The strong relationship between adult males and adult females form the basis of gorilla social groups. Relationships among female gorillas are considered to be relatively weak, which may be due to their diet and dispersal patterns. Female mountain gorillas are thought to gain little by cooperating to defend their food as their major food resource is widely abundant. Furthermore, both males and females can disperse and female gorillas might transfer to a group with no co-resident kin. However, a recent study found that maternal kin spent more time in close proximity than other female pairs. Furthermore, it was shown that female gorillas stay in close proximity to certain preferred females and hence form friendships. These friendships between females usually lasted between 1 and 4 years. It remains unclear whether friendships between gorilla females can endure for more than 7 years as has been shown in other primate species. Additionally, the question as to whether these relationships are restricted to maternal kin arises.

Using long-term data from up to 13 years, a study by the Dian Fossey Gorilla Fund in collaboration with the Universities of Texas and Zurich examined whether female mountain gorillas in the Virunga Volcanoes, Rwanda, form strong and enduring friendships. Proximity between female gorillas was used to indicate friendship. Furthermore, the researchers investigated whether friendship was influenced by kinship and age and whether friends also displayed high rates of grooming. Grooming is widely used as an affiliative behaviour to quantify the quality of social relationships or friendship in primates but it is rarely observed in gorillas.

The majority of females within the five groups studied had at least one female friend. Most relationships between female gorillas did not last longer than 2 years but 13 pairs of female mountain gorillas had strong and enduring friendships lasting more than 4 years. Additionally, long-term friendships of up to 12 years were found between mothers and daughters as well as maternal sisters. However, the researchers also found friendships between non-related females. The frequency of grooming was not associated with friendship between female mountain gorillas.

The study showed that female mountain gorillas can form strong and enduring friendships with each other, despite the low occurrence of grooming. The researchers argue that behaviours other than grooming might better reflect friendship in gorillas as well as in other species. Furthermore, friendship between gorilla females might impact their dispersal decisions and therefore explain the variable dispersal patterns observed in female mountain gorillas. Ultimately, strong and enduring friendships between gorilla females might influence fitness, as it has been shown in many other primate species.

Summary of this publication:
Derby, R. N., Eckardt, W., Stoinski, T. S., Morrison, R. E. Sandel, A. A. (2024): Female mountain gorillas form enduring social relationships. Animal Behaviour 213, 139-147