Gorillas adjust their aggression based on risk and reward

Female gorillas adjust their aggression depending on their needs and group structure. This finding comes from a 25-year-long study by Nikolaos Smit and Martha Robbins.

The research shows that female gorillas use simple but flexible rules, called aggression heuristics, based on risk and social dynamics.

“Animals that live in groups often compete for access to resources such as food and mates. The potential costs of this competition can drive the formation of hierarchies that determine priority of access to resources without superfluous conflicts,” wrote the study authors.

“Accordingly, individuals may choose strategically who they compete with, in order to minimize costs and maximize gains.”

Pregnant gorillas take more risks

The study includes data from five wild gorilla groups across Gabon and Uganda. The researchers found that most aggression comes from higher-ranking females targeting those just below them. However, 42% of aggression was aimed upward, toward more powerful females.

“Aggression, a proxy for competition, tends to increase when resources are limited and is usually directed toward lower-ranking individuals but can vary toward groupmates of different ranks, and variation is observed even within species,” said lead author Nikos Smit from the University of Turku.

Pregnant and lactating females showed more aggression toward higher-ranking groupmates. Among them, females in their third trimester took the most risk. These reproductive states require more energy, which may drive these females to act more boldly, especially when food is scarce.

Interestingly, lactating females, despite higher energy needs, were less aggressive toward powerful rivals than pregnant ones. This may reflect caution to protect their infants from retaliation.

Gorilla aggression and risky behaviors

Group makeup plays a big role in aggression direction. Female gorillas living in groups with more adult males tended to direct aggression toward more dominant females. This might be because male presence offers a protective buffer and reduces retaliation risks.

In contrast, in groups with more females, aggression was usually aimed downward. With more competition, females might prefer safer confrontations with weaker individuals.

These results show how gorillas balance risk and reward. “Overall, our results confirm that an individual’s circumstantial needs and their social environment may influence individual decisions to engage in more risky behaviors such as aggression toward more powerful groupmates,” said study senior author Martha Robbins.

Weak gorillas use light aggression

The researchers analyzed 6,871 aggressive interactions. Mild forms of aggression, such as vocalizations or vegetation pulling, were more common in upward aggression. Severe aggression, such as hitting or biting, was less frequent but present across the board.

The team quantified each aggressive act using a score. This score measured the difference in rank between the aggressor and the target. A high positive score meant a low-ranking female attacked a high-ranking one. A negative score meant aggression went down the hierarchy.

Mild aggression had higher average scores. This shows females prefer safer, non-escalating tactics when confronting stronger groupmates.

Gorillas adjust aggression to the situation

This study reveals that female gorillas follow simple rules when managing conflict. These rules change with the number of males and females in the group, reproductive status, and other social factors.

The authors highlight that such flexible aggression strategies may not just apply to gorillas. Other species, including humans, may also shift behavior based on risk, reward, and group dynamics.

Their work adds to growing evidence showing that social animals, like gorillas, make strategic decisions. They do not just follow fixed species-wide instincts. Instead, they adapt based on the challenges they face.

Research transparency and broader impact

This study not only opens new questions about the evolution of social rules in primates but also makes its data and code public.

Funded by Max Planck Society and several global conservation groups, this research highlights how long-term behavioral monitoring helps us understand animal societies and perhaps our own.

“Our study provides critical insights into the evolution of competitive behavior, showing that aggression heuristics, the simple rules that animals use to guide their aggressive interactions, are not simply species-specific but also dependent on the conditions that populations and individuals experience,” noted the researchers.

The study is published in the journal eLife.

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