Inter-group relationships influence territorial defence in mountain gorillas

1. Many species show territoriality, in which territory owners have
exclusive or priority use of a region. In humans, tolerance of others
within our space also depends greatly on our social relationships with
them. This has been hypothesised as one potential driver of the evolution
of long-term, inter-group relationships, through enabling shared access of
resources and easing disputes over space. 2. However, extremely little is
known about the importance of social relationships between neighbouring
groups in non-humans for how space is used and shared. 3. Using 16 years
of data on the simultaneous movement and interaction patterns of 17
mountain gorilla groups, we investigated how the occurrence of aggressive
and affiliative behaviour during inter-group encounters was influenced by
both their social and spatial context. 4. We found evidence of territorial
defence, with rates of aggression increasing towards the centre of home
ranges. Groups which had previously split from each other showed higher
levels of affiliation during encounters with each other and experienced
lower levels of aggression when within the other’s peripheral home range.
However, encounters within core areas of the home range consistently
elicited higher aggression, regardless of the groups’ history. Our
findings indicate that not only are the social relationships between
groups retained after they split from one another, but that these
relationships enable groups to access certain areas with a reduced risk of
aggression. 5. This suggests that reduced aggression when accessing areas
within neighbours’ home ranges may be an advantage for the maintenance of
inter-group relationships and a potential driver in the evolution of
long-term, post-dispersal relationships and complex multi-level societies.