Sex Differences in Parental Investment and Chick Growth in Wandering Albatrosses: Fitness Consequences

Abstract

Parents should adjust expenditure on parental care so as to maximize fitness, but quantitative data are sparse, particularly for sexually dimorphic species. We use data from two breeding seasons to investigate the fitness consequences of variation in parent provisioning behavior and chick growth in a sexually dimorphic seabird, the Wandering Albatross (Diomedea exulans). Male parents brought more food to their single offspring than females did, and male chicks (sons) received more food than female chicks (daughters). The study of growth parameters indicates that sons had faster growth rates, reached higher asymptotic mass, and were heavier and larger at fledging than daughters. Male chicks that survived to adult age were larger than those that disappeared, whereas the females that survived were those in better condition at fledging, indicating that different factors affect survival of each sex during the first years of independence at sea. Survival to adult age seems to be influenced by the ability of parents to provision the offspring adequately, especially in the case of male chicks, whose costs appear to be higher. The age and experience of parents did not influence the amount of food delivered to the chick, but older birds rearing male chicks were more synchronized on a within-pair basis than younger parents, and consequently their sons grew faster, attaining both higher asymptotic mass and higher mass at fledging. Old adult males appeared to have a higher mortality rate than younger males when rearing a son. There was no such tendency for adult males rearing a daughter or for females rearing a son. Younger, less experienced pairs may tend to produce more sons than daughters when compared to older and more experienced pairs. If valid, this tendency for an age-related sex ratio could be the result of higher costs of raising the more expensive sex. These findings indicate that the optimal age-related investment by parents varies between males and females but also depends to a large degree on whether they are rearing a son or a daughter.