Motherless Gorillas Beat the Odds
Researchers at the Dian Fossey Gorilla Fund have used more than 50 years of data from Rwanda to discover how maternal loss influences young gorillas' social relationships, survival and future reproduction. The study shows that when young mountain gorillas lose their mothers, the rest of the group helps buffer the loss by strengthening their social relationships with the orphans.
Mothers are incredibly important for survival early in life - this is something that is shared across all mammals. But in social mammals, like ourselves, mothers often continue to provide vital support up to adulthood and even beyond. In many species, like our close relatives, chimpanzees, individuals without mothers suffer higher mortality or may be less successful parents themselves, and this finding can hold even if the loss occurs in early adulthood. But these new findings show that mountain gorillas really go against this trend.
Since 1967, 59 gorillas studied by the Dian Fossey Gorilla fund have lost their mothers after they were able to feed themselves but before they were fully mature (between the ages of two and eight). This happened either when the mother died (30.5 % of cases) or when she transferred into another group without them (69.5 % of cases). The study shows that these gorillas do not have any greater risk of dying than those whose mothers are still around. In addition, maternal loss does not appear to have any long-term effect on the young gorillas' eventual ability to produce and rear offspring themselves.
What does change, however, is the number of affiliative interactions they have with other group members, which increases dramatically after they are orphaned. This causes them to become better integrated in the social group and may help buffer the social adversity experienced after maternal loss. This support from other group members may be similar to what we see in humans, where other family members and even non-relatives can step into key roles in caring for children.
Mountain gorillas live in close-knit "family" groups including a dominant silverback male who leads the group, multiple adult females, their immature offspring and, in some cases, subordinate adult males. Male gorillas are known to care for young members of their group, regardless of paternity. In her lifetime, Dian Fossey noted, "The extraordinary gentleness of the adult male with his young dispels all the King Kong mythology" - an observation reinforced by this study, which found that the dominant silverback male plays a particularly important role in supporting young motherless gorillas, spending more time close to them and increasing the time spent resting and grooming together. This response was common across all group leaders whether or not they were the genetic fathers. Access to the highest-ranking individual likely ensures that orphans do not become socially isolated and continue to have access to food and other resources.
Our capacity to care for other group and family members in times of need may be deeply rooted within our DNA and something we share with gorillas. Just like us, gorillas live long lives, so it takes years for researchers to record the rare and fascinating behaviours that occur over a gorilla's lifetime. The Dian Fossey Gorilla Fund's dataset, one of the longest of any animal species, stretches back more than 50 years, helping us understand how much we share with one of our closest relatives as we work to protect them and their biodiverse habitat.
Robin E. Morrison
Morrison, R. E., Eckardt, W., Colchero, F., Vecellio, V. & Stoinski, T. S. (2021): Social groups buffer maternal loss in mountain gorillas. eLife 10, e62939