Passive segregation and multi-scale space-use adjustments to socio-ecological variables in western gorillas

Abstract

Abstract Understanding what are the proximate mechanisms shaping species’ space-use dynamics, and at which scale, can provide insights into species socio-ecology. This is crucial information for both applied (e.g., conservation) and theoretical questions (e.g. individuals’ coexistence/segregation). Yet, longitudinal views of the space-use dynamics of animal species are generally lacking. Taking advantage of an unprecedented long-term data set (up to 8 years) on ranging patterns of wild western gorillas (Gorilla gorilla), we investigated the dynamics of home ranges in relation to socio-ecological variables in five habituated groups in Central Africa. First, we looked at the scale at which their ranging movements are stationary. Second, we studied how space use is shaped by (i) foraging and intra-group constraints (e.g., diet, group size, presence of offspring) by focusing on group daily path lengths, and by (ii) inter-group competition (direct or indirect), by quantifying static and dynamic interactions in neighbouring group-dyads. We found that gorilla groups shifted the barycenter of their home range 0.17 per year by a distance larger than the mean home range radius. Movements within the home range were highly dependent on an ecological factor (seasonal diet) and on the group composition, particularly on the presence of dependent infants. Repeatedly visited sites were also used the longest, but their locations differed seasonally, suggesting use of mnesic abilities for foraging. There was little overlap between the home ranges of neighbouring groups that did not tend to move away from each other more than expected at random. Groups also ventured into, and slept in, the central part of the home range of a neighbouring group. Overall, this highlights the absence of territoriality and suggests that spatial segregation should be the simple consequence of avoidance of depleted areas. This study shows how social and ecological constraints may shape space-use patterns at different scales in a non-territorial species. It also raises questions on the cognitive mechanisms underlying high foraging efficiency in environments shared with competitors.