Evolution of Reversed Sexual Size Dimorphism in Skuas and Jaegers

Abstract

Skuas and jaegers (Stercorariinae), like other birds with a raptorial lifestyle, display reversed sexual dimorphism in size (RSD) in which females are larger than males. This condition is commonly believed to be associated with the morphology or behavioral ecology of avian predators, and numerous theoretical explanations have been presented to account for its evolution. Most tests of these ideas have been theoretical or based on com- parative analyses. We present tests of specific hypotheses of RSD based on field studies of Great Skuas (Catharacta skua) and Parasitic Jaegers (Stercorarius parasiticus) breeding in Shet- land. We also present a comparative analysis of foraging behavior and degree of RSD within the Stercorariinae. We found no support for the popular theories that claim that RSD evolved as a result of specialized roles during breeding. Large females were not more fecund, nor were they the main defenders of the nest. Male body size was not correlated with any breed- ing parameter. In addition, interspecific comparisons did not support the idea that the de- gree of RSD is related to foraging behavior during the breeding season (as opposed to mor- phology). Female dominance within pairs did not affect any breeding parameters. Support was found for theories that RSD is linked to sexual selection. Large females had an advantage when pairing with older males, which could be a result of intrasexual competition for mates. There was assortative mating for size, indicating that small males could also have an advan- tage in pairing, because large males seem to be rejected by small females. Given the disparate results of several previous studies, we conclude that a large body of empirical evidence will be needed before we can draw firm conclusions about the main selective factors that act to maintain current levels of RSD in natural populations of raptorial birds. Received 24 July 1997,