The evolution of reversed sexual size dimorphism in hawks, falcons and owls:: A comparative study

Many hypotheses have been proposed to account for the origin and maintenance of reversed size dimorphism (RSD, females being larger than males) in hawks, falcons and owls, but no consensus has been reached. I performed comparative analyses, using both cross-taxa data and phylogenetically independent contrasts, to investigate potential correlates of reversed size dimorphism. Using a similar set of explanatory variables, covering morphology, life history and ecology, I tested whether any trait coevolved with size dimorphism in all three groups and hence provided a general explanation for the evolution of RSD. For hawks, strong correlates were found in the foraging-variable complex, so RSD might have evolved in species hunting large and agile prey. This is consistent with the intersexual-competition hypothesis (sexes have evolved different sizes to lessen intersexual competition for food), but especially the small-male hypothesis (males have evolved to be smaller to be more efficient foragers). Evolutionary pathway analyses suggest that RSD evolved most likely as a precursor of changes in hunting strategy but as a consequence of high reproduction. The falcons showed a similar pattern: species with strong RSD hunted larger and more agile prey. The evolutionary pathway analysis supported the idea that RSD evolved before the specialisation on more agile and/or larger prey. Finally for owls, the results showed clear parallels. RSD increased with prey size, consistent with the small-male hypothesis. Evolutionary pathway analysis suggests that RSD in owls has most likely evolved before specialisation on large prey, so a small and more agile male might be advantageous even when hunting small prey. These results suggest that RSD in hawks, falcons and owls evolved due to natural-selection pressures rather than sexual-selection pressures.