Why female birds of prey are larger than males

Abstract

The causes of the reversed sexual size dimorphism (RSD; females larger than males) in birds of prey are subject to a centuries-old, passionate debate. A crucial difficulty is to distinguish whether the postulated benefits derive from the proposed causal process(es) or are incidental. After reviewing the existing literature, we present a methodology that overcomes this difficulty and renders unnecessary any speculative a priori distinctions between evolved function and incidental effects. We can thus justify the following novel version of the well-known nest defence hypothesis as the most likely to explain the phenomenon in all birds of prey that show RSD: if the female predominates in actively defending the eggs and young against predators, then she is the heavier sex, and her relatively greater body mass is adaptive. That is, heavier females are favoured (independently of males) by natural selection. The attractiveness of this hypothesis is that it has the potential to explain the phenomenon in all raptors exhibiting RSD, can deal with the exceptional cases in this group, explains the direction of the dimorphism, focuses on a key factor in the reproductive success of most raptors, is parsimonious, i.e. does not require supporting hypotheses, and is supported by a substantial body of evidence.

Tri-axial accelerometry shows differences in energy expenditure and parental effort throughout the breeding season in long-lived raptors

“…This suggests that the female carries out almost all the incubation and egg turning as well as other tasks for chick-rearing. Moreover, in agreement with the reverse size dimorphism theory, the presence of the female in the nest would be favored to provide major protection to the nestlings due to its larger body size ( Schoenjahn et al 2020 ). Large raptors tend to take large prey, which cannot be swallowed by the nestlings and need to be processed by an adult ( Newton 1979 ; Cramp and Simmons 1980 ; Sonerud et al 2013 ).…”

“…In the case of birds, the majority of the species exhibit biparental care ( Lack 1968 ), with both sexes contributing to chick rearing ( Cockburn 2006 ). Nonetheless, the sort and amount of care effort could differ between sexes ( Webb et al 2010 ; Goymann et al 2016 ), some of them as a consequence of size dimorphism ( Schoenjahn et al 2020 ), age ( Møller and Nielsen 2014 ), and phylogenetic affiliation ( Cockburn 2006 ). This cooperation has a synergistic effect on the offspring’s biological fitness, particularly when there is sex-biased task specialization, leading the progeny to a better chance of survival ( Pilakouta et al 2018 ).…”

Substantial but Misunderstood Human Sexual Dimorphism Results Mainly From Sexual Selection on Males and Natural Selection on Females

“…For example, in raptorial birds the conflicting effects of larger size—increasing fecundity in the sex that lays the eggs, but compromising agility in the sex doing (more of) the hunting—has led to disruptive natural selection such that females are larger than males in many species. This females-larger pattern has evolved independently in the Accipitriformes, Falconiformes, and Strigiformes, groups now known to not be closely related ( Schoenjahn et al, 2020 ).…”

“…For example, natural (i.e., fecundity) selection can favor larger female size, notably in insects and fishes ( Perrone, 1978 ; Thornhill and Alcock, 1983 ; Andersson, 1994 ), as realized by Darwin (1871) . Likewise, divergent foraging niches can also produce sexually dimorphic body size, perhaps the best examples being raptorial birds where, again, females are consistently larger than males ( Newton, 1979 ; Andersson and Norberg, 1981 ; Schoenjahn et al, 2020 ).…”

Temporal patterns in prey size between sexes in a raptor with extreme size dimorphism: testing the intersexual competition hypothesis using web-sourced photographs

“…In many vertebrates, males are larger than females (Hedrick and Temeles 1989), a trait typically considered to be driven by sexual selection with larger males having a selective advantage when competing for mates (Andersson and Iwasa 1996). In raptors and several other predatory birds, size dimorphisms are reversed with females consistently being the larger sex (Andersson and Norberg 1981, Massemin et al 2000, Schoenjahn et al 2020.…”