Effects of sex-associated competitive asymmetries on foraging group structure and despotic distribution in Andean condors

Phenotype-limited interference models assume competitive asymmetries among conspecifics and unequal sharing of resources. Their main prediction is a c orrelation between dominance status and patch quality: dominant individuals should preferentially exploit better-quality habitats. We tested assumptio ns and predictions of the phenotype-limited interference model in Andean co ndors (Vultur gryphus), a New World vulture with strong sexual size dimorph ism (males are 30-40% heavier than females). We recorded searching birds in habitats differing in quality: mountains and plains. We also observed scav enging behaviour at 20 sheep carcasses, and videotaped 5 of them. Intraspec ific hierarchy at carcasses was based on size: males dominated females and, within each sex, older birds dominated younger ones. Adult males and juven ile females occupied extreme positions in the feeding hierarchy. Aggression was directed at those individuals belonging to lower hierarchical levels. In high-quality areas (mountains), more condors arrived at carcasses. Juven ile females were more often observed searching in low-quality areas (plains ), far from breeding areas and main roost sites. GLM analyses of individual behaviour showed that the hierarchy did not influence time of arrival, but low-ranking individuals spent more time at carcasses, especially if the nu mber of condors at arrival was high. Additionally, low-ranking condors spen t less time feeding at carcasses when individuals of higher hierarchical le vels were present. On the other hand, the number of condors present had a p ositive effect on feeding rates of dominant individuals;; probably because of a reduction in individual vigilance. These results support most of the a ssumptions :and predictions of the phenotype-limited distribution model, al though a spatial truncated distribution between phenotypes was not observed . Asymmetric feeding pay-off, unequal parental roles and sexual selection c onstraints could favour sexual divergence in body size in Andean condors.