Animals that live in groups often compete for access to resources, such as food and mates, forming hierarchies that determine priority of access. To maximise their gain of resources while minimising costs, such as energy expenditure and risk, individuals in social groups must choose strategically who to compete with at different times.
"Aggression, a proxy for competition, tends to increase when needs are high and/or resources are limited. While it is typically directed towards lower-ranking individuals, it can also be directed towards higher-ranking ones – and variation is observed even within species," says Nikos Smit, the study's lead author and a postdoctoral scientist at the Max Planck Institute for Evolutionary Anthropology in Leipzig and the University of Turku in Finland. "In this study, we tested the hypothesis that this variation arises due to the different conditions individual animals experience. We found that both an individual's needs and its social environment shape its aggression towards individuals of different ranks, as predicted by risk-taking theory."
Data from 25 years of behavioural observations
To test this hypothesis, the authors used 25 years of behavioural observations of a total of 31 adult female gorillas from five wild gorilla groups – one group of western gorillas in Gabon in Central Africa and four groups of mountain gorillas in Uganda in East Africa. Trained observers recorded behaviours 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 the direction of aggression and whether it was towards individuals who were ranked more powerful or less powerful, according to the aggressor-recipient rank relationship.
After analysing 6,871 aggressive interactions, the researchers found that most of the aggression was directed from higher-ranking individuals to lower-ranking ones. This is 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 percent, but still higher than in many other animals.
Female gorillas take risks in pursuit of energy and safety
Compared to other females, pregnant and lactating females showed aggression towards more powerful group members, reflecting their increased energy requirements. Despite the fact that lactating females may have even higher energetic needs than pregnant ones, they generally targeted less powerful group members than those pregnant females targeted. The authors suggest that this is because lactating females need to take risks to fulfill their increased needs, but they might limit these risks to protect their dependent infants.
Furthermore, female gorillas directed more aggression to more powerful, higher-ranking females when there were more males in the group, suggesting that male protection creates an environment in which females take greater risks. Conversely, female gorillas directed more aggression to lower-ranking, less powerful females when there were more females in the group, suggesting that they prefer to target weaker opponents when given the option. Together, these results confirm that gorillas can adapt their aggression to their social context.
"Overall, our results confirm that an individual's circumstantial needs and their social environment may influence their decision to engage in riskier behaviours, such as aggression towards more powerful group members," concludes Martha Robbins, the study's senior author and Group Leader at the Max Planck Institute for Evolutionary Anthropology.
