On Mating Systems and Sexual Selection

Abstract

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...