4 - Mating systems

Abstract

The vast majority of birds are socially monogamous, where one male and female form a pair bond and raise the young together. One of the most exciting recent discoveries is that many of these species are not actually genetically monogamous. The extra-pair mating system of socially monogamous birds has many similarities to leks because sexual selection is strong and females obtain only sperm through copulations with extra-pair partners. The evolution of extra-pair mating systems is not favored under conditions of low breeding synchrony. Because of low levels of extra-pair behavior, selection does not favor high levels of testosterone. Mating systems are intricately tied to sex roles in parental care and territory defense. Sex roles and sexual selection are tied in a feedback loop, reinforcing each other. When sex roles are highly divergent, selection pressures are divergent, and one sees sexually selected traits and behaviors in males but not in females. Sex roles are diverse and variable in tropical birds but are more often similar than in the case with temperate passerines. Sexual selection in monogamous birds can also arise when individuals, because of the high quality of their mates, increase their parental effort. This “differential allocation” is thought to be an adaptation by females that depend on mate quality. Although social monogamy predominates in tropical and temperate birds, tropical birds are well-known for their promiscuous mating systems. Cooperating breeding lies at the other extreme of sociality from promiscuous species.