Male choice and competition in Tetraopes tetraophthalmus: effects of local sex ratio variation

Abstract

Male milkweed beetles (Tetraopes tetraophthalmus) are shown to exhibit choice between potential mates. Males, however, also exhibit direct competition for access to potential mates. These differences in male behavior are dependent on the local sex ratio, with males becoming increasingly choosy when the sex ratio is female-biased, and more competitive when the sex ratio is malebiased. Females show neither choice nor competition. In both the laboratory and in the field, body size is important in determining patterns of competition and choice. Larger males win most aggressive encounters, and males generally prefer large females as mates. The combination in a single sex, of competition for mates and mate choice is not predicted by classical sexual selection theory. Male choice and competition may have evolved in T. tetraophthalmus due to the combination of (1) the mating system (2) the pattern of variation in female fecundity, and (3) the spatially and temporally variable operational sex ratios. This explanation is not dependent on high heritability of body size.