Gorillas live in close-knit groups, which provide them with strong and stable social relationships that benefit all their members. However, in mountain gorillas this includes relationships among related individuals, which poses a potential problem: mating among close relatives could negatively affect the population’s genetic fitness. This is why most group-living wild animals have evolved ways to avoid inbreeding, with the most common method involving dispersal by males, females or both, into new groups as they mature.

In gorillas either sex can disperse, with females often transferring groups several times during their lives, and males becoming solitary, joining bachelor groups or forming their own new groups. However, the Fossey Fund’s more than five decades of data show that mountain gorillas have an unusually flexible dispersal pattern, with only around half of the individuals of either sex leaving their birth group. This means that about 60% of reproductive mountain gorilla groups have a single adult male. The rest of the groups often contain adult males and females who may be closely related.

 

Living with relatives can provide a range of social benefits, but mating with relatives can reduce offspring survival and fertility and affect the overall population’s health. So a recent study by Fossey Fund scientists looked at 13 years of detailed data on copulation patterns and offspring paternity to help answer the question of what tactics mountain gorillas might use to recognize and avoid mating with close kin, aside from dispersing to other groups. This was aided as well by our 55+ years of data on dispersal patterns in each sex.

The study is especially important because the mountain gorilla population is very small, with only about 600 individuals in the Virunga mountain region, where this study took place. (There are about another 400 mountain gorillas living in a non-contiguous area in Uganda.)  So the genetic diversity in the population as a whole is already low.

Females drive gorilla choices

Interestingly, in mountain gorillas most copulations are initiated by females, at more than 60%, and when there are multiple males in a group, most females mate with multiple males each year, some of whom could be close relatives, raising the question of whether and how they avoid inbreeding. 

Our researchers found that mountain gorillas are able to recognize relatives on their maternal sides fairly easily, due to the familiarity gained through close social relationships maintained between mothers and offspring and between maternal siblings as they grow up and often beyond. And this study showed they had a significant bias against mating with any maternal kin.

However, identifying paternal kin within a multi-male group is not so easy. In this case, simpler methods may be used, such as our finding that females tend to avoid mating with any males old enough to be their fathers in such a group. Interestingly, this pattern does not hold when females transfer to another group. In that case they actually prefer mating with older males who likely have higher status in the group. This flexibility really highlights the intelligence of gorillas to use different strategies depending on the circumstances.

 

It’s also possible that gorillas simply recognize what their close kin look like – or smell like –  which is called “phenotypic matching.” It is a bit like how we can look at photos of people and see physical resemblances. This capacity has been shown in chimpanzees, but we didn’t find strong evidence for it in this study on gorillas. 

Overall, our research found that the mate choice strategies these gorillas use do considerably reduce the amount of inbreeding in groups where relatives live together and may be an important step in having enabled their more flexible dispersal patterns to evolve. 

This study was led by Fossey Fund affiliate scientist Dr. Robin Morrison. The other authors included Fossey Fund Research Assistant and Ph.D. candidate Eric Ndayishimiye; CEO and President/Chief Scientific Officer Dr. Tara Stoinski, and Senior Manager of Primate Research Dr. Winnie Eckardt. The study was published in the Proceedings of the Royal Society B, Biological Sciences