Long-term dominance relationships in female mountain gorillas: strength, stability and determinants of rank

Abstract

A common practice in studies of social animals is to rank individuals according to dominance status, which has been shown to influence access to limited resources and stability of social relationships, and may in turn correlate with reproductive success. According to the socioecological model for primates, most female dominance relationships are either nepotistic or virtually undetectable (egalitarian), with nepotistic species being philopatric, and dispersing females being egalitarian. Female mountain gorillas (Gorilla beringei beringei) disperse, and they have been characterized as being egalitarian, but previous studies have not examined their dominance relationships from a long-term perspective. We evaluated 15 matrices of displacement/supplantation interactions that spanned 30 years of observations in the Virunga Volcanoes region, and included 51 female mountain gorillas in six groups. Only 4% of displacements were directed against higher ranking females, and when matrices had less than 5% unknown dyads, linearity indices were consistently greater than 0.95. Therefore, previous results suggesting undetectable dominance relationships may have reflected an insufficient quantity of data for this species, rather than actual nonlinearity in its hierarchies. Dominance depended on age and group tenure rather than nepotism, yet some females maintained a high ranking for most of adulthood (15-25 years). Most rank shifts occurred through changes in group composition, rather than switches in established relationships. These results fit within growing evidence for linear individualistic hierarchies in some primates, often coupled with dispersal, as commonly found in ungulates. In light of these results, we propose that the dominance relationships of female mountain gorilla are best characterized as 'Dispersal-Individualistic' instead of the previously suggested 'Dispersal-Egalitarian'.

The lost dream of ecological determinism: Time to say goodbye? … Or a White Queen's proposal?

“…For example, linear hierarchies, which are predicted for despotic societies, have been taken to indicate contest competition. 43,44 Similarly, frequent agonism has been considered evidence of contest competition, while infrequent agonism has been considered evidence of the absence thereof. 26,27,45,46 This practice might be justified under certain narrow conditions, particularly if ''contest competition'' is defined as ''frequent agonism'' or if frequency of agonism differs by several orders of magnitude between populations, possibly indicating different forms of competition.…”

“…57 In the case of female dispersal, which is affected by various ultimate and proximate factors, it seems to us that the model was trying to bite off more than it could chew. Consequently, it might be time to drop this factor (see also Robbins and coworkers 44 ) and to restrict the predicted social consequence to social relationships.…”

Individualistic female dominance hierarchies with varying strength in a highly folivorous population of black-and-white colobus

“…However, recent studies of Gorilla beringei beringei (mountain gorilla) have documented relatively strong, linear female dominance hierarchies despite them having a highly folivorous diet that generates relatively low levels of within-group contest competition (Robbins et al, 2005(Robbins et al, , 2007. To investigate if this pattern holds true for other highly folivorous species that may experience low levels of contest competition, we examined the linearity and strength of female dominance hierarchies in a population of Colobus vellerosus (ursine colobus or white-thighed colobus) at Boabeng-Fiema, Ghana.…”

“…Some colobines, gorillas and elephants that eat mostly leaves or grass were thought to fit this pattern because of low rates of agonistic interactions. However, detailed, long-term observations show that these species form stable and/or consistent female dominance hierarchies (e.g., Semnopithecus entellus: Koenig, 2000; Trachypithecus phayrei: Koenig et al, 2004; Gorilla beringei beringei: Robbins et al, 2005; Loxodonta africana: Archie et al, 2006). Many of these species occasionally forage on high-quality food items, which may only be available seasonally (reviewed in Snaith & Chapman, 2007).…”

“…(Appleby, 1983) and highly linear dominance hierarchies characterize despotic rather than egalitarian species (van Schaik, 1989;. However, apparent non-linear hierarchies are often an artefact of insufficient data, because the indices that measure linearity are sensitive to the presence of dyads without any recorded interactions (Galimberti et al, 2003;Robbins et al, 2005;Koenig & Borries, 2006). Therefore, a high hierarchical strength (i.e., how strictly enforced the dominance hierarchy is) may be a more informative indicator of despotic relationships.…”