On Mating Systems and Sexual Selection

Certain aspects of the relationships of mating systems, sexual selection, sex ratios, rates of maturation in males, and behavior in grackles and other icterids have been considered. In the promiscuous grackles of the genus Quiscalus, the nestling sex ratio is balanced but differential mortality in the sexes following the nestling stage produces a marked imbalance in the tertiary sex ratio in favor of females. Further imbalance in the sex ratio at the breeding colonies results from the failure of first-year males to breed. Imbalances in the tertiary sex ratio of the population and in the sex ratio at breeding colonies are viewed as consequences of the promiscuous mating system rather than as causal factors promoting promiscuity. An explanation for the failure of first-year males of polygynous and promiscuous species to participate in the breeding effort of the population is sought in terms of individual rather than group selection. Because the probability of a young male obtaining mates in competition with older, experienced males is very low, males which postpone breeding may ultimately have greater total reproduction; and through selection first-year males have become adaptively "handicapped" physiologically and morphologically in competition with adult males for mates. The fact that sympatric hybridization is more frequent among non-monogamous species than among those having a monogamous mating system is attributed in part to the higher sex "drive" and the lower thresholds of response of males to feminine releasers, which arise through strong sexual selection in polygamous and promiscuous mating systems. It is also suggested that the heightened aggressive tendencies of non-mono gamous males place a premium on feminine characters which minimize the chances of the female eliciting aggressive responses on the part of the male at the time of mating.