- A 21-year study of 164 wild mountain gorillas found that strong social bonds produce different health effects for males and females, with males experiencing more illness but fewer injuries when they have close friendships.
- Female gorillas with strong social bonds generally had better health outcomes, experiencing 19% fewer injuries and 14% less illness compared to those with weaker social bonds.
- The benefits and costs of friendship varied based on group size, with females in small groups having fewer offspring despite better health, while those in large groups had more babies but higher rates of illness.
- The research helps explain why animals exhibit such diverse social behaviors and may inform conservation efforts for this endangered species, which numbers just over 1,000 individuals.
Titus was just 4 years old when poachers killed his father and several other close adults in his group. Despite the violence, the young mountain gorilla developed an approach to leadership characterized by gentleness and calm behavior, building close bonds with the females in his group, who often stayed in physical contact with him. This friendship-focused style worked out for Titus. He remained the dominant male for two decades, until his death in 2009.
A 21-year study of 164 wild mountain gorillas (Gorilla beringei beringei), including Titus, found that strong friendships produce both advantages and disadvantages, which differ based on sex and environment.
The study, published in Proceedings of the National Academy of Sciences, analyzed decades of behavioral and health records to examine how social bonds influence survival and reproduction in one of the world’s most endangered species, the mountain gorillas of the Virunga Mountains in East Africa.
There are only around 1,000 living mountain gorillas. Just over half live in the Virunga Mountains, a range of extinct volcanoes that straddle the borders of the Democratic Republic of Congo, Rwanda and Uganda. The remainder inhabit Uganda’s Bwindi Impenetrable National Park.
Past studies in humans and other animals have shown that strong social bonds typically improve health and help animals live longer; however, this study found that these relationships can sometimes work differently than researchers initially thought.
“Having a lot of strong social relationships is often really good, but sometimes it isn’t,” said study lead author Robin Morrison, a senior researcher at the University of Zürich. “For example, our study found that strong and stable social bonds are generally linked to less illness in female gorillas, but more illness in males.”
The team found notable differences between the sexes. Males who maintained stronger social bonds faced higher rates of illness but lower rates of injury from conflicts. When male social bond strength increased by one standard deviation, injuries dropped by 50% while illness increased by 32%.
“It’s possible that males expend more energy by having close social ties, as they have to defend females and offspring, and the stress of this may reduce their immune function,” Morrison said.
Females showed the opposite pattern. Those who formed stronger bonds experienced fewer instances of both illness and injury, with stronger social ties reducing female injury by 19% and illness by 14%.
Females with strong social bonds, living in small groups, had fewer illnesses but also produced fewer offspring. In contrast, well-connected females in larger groups got sick more often but achieved greater reproductive success.
“It’s super interesting that the closeness of their friendships really mattered in this context,” Dominick Shattuck, an associate professor at Johns Hopkins who researches human social relationships and was not part of the study, told Mongabay. “How close the relationships are really impacts the overall well-being of the gorillas.”
The research on mountain gorillas builds on decades of observation by the Dian Fossey Gorilla Fund in Rwanda’s Volcanoes National Park. “This paper highlights the incredible value of long-term studies to furthering our understanding of the evolution of sociality and how the benefits or costs of sociality can vary considerably across different environments,” Tara Stoinski, president and CEO of the Dian Fossey Gorilla Fund and a co-author of the study, said in a statement.
Mountain gorillas remain endangered despite recent population growth. Their recovery in past decades stems from ecotourism initiatives created in the 1980s by conservationists who persuaded Rwanda’s government that gorilla tourism would generate more income than clearing forests for cattle pasture. Tourism now generates more than $400 million annually for Rwanda, with gorilla visits representing the top attraction.
The recovery of mountain gorilla populations is a conservation achievement, occurring despite ongoing civil conflict, poaching, and an encroaching human population. Although they are the only great apes whose population is increasing, they still face ongoing threats from human activities and the climate crisis.
Banner image of a family of mountain gorillas resting together, in Volcanoes National Park, Rwanda. Resting in physical contact is an indicator of close social relationships and tolerance in gorillas. Image credit: Dian Fossey Gorilla Fund.
Liz Kimbrough is a senior staff writer for Mongabay and holds a Ph.D. in Ecology and Evolutionary Biology from Tulane University, in New Orleans, Louisiana where she studied the microbiomes of trees. View more of her reporting here.
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Citation:
Morrison, R. E., Ellis, S., Martignac, V., Stoinski, T. S., & Eckardt, W. (2025). Group traits moderate the relationship between individual social traits and fitness in gorillas. Proceedings of the National Academy of Sciences, 122(20), e2421539122. doi:10.1073/pnas.2421539122
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