Foraging segregation in tropical and polar seabirds: Testing the Intersexual Competition Hypothesis

Abstract

The Intersexual Competition Hypothesis (ICH) predicts that sexual size dimorphism (SSD) in seabirds may reduce intraspecific food competition through the exploitation of different trophic niches by each sex. We tested the ICH using stable isotopes (δ15N and δ13C) from whole blood and muscle from six tropical and five polar seabird species sampled at breeding sites. We expected that greater morphological differences between sexes would be related to larger differences in δ15N and δ13C values, reflecting potential diet and spatial segregation between males and females. We also compared trophic segregation in non-tropical and tropical seabirds to determine if there was more intense feeding competition during the generally shorter breeding season in non-tropical areas, leading to more pronounced segregation mechanisms; alternatively, more abundant food resources during breeding at temperate and polar areas, in contrast to oligotrophic tropical areas, could lead to a relaxing of segregation. No significant differences in δ15N or δ13C were found between sexes in seabird species from tropical or polar regions. In addition, there was no correlation between total dimorphism index and differences in mean δ15N or δ13C values of females and males for each species. Analysis of data from the literature, accounting for phylogeny, indicated that size-dimorphic seabird species from temperate and polar regions tend to show trophic (δ15N) or spatial (δ13C) segregation (71%; 30 out of 42 study cases) more often than tropical dimorphic species (19%; 3 out of 16 study cases). Overall, SSD seems to facilitate trophic or spatial segregation in non-tropical seabirds, but not in tropical species. Further investigations are necessary to confirm the lack of this pattern in tropical seabirds.