Female mountain gorillas form enduring relationships

Strong, affiliative relationships are important across social mammals, and in many species, relationships between female kin form the basis of group life. Relationships are expected to be weaker in cases where females disperse or do not cooperatively defend resources. Mountain gorillas (<i>Gorilla beringei beringei</i>) seem to support this, as females can emigrate multiple times throughout their life and do not jointly defend the abundant vegetation they feed on. Unsurprisingly, mountain gorillas form variable or weak relationships with other females and seemingly prioritize relationships with adult males. But prior studies may have misinterpreted relationships due to a focus on grooming and due to understandable limitations of small sample sizes and short study periods. Here, we examine proximity and grooming between 47 adult female mountain gorillas in five groups over 5 to 13 years. We analyze 2m, 5m, and grooming relationships between 366 individual dyads to determine (1) if proximity relationships are preferential, and (2) if they endure across time. Most females formed at least one preferential 2m and 5m proximity relationship (43 of 47), which both lasted a mean of 2.1 years. Additionally, 3.6% of dyads (n=13/366) formed enduring 2m proximity preferences that exceeded four years with the longest lasting at least 12 years. Maternal kin had the most enduring proximity and grooming relationships, although grooming was rare overall. The enduring proximity relationships between some female gorillas in our sample are similar in length to those of female baboons and male chimpanzees, which are considered social bonds. Mountain gorillas provide a system in which factors influencing bonds can be better understood, and our study highlights a need to refine definitions of social bonds while potentially reassessing their evolutionary function.