Sexual size dimorphism in birds

Abstract

Birds provide excellent model organisms for testing functional explanations of sexual size dimorphism (SSD), since many species are exceptionally well-studied in nature. This chapter reviews four major functional hypotheses of SSD, and tests these using data on five morphometric traits from over 2,500 bird species. This comprehensive analysis reveals that SSD is male-biased in most avian species and families, and that allometry consistent with Rensch's rule occurs in significantly more avian families than expected by chance. Using cross-species analyses, the chapter shows that the pattern of SSD is most consistent with patterns of sexual selection, specifically with the Mating competition and the Display agility hypotheses. Sexual selection, however, is unlikely to explain all variation in SSD, and further work is essential on ecological use of resources and fecundity selection.

The evolution of sexual dimorphism in New Zealand giant moa (Dinornis) and other ratites

“…Conversely, female-based intrasexual competition and dominance in intraspecific interactions (e.g. selection for fecundity, offspring investment and/or defence of breeding territory) have been invoked to explain RSD in mammals and birds [1,7,28]. These hypotheses are consistent with past selective pressures on New Zealand's K-selected avifauna, which evolved in the absence of mammalian predators; many New Zealand birds are not only flightless and largebodied, but also have low reproductive rates, prolonged immaturity and high investment in individual young [14,54].…”

“…females are more than twice the size of males), whereas the maximum SDI value for jacanas is ca 0.8. This level of sexual dimorphism is almost as great as the highest ratio of male-biased size dimorphism, shown by great bustard Otis tarda (SDI ¼ 1.48 or higher; [1,4,6,12,13]). …”

“…Many different forms of sexual dimorphism are displayed among birds [1], and attempts to explain complex patterns of phylogenetic variation in avian dimorphism have been the subject of considerable debate ever since Darwin [2]. Together with plumage dichromatism, the most obvious difference between males and females of many bird species is sexual body size dimorphism (SSD).…”

Comparative Tests of Birds Support a Link between Sex-Biased Dispersal and Body Size

“…The log-transformed mass ratio character is a measure of a species' female-to-male SSD with values of <0, >0, and 0 corresponding to male-biased SSD, female-biased SSD, and sexual size monomorphism, respectively (cf. [16]). Thus, these negative associations indicate that species with greater male-biased SSD are more likely to have malebiased dispersal.…”

“…If distance and mass for females co-vary in a manner different from males, then this discrepancy may also lead to a relationship between the sex-biased dispersal and body sizes of their species. Such interrelationships may be especially likely for birds as they exhibit a sexual size dimorphism (SSD), whereby males are usually larger than females [16]. Birds also follow Rensch's [17] rule, whereby their male-biased SSD increases with the body size of the species.…”