Even in an extremely hierarchical monkey society, individuals help each other

Some individuals in an extremely hierarchical society of monkeys help others by providing them with food, without receiving a direct reward themselves. As this is the first time that such ‘prosocial’ behaviour is seen in a species that is not considered socially peaceful or tolerant, these results have consequences for the understanding of the evolution of cooperative behaviours. An international group of researchers, including Debottam Bhattacharjee and Jorg Massen of Utrecht University, made the discovery by testing a large group of Japanese macaques. The findings were published yesterday in the scientific journal iScience.

We have a general tendency to help others, for example our friends, relatives, and even strangers. This is not a uniquely human trait, as several other species like marmosets, bonobos and azure-winged magpies, are known to have such helping or ‘prosocial’ tendencies. Individuals with prosocial tendencies can improve the well-being of others.

These extremely hierarchical monkeys may well use this behaviour to ensure coalitionary support in the political games they play.

Extremely hierarchical

It has been argued that prosociality is restricted to the more socially tolerant species that have become less aggressive and more socially tolerant via domestication or ‘self’domestication (dogs and humans, for instance), or cooperatively breeding species where group members other than the parents also take care of the young (for example, marmosets).

However, Japanese macaques live in extremely hierarchical societies, where higher-ranking animals are much more dominant and where certain group members clearly have more power than others. “In line with previous research, one should not expect these macaques to have prosocial preferences. The complexity of a large group, like the one we tested, could however facilitate selective helping by individuals to specific group members, like relatives and friends,” says Bhattacharjee.

Experiment

The group of Japanese macaque that was studied lives ‘semi-free’ in a large enclosed, naturally forested area. By using a simple seesaw food provision mechanism, the researchers tested whether individuals within this group proactively provision food to group members: by moving a lever, one animal allowed a group member that was a short distance away to obtain some peanuts. Due to the design of the testing apparatus, a ‘helping’ individual could never benefit from the experiment by obtaining a food reward itself.

It was found that 9 out of the 25 apes that voluntarily approached the testing apparatus had clear prosocial preferences, meaning they provided food to group members. These individuals provisioned food in almost 69 percent of the trials, which is comparable to the more socially peaceful species like the marmosets and azure-winged magpies of which prosociality was already known. Japanese macaques are neither self-domesticated nor cooperatively breeding, yet the evidence of prosocial preferences among them is unprecedented. The researchers do note, however, that the provisions of the Japanese macaques were almost exclusively restricted to close relatives and to group members with whom the helpers had friendly relationships.  

Current evidence seems to be in favor of an evolutionary framework where prosociality arises when individuals depend heavily on the collaboration of others.

Evolution of cooperative behaviours

The findings of the study are important concerning our understanding of the evolution of cooperative behaviours. “Whereas previous results suggested that prosociality was restricted to socially tolerant species, and that its evolution may have been due to selection for generally nice individuals, the current results show prosociality among these extremely hierarchical monkeys which may well use this behaviour to ensure coalitionary support in the political games they play in such a society", says Massen. He continues, “Therefore, current evidence seems to be more in favor of an evolutionary framework where prosociality arises when individuals depend heavily on the collaboration of others, be it for the sake of the whole group, as is the case in the self-domesticated and cooperative breeding species, or for ones own political interests.”

At the same time, Massen and colleagues acknowledge that more studies are needed. Massen: “Our study shows that research on prosociality should not be limited to only tolerant animal societies, but that it would be important to include other hierarchical species, or humans in hierarchical systems.”