Structural complexity of hunting habitat and territoriality increase the reversed sexual size dimorphism in diurnal raptors

Abstract

Despite numerous efforts and many hypotheses to explain the selective pressures thatmay have favoured reversed sexual dimorphism (RSD) in raptors &- i.e. that the femaleis larger than the male &- some drivers of RSD are still unknown.Here we analyse how much variation in RSD is explained by hunting habitatstructure, territoriality or territory size. We do so using data on diurnal raptors from theNew World and the Western Palearctic &- i.e. Cathartidae, Pandionidae, Accipitridaeand Falconidae, the largest bird group showing RSD &- taking into account thephylogenetic relationships among species.Our results identify the type of the main prey as a major factor explaining RSD inraptors. We also found RSD to increase with increasing structural complexity in thehunting habitat from open or semi-open habitats to forest interior. RSD also increasedwith increasing degree of territoriality of the species (non-territorial < facultative< territorial). Finally, for territorial species RSD increased with increasing size ofnesting territory. A model comprising only three predictor variables (prey type, structuralcomplexity of hunting habitat and territoriality) explained up to 50% of thevariation in RSD of European and American diurnal raptor species, and up to 40% ofthe variation in RSD when only territorial species were considered.Our results highlight the relevance of spatial facets of the niche &- e.g. huntinghabitat, territoriality and territory size &- in exerting selective pressures on the body sizeof diurnal raptors. These selective pressures, joint with already known trophic factors&- e.g. diet &- are decisive for the evolution of the RSD, a key trait in the functionalecology of raptors. Our findings open up new perspectives in the study of sexual sizedivergence in birds.