What happens when gorillas lose their mothers?

Losing a parent, and particularly your mother while you’re still a youngster, has well documented, profound lifelong impacts on a person’s life course. So the obvious question is, what happens to other social species that are closely related to us, like gorillas, for instance. As Chris Smith heard, Robyn Morrison has been combing through data collected across more than half a century by the Dian Fossey Gorilla Fund…

Robyn - It's one of the difficulties of studying wild populations, right? Especially endangered wild populations. You can't do any sort of experimenting yourself. You have to wait for these things to happen. And it is, of course, really rare that young gorillas lose their mothers. So the data that we use has been collected over 50 years. So it's taken all of this time to have kind of a big enough sample for us to really answer these questions and say, you know, what does happen to these gorillas when this rare thing does happen.

Chris - Was your starting point that we know from epidemiology and psychology and sociology, that when this happens to a human, it does have really very profound consequences on some individuals? Were you coming at it from that angle saying, well, are gorillas as another social species similar?

Robyn - Yes, exactly. That and, in humans, it's interesting because in some populations we see really, really detrimental effects of losing one's mother. And in other populations, we see much less dramatic effects. And part of this study we wanted to do is to kind of understand why that might happen looking at one of our closest relatives, the gorilla.

Chris - How did you get enough data to come up with with statistically robust findings then?

Robyn - I work with the Dian Fossey Gorilla Fund and in 1967 they started monitoring mountain gorillas in Rwanda, and they've been there ever since. So there's this really incredible dataset contributed to by hundreds of different researchers and fieldwork people. And so it was kind of all of that really long-term data, that was the only way we can really answer these sorts of questions.

Chris - What were the outcome measures then?

Robyn - So what we found is that in terms of fitness, we couldn't see any detrimental effects of young gorillas losing their mothers. So we looked at gorillas that were over the age of two and not quite eight, so eight is when they are kind of first classified as adults. And what we found is that they were just as likely to survive, they were just as likely to reproduce. So we looked at females and we found that they were actually reproducing ever so slightly earlier, so about six months earlier they had their first offspring. So they seem to be kind of succeeding in life all around. Not only are they surviving but they're also kind of reproducing and kind of contributing to the next generation of gorillas.

Chris - Well, that flies in the face of where I thought you were going to say it was going to go then, that you'd say it was the same as with other primates where there's a big catastrophe if you lose your mother. How do you think that they're doing so well when this happens to them then?

Robyn - Yeah, it's a really good question because we do see that this is really problematic, especially in chimpanzees as well, which are super closely related to gorillas. And I think part of it is to do with this really cohesive social structure that gorillas have, right? They live in these groups, they're usually about 12 individuals, but they move together. They feed together, they build nests at night together, and they're always in this really cohesive social group. And so part of our study was looking at "how did their social relationships change when they lose their mothers?" And what we found is that they kind of actually became better integrated in the group. They had stronger relationships with everyone else. They were really central. They had really strong relationships with the dominant male. And when we compare this with other primates, the problems that the other young primates face is that they actually kind of struggle to be part of this social group when they lose their mothers. So they struggled to get kind of access to food, access to mates, all of those sorts of things.

Chris - Is it that the group recognises that the individual is now vulnerable because it's lost its mother, or is it that the individual knows it's vulnerable because it's lost his mother and behaves differently to the group? Or is it both?

Robyn - That is a brilliant question, and we actually don't know. So from our data, we can see that these relationships are really strengthening, but what we haven't yet got into is who is causing it to change. Is it the young gorilla that's kind of seeking out much more social interaction with other individuals or is it all the group members that are noticing these changes and responding to it? And I think that's a really interesting question going forward. Particularly we found that the strongest relationship was with the dominant male. So it'll be really interesting to understand who is pushing that, who's kind of strengthening that relationship. And I think that's one of the next steps moving forward from this research.

Chris - And given that you're seeing this in these gorillas, does this argue then that this is some kind of evolutionary trait here because we see, you know, humans are also successful because we're pretty good at reconfiguring groups and opening up a gap to slot someone in when they're vulnerable? Sounds like the gorillas are doing the same sort of thing, which would suggest that this is something that's innate to our behaviours

Robyn - Potentially, yes. I think it seems to be a very successful strategy, right? If there is this risk of losing certain individuals, having this cohesive social group can be really beneficial, right? Because you've got other social relationships to turn to it kind of buffers you in these extreme cases where, you know, something really bad happens and you lose a really important social contact. I don't know whether it means it's kind of the best strategy. I suspect, you know, there are different strategies in different ecosystems, different environments, different species that are beneficial, right? So each species is finding its optimal way through this evolutionary landscape, but we don't seem to see it in chimpanzees as well. And that's kind of another of our really close relatives. So it's kind of complicated. It's kind of uncertain when these sorts of behaviours might have evolved. Maybe it's evolved kind of separately in humans and gorillas. Maybe it evolved kind of before we split from each other. There's some really interesting research in Bonobos as well. There's quite a lot of evidence of adoption and even adoption of individuals outside their own social group.