05-06-2025

Some gorillas get sick from being 'too social'

Earth.com staff writer

We often celebrate social friendship as a pillar of health, longevity, and happiness. Across species, from humans to gorillas and other animals, social bonds appear to enrich lives. Yet, what if these connections don’t always help? What if social behavior sometimes drains more than it gives?

A recent study from the Dian Fossey Gorilla Fund, in collaboration with the universities of Exeter and Zurich, uncovers this hidden complexity.

By tracking 164 wild mountain gorillas over 21 years in Rwanda’s Volcanoes National Park, researchers revealed a nuanced picture of how friendship and social roles influence survival and reproduction.

This research doesn’t just apply to gorillas. It reflects the deeper currents of our own evolutionary past. It forces us to rethink whether more friendship always means better health or longer life.

In humans, the social environment ranks among the strongest predictors of health and lifespan. Similar patterns emerge in other social mammals, suggesting ancient evolutionary links.

But even with the known benefits of connection, individuals often vary in how social they are. Some display behaviors that seem, at first glance, to defy evolutionary logic.

The mountain gorillas in this study helped clarify that contradiction. Scientists looked at both individual social traits and broader group characteristics to explore how these elements influence fitness.

The findings reveal a pattern of highly context-dependent outcomes. The costs and benefits of sociality change with sex, group size, and the nature of relationships.

Despite strong fitness benefits tied to social bonds, not everyone gains equally. A female gorilla’s experience of friendship might be very different from a male’s – and what works in one group may not work in another.

The shifting equation of friendship

The research uncovered a striking set of contrasts. Female gorillas in smaller groups who maintained strong bonds were less likely to fall ill.

However, these same females had fewer offspring. On the other hand, females in larger groups who had strong ties were more likely to get sick but also had higher birth rates. Health and reproduction pulled in opposite directions.

For males, the trade-offs played out differently. Those with strong, stable bonds had fewer injuries – perhaps because allies provided safety – but experienced higher rates of illness. These findings shake the assumption that sociality always leads to better outcomes.

“Having a lot of strong social relationships is often really good – but sometimes it isn’t,” said Dr. Robin Morrison, lead author and senior researcher at the University of Zurich.

Why would strong relationships lead to more illness in males? The answer may lie in the costs of bonding itself.

“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,” explained Dr. Morrison.

In other words, it isn’t always the risk of disease transmission that causes illness. The emotional and physical toll of guarding others, staying vigilant, and maintaining dominance might chip away at the immune system.

This insight links emotional labor with biological consequences – a concept that resonates far beyond the gorilla world.

The gorillas remind us that relationships, while powerful, demand energy. Sometimes, the social load can overwhelm the very systems meant to protect us.

Group traits shift the balance

The study didn’t stop at individual behavior. The researchers looked closely at the traits of entire gorilla groups – size, cohesion, conflict, and stability – and how they altered the effects of personal social styles.

What they found was a dynamic interaction. Group context shaped whether a gorilla’s social strategies helped or harmed.

In small groups, friendly females enjoyed low illness rates but had fewer babies. In large groups, the same friendly behavior led to more sickness but also higher reproduction. These trends show that social advantages are not fixed – they shift depending on who surrounds you.

“With these forces pushing in different directions, the ‘optimal’ social type will depend on an individual’s sex, age, offspring and wider social group,” said Dr. Sam Ellis from the University of Exeter.

The implications stretch beyond the animal kingdom. In humans and other social mammals, researchers often treat connection as inherently good. But the gorilla study highlights a deeper complexity. In some cases, behaviors once considered maladaptive – like avoiding others – can yield unexpected benefits.

“In some situations social traits that we’ve previously thought of as maladaptive can have important benefits,” said Dr. Ellis.

The paper’s title, “Group traits moderate the relationship between individual social traits and fitness in gorillas,” captures this balance. It shows that both personal and social group characteristics shape the path to survival and reproductive success.

A long-term look into gorilla life

This research would not have been possible without long-term observation. For over two decades, scientists tracked interactions among gorillas in Volcanoes National Park. The study paints a detailed portrait of lives marked by leadership, loyalty, loss, and resilience.

“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,” said Dr. Tara Stoinski, CEO and Chief Scientific Officer of the Dian Fossey Gorilla Fund.

Funded by the Swiss National Science Foundation (SNSF) and the Dian Fossey Gorilla Fund, the project demonstrates how essential sustained research is in revealing the deeper workings of social evolution.

The gorillas behind the research

The study includes unforgettable case studies that bring the numbers to life. Gutangara, a successful mother, lives in one of the park’s largest groups. Though she forms many friendships, Gutangara spends most of her time with her offspring, both young and grown. She holds the record among the observed gorillas with eight surviving children.

Maggie, once the top-ranking female in the Bwenge group, was both fierce and nurturing. She often initiated grooming but could also be aggressive. After the dominant male’s sudden death, she led the group. However, after merging with a neighboring group, she could not adjust and chose to leave. She vanished into Congo, her fate unknown.

Titus, a silverback male, lost his parents early to poaching. Despite this, he became the dominant male at just 15. Known for his gentleness, he had deep bonds with the females in his group.

They often stayed in physical contact with him – an unusual trait among gorillas. His soft leadership helped him maintain dominance for 20 years.

Cantsbee, another silverback, ruled his group for 22 years and fathered at least 28 children. Known for his peaceful authority, he rarely fought but always protected others. He shared a deep bond with his son Gicurasi, who took over as leader later. When Cantsbee grew ill, he left the group, returning only once before passing away in solitude.

What this study reveals is the delicate, ever-shifting calculus of social behavior. Fitness doesn’t rely solely on closeness. It emerges from how an individual fits into their specific group, under particular conditions, during specific life stages.

By revealing how personal and group traits interact, this research offers a new view of evolution. It suggests that diversity in social strategies isn’t just tolerated – it may be necessary. In gorillas, as in humans, there is no one right way to connect.

Sometimes, stepping back is just as important as reaching out.

The study is published in the journal Proceedings of the National Academy of Sciences.

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