revisiting niche divergence hypothesis in dimorphic birds: is diet overlap correlated with sexual size dimorphism?

The evolution of sexual size dimorphism (SSD) is a long-standing topic in evolutionary biology, but there is little agreement on the extent to which SSD is driven by the different selective forces. While sexual selection and fecundity selection have traditionally been proposed as the two leading hypotheses, SSD may also result from natural selection through mechanisms such as sexual niche divergence, which might have reduced resource competition between sexes. Here, we revisited the niche divergence hypothesis by testing the relationship between the sexual overlap in diet and SSD of 56 bird species using phylogenetic comparative analyses. We then assessed how SSD variation relates to the three main hypotheses: sexual selection, fecundity selection, and sexual niche divergence using phylogenetic generalized least squares (PGLS). Then, we compared sexual selection, fecundity selection, and niche divergence selection as SSD drivers through phylogenetic confirmatory path analyses to disentangle the possible causal evolutionary relationships between SSD and the three hypotheses. Phylogenetic generalized least squares showed that SSD was negatively correlated with diet overlap, i. e., the greater the difference in body size between males and females, the less diet overlap. As predicted by sexual selection theory, the difference in body size between sexes was higher in polygynous species. Confirmatory phylogenetic path analyses suggested that the most likely evolutionary path might include mating system as a main driver in SSD and niche divergence as a result of SSD. We found no evidence of a role of fecundity selection in the evolution of female-biased SSD. Our study provides evidence that sexual selection has likely been the main cause of SSD and that dietary divergence is likely an indirect effect of SSD.

Methods
The database of diet overlap was compiled through a literature survey of all major ornithological journals. The ISI Web of Knowledge was systematically searched for original case studies in which data on the diet composition of males and females were provided. A total of 289 references were examined, 62 of which provided data on the diet of both sexes for 58 species belonging to 32 families. We discarded studies based on prey types brought to the nest by adults (n=8) because food provisioning to chicks does not represent the actual nutritional requirements of males and females, or those based on isotope analyses (n=10), since both sexes might show overlapping isotopic niches, while having a different trophic niche, for instance, if both sexes feed on different prey with similar δ15N values (Bearhop et al., 2004). Diet composition was described using stomachs, esophageal contents, and regurgitations (80.3% of the studies), feces (13.1%), and direct observation (6.6%).

The description of the mating system, male display type, and clutch size were obtained from the Handbook of the birds of the world (del Hoyo et al., 1992), Lislevand et al. (2007) and BoW (Billerman et al., 2022).