Long‐term group membership and dynamics in a wild western lowland gorilla population (Gorilla gorilla gorilla) inferred using non‐invasive genetics

Abstract

The social organization of a group-living animal is defined by a balance between group dynamic events such as group formation, group dissolution, and dispersal events and group stability in membership and over time. Understanding these processes, which are relevant for questions ranging from disease transmission patterns to the evolution of polygyny, requires long-term monitoring of multiple social units over time. Because all great ape species are long-lived and elusive, the number of studies on these key aspects of social organization are limited, especially for western lowland gorillas (Gorilla gorilla gorilla). In this study, we used non-invasive genetic samples collected within an approximately 100 km area of Loango National Park, Gabon to reconstruct group compositions and changes in composition over more than a decade. We identified 98 gorillas and 11 mixed sex groups sampled during 2014-2017. Using published data from 85 individuals and 12 groups surveyed between 2005 and 2009 at the same locality, we tracked groups and individuals back in time. The identification of 11 silverbacks via parentage analyses and the genetic tracking of 39 individuals across studies allowed us to infer six group formations, five group dissolutions, and 40 dispersal events within 12 years. We also observed four groups persisting across the sampling periods with a maximum inferred existence of nearly 17 years and exhibiting variation in membership stability. Our results highlight the variation in composition and stability among groups of western lowland gorillas and illustrate the power of non-invasive genetic sampling for long-term monitoring.

Variation in the social organization of gorillas: Life history and socioecological perspectives

“…Such research could be done in conjunction with studies on how the lifetime reproductive success of males is influenced by dispersal patterns of males and females, routes to attaining alpha status, group transitions (group formations, disintegrations, and transitions between one‐male and multimale organization via usurpation or inheritance), male lifespan, body size, and causes of mortality. Determining the fate of males that disperse to become solitary is challenging, because of the difficulties of following single individuals, but studies conducted at open clearings, such as Mbeli Bai as well as genetic tracking studies that cover several years provide a means for monitoring solitary individuals . Genetic studies also enable us to examine group transitions, individual trajectories, and reproductive success of males on a larger spatial scale than what is typically possible with habituated groups.…”

“…An evolutionary disequilibrium might be the latest cycle in a longer-term pattern of fluctuating selection, or it could represent the first time that the mountain gorillas are diverging from the one-male organization of western gorillas. 85,86,89,192 Genetic studies also enable us to examine group transitions, individual trajectories, and reproductive success of males on a larger spatial scale than what is typically possible with habituated groups. Modeling can also be a fruitful approach to understanding the relationships between life history characteristics and reproductive success in social species that have long life spans.…”

Dispersal and reproductive careers of male mountain gorillas in Bwindi Impenetrable National Park, Uganda

“…The genetic tracking of dispersing males in this study provides some of the first information on the ability of these males to acquire mates and form new groups (see Hagemann et al 2018 for western gorillas). Assuming that males did not briefly acquire females and then return to a solitary state if they all dispersed again, our results show that males may be solitary for as much as 5 years, suggesting either that they need a significant amount of time to continue developing before they are able to acquire groups or that there are few opportunities for these males.…”

Intersexual conflict influences female reproductive success in a female-dispersing primate

“…2). For long-lived species such as gorillas which can have tenures of more than 17 years (Hagemann et al 2018), examining the beginning and the end of the tenures separately can be a useful way of circumventing the lack of complete tenures. Therefore, we used two variables as estimates of tenure duration: the elapsed tenure: the duration since the tenure began and the remaining tenure: the time until the tenure ended (see Figs.…”