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Nonhuman animals can adapt the intensity and direction of aggression to suit their individual needs and social contexts, according to a recent study.

The study, as a Reviewed Preprint in eLife, with the revised version appearing July 23, is described by editors as important research that uses long-term behavioral observations to understand the factors that influence female-on-female aggression in gorilla social groups.

They say the evidence supporting the claims is convincing, as it includes novel methods of assessing aggression and considers other potential factors. The work will be of interest to biologists and ecologists working on the social interactions of animals and on broader human societal relationships.

Animals that live in groups often compete for access to resources such as food and mates, forming hierarchies that determine priority of access. Individuals in the group must choose strategically who to compete with, at different timepoints, to maximize gain of resources and status while minimizing costs—such as energy expenditure and risk.

"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," says lead author Nikos Smit, a postdoctoral scientist at the Max Planck Institute for Evolutionary Anthropology, Germany, and the University of Turku, Finland.

"In this study, we tested the hypothesis that this variation arises due to different conditions individual animals experience, and that both an individual's needs and social environment shape their aggression toward different ranking individuals, as predicted by risk-taking theory."

To test this hypothesis, the authors used 25-year-long behavioral observations among a total of 31 adult female gorillas from five wild gorilla groups—one group of western gorillas in Gabon, Central Africa, and four groups of mountain gorillas in Uganda, East Africa. Trained observers recorded behaviors used to infer female gorilla hierarchies (such as decided avoidance—walking away from an approaching individual) and assigned each female gorilla a rank in the hierarchy.

The observers also recorded aggressive interactions among adult females, and assigned a score to each of these interactions to quantify direction of aggression, and specifically if aggression was directed toward more or less powerful individuals, according to aggressor–recipient rank relationship.

After analyzing 6,871 aggressive interactions, most of the aggression was directed from higher-ranking to lower-ranking individuals, consistent with the hypothesis that high-ranking individuals commonly use aggression to reinforce their status. The percentage of aggressive interactions from lower-to-higher ranking individuals was lower, at 42%, but higher than observed in previous studies and higher than in many other animals.

In comparison to other females, pregnant and lactating ones directed aggression toward more powerful groupmates—reflecting their increased energetic needs. However, despite the potentially higher energetic needs of lactating females, pregnant females directed aggression toward more powerful groupmates in comparison to them. The authors suggest this might be because lactating females are less likely to take the risk of aggression toward superior groupmates because they need to protect their dependent infants.

Female gorillas directed more aggression to more powerful, higher-ranking, females when there were more males in the group, suggesting that male protection affords an environment in which females will take greater risks. By contrast, female gorillas directed more aggression to lower-ranking, less powerful females when there were more females in the group, suggesting that females prefer to target weaker opponents when they have such an option. Together, these results confirm that gorillas can adapt aggression to their social context.

"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," concludes senior author Martha Robbins, Group Leader at the Max Planck Institute for Evolutionary Anthropology.