A PHYLOGENETIC APPROACH TO REVERSED SIZE DIMORPHISM IN DIURNAL RAPTORS

Several adaptive hypotheses have been proposed to account for reversed size dimorphism (females larger than males) in diurnal raptors, but no consensus has emerged. We adopted a phylogenetic approach by mapping size ratios onto Sibley et aI.'s phylogeny for the Ciconiiformes, a diverse order that includes diurnal raptors. Reversed size dimorphism (RSD) has apparently evolved at least five times in the Ciconiiformes: four times among aerial-pursuit predators (the diurnal raptors, skuas, boobies, and frigatebirds), and once in a largely polyandrous group that includes the jacanas and phalaropes. Among diurnal raptors, RSD is likely to represent the ancestral condition, but its magnitude depends on feeding habit. It is most extreme in taxa that feed on agile, avian prey (Falconidae and some Accipitridae), and has been secondarily lost in taxa that feed on slow-moving prey or carrion (Sagittariidae and Old-world vultures within the Accipitridae). The importance of foraging mode is further suggested by the case of the Hawaiian hawk (Buteo solitarius), a highly dimorphic species that was probably restricted to agile, avian prey upon colonization of Hawaii. Comparison of the Hawaiian hawk with its putative, continental ancestor suggests that bath males and females became smaller on Hawaii, but the reduction in male size was greater. This differential size reduction is not explained by an intersexual allometry among Buteo species, but may reflect constraints that set a lower limit on female size.