WESTERN LOWLAND GORILLA POPULATION 2015 DRIVERS

Nevertheless, bais are sites where gorillas spend just 1% of their time and not all groups have access to them. WLG groups have just one silverback, in contrast with the frequent multi-silverback groups of mountain gorillas, where more than 15% of the infants are not sired by the dominant male. Also, group takeovers by outside males do not occur in WLG as opposed to mountain gorillas. Concordantly, infanticide is frequently observed in mountain gorillas, while it has never been reported in WLG. While mountain gorilla group interactions are frequently aggressive, WLG groups interact non-aggressively. These observations suggest that one of the most striking differences between the two gorilla species is in their social behaviour. For this reason, most of the information of social interactions in WLG have been gathered in bais, which are easily monitored but rare swampy clearings in the forest where groups commingle while feeding on grasses rich in salts. This bias is due to the higher mobility and lower observability of WLG, impairing simultaneous monitoring of multiple groups. beringei beringei), the structure and dynamics of social groups in WLG are poorly understood. Ĭompared to the better-studied mountain gorilla ( G. This great ape from the lowland forests and swamps of western central Africa (see electronic supplementary material, figure S1) lives in groups generally consisting of one fully mature male (silverback) and several adult females with their offspring, or in non-breeding groups. The global population of this primate, recently estimated at about 360 000 individuals, has suffered a dramatic decline mainly due to massive die-offs caused by Ebolavirus outbreaks, and forecasts predict further sharp declines. Western lowland gorillas (WLG Gorilla gorilla gorilla) offer the possibility of studying the potentially complex social structure in a great ape in areas with minimal human impact. However, highly dynamic social structures can make the interpretation of social processes and their evolutionary significance a challenging task. Information on social structure is highly relevant in wildlife ecology, conservation, and management. Social structure and behaviour are adaptive responses to environmental pressures, and flexibility in social organization may facilitate reactions to changing environmental conditions. These lead to social structures above the group level with varying degrees of complexity and dynamism.

WESTERN LOWLAND GORILLA POPULATION 2015 DRIVERS

In this context, temporal demographic changes and dispersal are the major drivers of variability in social group size, but are complemented with short-term segregation/aggregation events and intergroup interactions. Understanding the processes driving the structure of animal societies is a non-trivial exercise which requires disentangling stable social networks from dynamic spatio-temporal patterns. Such extended sociality can promote the sharing of behavioural and cultural traits, but might also increase the susceptibility of western lowland gorillas to infectious diseases that have decimated their populations in recent times. This resulted in a community composed of groups that interacted frequently and not-aggressively, contrasting with the more fragmented and aggressive mountain gorilla ( G. Both approaches revealed a social system much more dynamic than anticipated, with non-aggressive intergroup encounters that involved social play by immature individuals, exchanges of members between groups likely modulated by kinship, and absence of infanticide evidenced by infants not fathered by the silverback of the group where they were found. We assessed the rate of intergroup encounters for three western lowland gorilla ( Gorilla gorilla gorilla) groups with daily observations over 5 years, and non-invasively genotyped a larger population over four months.

Thus, sporadic intergroup encounters and individual forays play a central role in defining the dynamics of populations in social species. Social networks are the result of interactions between individuals at different temporal scales.

Western lowland gorilla population 2015