Sex-specific behavior by a monomorphic seabird represents risk partitioning

The presence of sex-stereotyped behavior in monomorphic animals, where there are no sexual differences in form to account for sexual differences in function, is often attributed to intraspecific competition or to differential parental investment. The possibility that the use of different behavioral strategies by each parent may increase reproductive success for both partners through risk partitioning is seldom considered. We studied thick-billed murres (Uria lomvia), where the male exclusively feeds the offspring during the late chick rearing. During the period of biparental care, males fed on "risk-averse" prey (consistent across time and space; unitized risk = 0.29), whereas females fed on "risk-prone" prey (risk = 0.59). Males fed at night at 1 colony, during the day at 2 colonies, and there was no pattern at another colony. We suggest that these differences reflect the availability of risk-prone prey. Modeling suggested that mixed-risk pairs had higher success than "risky" or "riskless" pairs. Males accumulated reserves and reduced chick provisioning just prior to fledging. Thus, sex-specific patterns at 1 period (male-only care during postfledging) may have led to sex-specific patterns at earlier periods through the need for specialization in foraging habits and risk. We propose that risk partitioning may contribute to the prevalence of sex-specific behaviors in monomorphic animals and that patterns are likely context specific rather than species specific. Copyright 2010, Oxford University Press.