Sex-specific growth and effects of hatching condition in the reversed sexually size-dimorphic great skua

Increased environmental sensitivity of the larger sex has been reported in several size-dimorphic vertebrate species. It is generally assumed that this is due to a higher energy demand of the larger sex, thus increasing susceptibility to food shortage. However, sex-specific growth rates might act to reduce discrepancies in energy demands of differently sized chicks. To test these ideas, we compared growth of male and female chicks in a reversed size-dimorphic seabird, the great skua Stercorarius skua, for which a higher mortality rate of female chicks has been shown. We used chick growth data of mass, tarsus and wing from unmanipulated nests, as well as from an egg removal experiment in which chicks hatched from smaller eggs and in poor body condition. Experimental chicks were raised by either of two groups of parents with differing condition. We fitted logistic growth curves to the data using non-linear mixed models. In the experimental treatments growth was only impaired in the group which was raised by poor condition parents, which underlines the importance of parental quality for chick development. At the end of the nesting period, great skua chicks exhibited a comparable degree of size dimorphism as is found in adults, although neither sex had reached final adult size. Despite females reaching larger asymptotic values, timing of growth was not different between the sexes. However, we found a sex-specific effect of hatching condition: improved hatching condition correlated with faster growth of mass and tarsus in females, but did not affect male growth. The instantaneous growth rates suggest that females face higher energetic demands during growth, especially in the latter part of chick development when size-dimorphism is most apparent. However, vulnerability appears to be connected to circumstances during early development, a time at which the dimorphism is not yet apparent in terms of size or weight. We propose that nutritional constraints during early life negatively affect the development of structures or processes that are necessary to sustain high growth rates later. We found no evidence that growth rates are adapted in order to reduce differences in sex-specific offspring cost in this size dimorphic species.