Date Published: October 24, 2018
Publisher: Public Library of Science
Author(s): Nancy V. Marinero, Verónica B. Cailly-Arnulphi, Sergio A. Lambertucci, Carlos E. Borghi, Antoni Margalida.
Attributes such as sex, age and pigmentation of individuals could correspond to the competitive skills they use to access resources and, consequently, determine their social status when a hierarchy of dominance is established. We analysed patterns of social dominance in relation to sex, age and, for the first time, according to face pigmentation in a large scavenger bird species, the Andean condor (Vultur gryphus). This species displays extreme sexual dimorphism, with males being up to 50% heavier than females. Associated to this, strong hierarchical relationships characterize foraging, roosting and breeding. We recorded agonistic interactions within condor groups while foraging through video recordings in experimental stations. We corroborated a strong despotism by the adult males to the rest of the categories. More interestingly we found this despotism was also expressed by most pigmented birds; juvenile females being completely subordinated and, at the same time, not expressing pigmentation. Importantly, when condors of equal sex and age category fought, the more pigmented individuals were successful. Our results highlight that pigmentation, besides sex and age, is an attribute that also corresponds with social status in the Andean condor, making its hierarchical system more complex.
Feeding in groups is a foraging strategy that brings numerous benefits to group members, such as increasing the likelihood of locating food and enhancing vigilance for predator detection . However, this social behavior also increases the likelihood of individuals competing for food resources . Such competition involves great cost to those individuals engaging in fights because of the expenditure of time and energy involved and possible injuries .
This study was approved by the Comité de Bioética from the department of Biology of the Universidad Nacional de San Juan, Acta N°17, Exp. 02-3243-C.
A total of 468 interactions were recorded in Andean condor feeding groups, of which 282 were aggressions. The greatest number of interactions was initiated by adult males (41.03%), followed by adult females (32.05%), sub-adult males (10.26%), sub-adult females (7.05%) and, finally, by juvenile males and females (5.34% and 4.27%, respectively).
As expected, we found that agonistic interactions between condors feeding at carcasses were related to the sex and the age categories, but interestingly also to their pigmentation. Adult males are major aggressors when feeding since they displace all individuals within the group, dominating all other categories. Possibly, this is due to the sex-specific differences in body size that drive dominance hierarchies in accessing resources , since the male condor can be up to 50% heavier than females , . Sexual dimorphism in the Andean Condor is also responsible for partial temporal segregation between sexes to reduce agonistic encounters, which could lead to females programming routines that are not efficient in terms of energy, undertaking greater risks at feeding sites , . The pattern of hierarchies dominated by size has been well observed in other social foraging species with marked sexual dimorphism (e.g. , ). In turn, the dominance relationship is clearer in female than in male Andean condors, and is determined by age. In this sense, juvenile females are completely subordinated to the other female categories (and also by males in general), and are expected to have a lower food intake according to the phenotypic limitation model .