Western lowland gorillas may be territorial, a new study finds

Western lowland gorillas (Gorilla gorilla gorilla) may be avoiding their neighbors, according to a new study that presents evidence of territoriality among gorilla groups in the Republic of Congo.

Using large-scale camera trapping, researchers observed eight groups of gorillas across a 60-square-kilometer (23-square-mile) area in their dense forest habitat. The images revealed that groups avoided one another. Groups also stayed away from the central area of each other’s home ranges — evidence that western lowland gorillas may be more territorial than previously thought.

Territoriality is common among apes, including chimpanzees and gibbons, and there is some evidence of territoriality in the mountain gorilla (Gorilla beringei beringei), so the authors of the new paper in the journal Scientific Reports were not entirely surprised by these findings. However, gathering enough data on gorilla group interactions to support this idea has proved challenging.

“Western gorillas are particularly difficult to study in the wild because they live in dense forest and will usually run away from and/or be extremely aggressive towards humans if they are not habituated,” study author Robin Morrison, a researcher with the Diane Fossey Gorilla Fund, told Mongabay. “We know little about how multiple groups share space.”

Habituating a group of gorillas to human contact can take up to five years, and only a few wild groups are habituated. So the team used another method, camera trapping, which allowed them to observe the gorillas in a less invasive way.

Camera traps were placed at 36 natural feeding locations frequented by the gorillas on the outskirts of Odzala-Kokoua National Park in the Republic of Congo and recorded 568 distinct gorillas from 24 different groups. Groups recorded 10 or more times were chosen as focal groups for the study.

Territoriality comes in many forms. Species like chimpanzees (Pan troglodytes) patrol the boundaries of their territories and violently defend them. This type of behavior was not expected in western lowland gorillas, as groups sometimes interact in a positive manner and there is no evidence that they patrol their boundaries.

“We expected there might be a more gradual transition from strongly defended space at the core of their home range to weak or no defense of space at the peripheries, which is what the movement patterns we detected suggest,” Morrison said.

Many definitions of territoriality require evidence of groups actively excluding other groups. The lack of data on western lowland gorilla behavior, in general, has made it difficult to determine to what extent the species actively excludes others. So instead of focusing on active exclusion behaviors, the researchers focused on avoidance behavior.

The way gorilla groups avoided certain areas demonstrated an understanding of the “ownership” of specific regions, with greater avoidance of neighbors occurring the closer groups were to the center of another group’s home range. Western lowland gorillas were also less likely to feed in an area that had been visited by another group that day, which, the paper suggests, is a way to avoid conflict.

“I expect these findings of territoriality in Western gorillas to spur future research to understand the nature of how this territoriality actually plays out when gorillas groups meet — something not observed in this study,” Jason Hodgson, a primate evolution specialist at the University of Cambridge, who was not involved in the study, told Mongabay.

Western lowland gorillas are threatened by habitat loss, hunting, and armed human conflict in some areas (though not in the study region). Shrinking habitats could also increase the spread of the Ebola virus, transmitted by direct contact among gorillas. Ebola killed an estimated 5,500 western gorillas between 2002 and 2004 and continues to pose a serious threat.

The western gorilla is classified as critically endangered on the IUCN Red List, with an estimated 125,000 remaining in the wild. The Eastern gorilla (Gorilla beringei) is also critically endangered, largely due to habitat loss and hunting. One subspecies of eastern gorilla, the mountain gorilla (Gorilla beringei beringei), has increased in number and is now listed as endangered, but the population is only an estimated 1,069 individuals.

Roughly 80% of western gorillas live outside of protected areas, where shrinking territory due to forest loss and habitat fragmentation is a big problem. This new information on territoriality, combined with the shrinking habitat, means gorillas may experience increased competition for food as well as for the limited space.

“Aggression, whilst usually rare, has been lethal in some cases and could pose an issue for maintaining gorilla numbers in their ever-decreasing habitat,” Morrison said.

“Though not discussed directly in the paper, these findings have important implications for the conservation of this species,” Hodgson said. “Understanding how gorilla groups interact with each other is key to both designing the most effective conservation strategies, and also predicting how Western gorilla populations will respond to habitat loss in the future.”

Banner image of gorilla group courtesy of Germán Illera of SPAC Scientific Field Station Network.

Citation: Morrison, R. E., Dunn, J. C., Illera, G., Walsh, P. D., & Bermejo, M. (2020). Western gorilla space use suggests territoriality. Scientific Reports, 10(3692). doi:10.1038/s41598-020-60504-6

Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough_

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