Strict hierarchies boost cooperation among monkeys

Cooperation is often associated with friendly, equal societies. But new research shows that it can actually thrive in strict hierarchies. An international team of biologists led by Utrecht University found that monkeys in highly structured groups cooperate more effectively. Their study is published today in the scientific journal Nature Communications.

Until now, most biologists assumed that cooperation in primates mainly occurs in tolerant, egalitarian groups with little hierarchy. The new study paints a different picture.

“It may sound counterintuitive,” says behavioural biologist Jorg Massen, who led the research. “But in a hierarchical group, everyone knows where they stand. That creates clarity and trust. And that actually helps cooperation.”

Fewer partners, stronger bonds

The researchers draw this conclusion after studying over one hundred macaques living in groups of six different species. These species vary widely in how strict their social hierarchies are: some have clear, steep hierarchies, while others are more egalitarian.

In experiments, the animals had to work together to obtain food rewards. The results were striking: monkeys in hierarchical groups did not cooperate with everyone, but instead chose a small number of consistent partners. And these partnerships proved highly effective.

“In strongly hierarchical groups, individuals often have one or two key cooperation partners,” says biologist Debottam Bhattacharjee, co-lead of the study, and the one who conducted most of the experiments. “Because they depend on each other in relation to higher-ranking individuals, they build strong and stable relationships. The social structure is clear, and you know what to expect from one another.”

In more egalitarian groups, the opposite pattern emerged. Animals had more social interactions, but these were less stable. “In egalitarian societies, someone may be your ally today, but not tomorrow,” says Bhattacharjee. “That unpredictability makes it harder to build lasting cooperative relationships.”

Dependence drives cooperation

According to the researchers, not only predictability but also mutual dependence plays a crucial role.

In a hierarchical society, the rules are clear. That creates confidence in your relationships.

“In a hierarchical society, the rules are clear,” Massen says. “You know who is dominant and how to behave. That creates confidence in your relationships.” At the same time, individuals depend more strongly on one another. “Without cooperation, you cannot move up within such a group. That dependence stimulates cooperation, and may even have contributed to how cooperation evolved.”

Even ‘strict’ monkeys are social

Another surprising finding is that hierarchical groups are not necessarily less tolerant. In some experiments, these monkeys were perfectly able to share space and food.

“We did not expect that,” says Bhattacharjee. “But clear social relationships likely help. Animals know who to avoid and who they can safely cooperate with.”

Hundreds of hours of observations

The researchers studied macaques in several zoos in the Netherlands, Belgium and Austria, including Affenberg Landskron, Apenheul Primate Park, Artis, Diergaarde Blijdorp, Gaia Zoo, and Zoo Planckendael. In one experiment, animals could only obtain a food reward if they worked together. For example, they had to pull two ends of a rope at the same time.

The team also measured how willing the animals were to share food and how tolerant they were when feeding in close proximity. In total, the researchers collected nearly five hundred hours of behavioural observations.

Virtual monkeys reveal long-term patterns

To better understand how social relationships develop over time, the researchers also used computer simulations. These simulations recreate a virtual monkey society in which individuals interact according to predefined rules.

Using computer models of monkey societies, we can simulate processes that would take years to observe in real life.

“Using these computer models, we can simulate processes that would take years to observe in real life,” says Massen. “You can see how relationships form and how stable they remain.”

The simulations confirmed the experimental findings: hierarchical societies produce fewer social bonds, but they are stronger and more stable. And these stable relationships are key to successful cooperation.

What does this mean for humans?

The findings naturally raise questions about human societies and organisations. While parallels are tempting, the researchers urge caution.

This show us that a certain level of structure and clarity can support cooperation.

“This does not mean that hierarchical societies are inherently better, not for humans either,” says Massen. “But it does show that a certain level of structure and clarity can support cooperation. It also helps us understand how complex cooperation may have evolved.”

According to Massen, similar patterns can be seen in organisations, where clear roles and responsibilities can improve collaboration. At the same time, human societies are shaped by many additional factors, such as culture, norms and individual choices.

“We study mechanisms that can support cooperation and how they may have evolved,” Massen concludes. “But how this translates to modern human societies is always more complex.”