Interactions between gorilla groups are endlessly fascinating. Interactions require considerable multi-tasking for males, as they have to protect their group, try to attract females from other groups, and try to prevent their females from leaving. Not surprisingly, interactions can be very intense, with a lot of male posturing and displaying. However, they can also be peaceful, with groups intermingling for hours. Understanding this variability was one of the aims of a series of papers Fossey Fund staff recently published in collaboration with the University of Western Australia.
Led by Dr. Melanie Mirville as part of her PhD research, the study and subsequent series of papers analyzed 13 years of data on group interactions and gorilla behavior in 14 separate groups of gorillas monitored by the Fossey Fund’s Karisoke Research Center.
Several interesting results were found. First, we looked at what influences interactions at the group level and found that when groups are comparable in size and the members do not know one another, there tend to be more frequent and more intense fights between the groups.
Second, we examined which gorillas were involved in interactions. Unsurprisingly, dominant males participated in conflicts most frequently, as they have the most mating opportunities to defend. Females tended to participate in aggressive interactions with lone silverbacks, possibly to avoid the risk of infanticide if the dominant male is killed or injured. When interactions were peaceful, those participating were most likely familiar individuals who were born in the same group before moving to separate groups.
Finally, we observed that fighting between groups influenced how group members interacted with each other. In particular, after an intergroup conflict, females increased their positive social interactions with their group members, while males decreased their aggressive interactions with group members. In addition, conflicts resulted in both involved groups roughly doubling the time they spent moving, with the losing group also spending less time resting.
Jean Pierre Samedi Mucyo, Fossey Fund’s monitoring and protection officer and co-author of the studies, says that the studies not only helped scientists understand gorilla behavior but also “provided insight on the space needed by gorillas as their population density continues to increase.”
Understanding the dynamics around group interactions is important for conservation. While the mountain gorilla population has slowly grown, says Mirville, their available habitat has not, and interactions between groups, including with lone silverbacks, have increased in frequency. “Increased interaction and increasing competition have the potential to affect future population growth,” Mirville says. “It’s important to understand the causes of intergroup aggression in order to predict the potential effects on an increasing population.”