaculates per male each with a relatively small number of sperm.