Selection on body size in a raptor with pronounced reversed sexual size dimorphism: are bigger females better?

Abstract

An overabundance of hypotheses have been proposed to account for reversed sexual size dimorphism (RSD; females the larger sex) in raptors. Previous research principally focused on examining interspecific patterns of RSD, rarely testing predictions of various hypotheses within populations. To redress this, we used data from both sexes of a large brown falcon, Falco berigora, population to evaluate the importance of size and body condition indices on the hunting prowess of males and the reproductive success, recruitment, and survival probabilities of both sexes. Female-female competition for territorial vacancies was likely to be intense as the floating population was female-biased and intrasexual agonistic interactions were frequently observed. In this competitive population, larger adult females were more likely to be recruited, indicating directional selection favoring increased female body size. Furthermore, after recruitment larger females were more likely to successfully fledge offspring, providing a mechanism by which RSD is maintained in the population. In contrast, male recruitment was unrelated to either body size or condition indices. Smaller immature males more often held their territories (survived) over two breeding seasons than did their larger counterparts; however, they also took small prey more frequently, a diet related to poor reproductive success. We argue that, together, these results are indicative of selection favoring an increase in female body size and a reduction or maintenance in male body size. Of all the hypotheses proposed to account for the maintenance and evolution of RSD in raptors, this scenario is consistent only with the predictions of the intrasexual competition hypothesis.