Inter‐group relationships influence territorial defence in mountain gorillas

Abstract

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Do we need to reclassify the social systems of gregarious apes?

“…The familiarity of interacting groups (i.e., whether they had split from a single group in the past) was the main determinants of peacefulness 69 . In a follow‐up study, Morrison and colleagues 70 refined this finding, showing that the familiarity effect applies only when interacting groups are within the periphery of their home ranges; encounters within core areas were consistently aggressive, irrespective of the groups' sociodemographic history. In an earlier study at the same site, 93% of close‐range encounters between groups were characterized by aggressive displays 71 .…”

Variation between species, populations, groups and individuals in the fitness consequences of out-group conflict

“…Here, resource context appears more important [ 143 ]; as another example of this type of variation, grey-cheeked mangabey, Lophocebus albigena , groups that had recently arrived at a location were more likely to approach playback of rivals than those who had been there longer, with site residency probably indicating the degree of short-term, local resource exploitation [ 144 ]. Rival identity is also known to affect the intensity of IGIs: for instance, mountain gorillas exhibit greater tolerance towards groups containing familiar or related individuals [ 142 , 145 ]; in pied babblers, conflicts with kin groups are shorter than those with non-kin rivals [ 62 ]. Moreover, for species that exhibit dear-enemy or nasty-neighbour relationships, interactions with groups representing the greater threat might be expected to be more intense [ 5 ].…”

“…Lastly, contest location is a key factor that can affect its intensity. Contests at the core of a territory, which is generally considered more valuable owing to plentiful resources [ 146 , 147 ], can be more intense than those in peripheral areas, as seen in mountain gorillas, black and white colobus, Colobus guereza and blue monkeys, Cercopithecus mitis [ 130 , 145 , 148 ].…”

“…For instance, while a group dominance hierarchy exists among neighbours in some species, including African lions, grey wolves, Verreaux's sifaka, Propithecus verreauxi and black and white colobus [ 44 , 154 , 158 , 159 ], we are only starting to understand how this may impact the level of out-group pressure experienced and its influence on variation in fitness consequences across groups [ 125 ]. In addition, it is becoming increasingly apparent that nuanced relationships can exist between neighbouring groups owing to other factors such as relatedness and familiarity [ 145 ]; it is important to capture these within the ‘intergroup dominance hypothesis' [ 160 ]. Information on the relative strengths of different groups potentially also extends beyond near neighbours.…”

Social groups buffer maternal loss in mountain gorillas

“…Females that disperse from their natal group tend to do so earlier (mean age of 7.9 years [ Robbins et al, 2009a ]) than males (mean age of 15.3 years [ Stoinski et al, 2009a ]) and therefore have a shorter period of potential maternal investment. The complexity of gorilla social structure with numerous types of differentiated social relationship both within and among groups ( Morrison et al, 2019 ; Mirville et al, 2018 ; Morrison et al, 2020a ; Morrison et al, 2020b ) suggests that detrimental long-term effects on individual gorillas’ social environments could have particularly negative fitness consequences. However, these stable, cohesive, social groups also have the potential to provide a social buffer to the negative consequences of maternal loss.…”