Western gorilla space use suggests territoriality

Abstract

The evolutionary origins of how modern humans share and use space are often modelled on the territorial-based violence of chimpanzees, with limited comparison to other apes. Gorillas are widely assumed to be non-territorial due to their large home ranges, extensive range overlap, and limited inter-group aggression. Using large-scale camera trapping, we monitored western gorillas in Republic of Congo across 60 km 2. Avoidance patterns between groups were consistent with an understanding of the "ownership" of specific regions, with greater avoidance of their neighbours the closer they were to their neighbours' home range centres. Groups also avoided larger groups' home ranges to a greater extent, consistent with stronger defensive responses from more dominant groups. Our results suggest that groups may show territoriality, defending core regions of their home ranges against neighbours, and mirror patterns common across human evolution, with core areas of resident dominance and larger zones of mutual tolerance. This implies western gorillas may be a key system for understanding how humans have evolved the capacity for extreme territorial-based violence and warfare, whilst also engaging in the strong affiliative inter-group relationships necessary for large-scale cooperation. Understanding how neighbours use and share space is fundamental to understanding a species' large-scale social system 1. Patterns of space use can have considerable impacts on the likelihood of neighbours encountering one another, and the location of such encounters can greatly influence the behaviour shown when they meet 2-4. One key example of this is in species showing territoriality, where rates of aggression shown to out-group individuals can differ drastically depending on the location relative to the group's territory 5-8. Territories are commonly defined as regions of a home range that are actively defended against intruders to enable exclusive use by the individual or social unit 9. However, there is considerable variation in how the term has been used in different study systems 10 and broader definitions of territoriality also include areas of priority use 1,11 , for example, through site-specific dominance 8,12. Territories can be defended using physical aggression or advertised using less costly alternatives such as scent marking, calls or displays; with a broad diversity of territorial behaviors observed both within and among species 11 , including humans 13. It has been increasingly suggested that defining territoriality as a binary trait cannot explain the full diversity of territorial behaviours observed 1,11,14-16. Territoriality may be better described by a continuum from extreme territoriality where neighbouring conspecifics impose a hard boundary on movement, to species with more flexible territory boundaries such as the black bear where territoriality can vary, geographically and temporally 11 , through to non-territorial species in which the ranges of neighbouring conspecifics do not constrain an individual or group's movem...

Inter‐group relationships influence territorial defence in mountain gorillas

“…However, this effect appears to be relatively weak. These findings support the hypothesis proposed in Morrison, Dunn, et al (2020) that gorillas show a form of territoriality where groups do not have hard territory borders, but rather,…”

“…If gorilla home ranges are aggressively defended, groups may avoid encountering their neighbours more when within their neighbours' home ranges. Indeed, there is evidence supporting this in western lowland gorillas (Morrison, Dunn, et al, 2020). The more effective territorial defence becomes at preventing neighbour intrusions, the harder it may become to detect from aggression.…”

“…They therefore concluded that mountain gorillas may be intermediate on a gradient between territoriality and non‐territoriality. Western lowland gorillas ( G. gorilla gorilla ) have also been found to actively avoid other groups' home ranges and avoid encountering other groups to a greater extent when closer to the centre of those other groups' home ranges (Morrison, Dunn, et al., 2020); another finding consistent with territoriality in gorillas. It was therefore suggested that gorillas may show a more flexible form of territoriality, in which the location, relative to the centre of a group's home range, is one of a number of important factors influencing how groups interact.…”

Lethal coalitionary attacks of chimpanzees (Pan troglodytes troglodytes) on gorillas (Gorilla gorilla gorilla) in the wild

“…The majority of studies have, however, only focused on one eastern sub-species ( Gorilla beringei beringei ), limiting an in-depth understanding of the behavioural diversity of gorillas. For instance, a recent study on western lowland gorillas ( Gorilla gorilla gorilla ) suggested that groups may show high levels of territoriality and actively defend core regions of their home ranges against neighbours 26 .…”

Ranging and territoriality in red‐shanked doucs (Pygathrix nemaeus) at Son Tra Nature Reserve, Vietnam

“…Thus, a territory is defined as a specific type of home range characterized by aggressive protection or defense of its borders by the occupants against conspecifics (Burt, 1943). Territoriality is hereafter discussed as a behavioral trait which, in the absence of direction behavioral observations, may be predicted by measures of economic defendability and assess with tools such as the Defendability Index (Bartlett & Light, 2017; Brown, 1964; Mitani & Rodman, 1979; Morrison et al, 2020). The Defendability Index, one of the earliest developed models for assessing the potential for territoriality within populations and species, mathematically represents the relationship between daily path length and home range to reflect daily mobility and degree of defendability (Mitani & Rodman, 1979).…”