Ja. Donazar et al., Effects of sex-associated competitive asymmetries on foraging group structure and despotic distribution in Andean condors, BEHAV ECO S, 45(1), 1999, pp. 55-65
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.