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 n o consensus has emerged. We adopted a phylogenetic approach by mapping size ratios onto Sibley et aI.'s phylogeny for the Ciconiiformes, a d iverse 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 t hat includes the jacanas and phalaropes. Among diurnal raptors, RSD is likely to represent the ancestral condition, but its magnitude depend s on feeding habit. It is most extreme in taxa that feed on agile, avi an prey (Falconidae and some Accipitridae), and has been secondarily l ost in taxa that feed on slow-moving prey or carrion (Sagittariidae an d Old-world vultures within the Accipitridae). The importance of forag ing mode is further suggested by the case of the Hawaiian hawk (Buteo solitarius), a highly dimorphic species that was probably restricted t o agile, avian prey upon colonization of Hawaii. Comparison of the Haw aiian 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 constra ints that set a lower limit on female size.