10th November 2011, 10:22am
Monkeys bully subordinates into grooming them, study finds
Aggression in Barbary macaque society Monkeys use violence to bully subordinates into grooming them and will launch repeated attacks if their demands are not immediately met, researchers have found.

A study of the Barbary macaque (Macaca sylvanus) revealed evidence that in the wild dominant individuals use aggression tactically against weaker group members to elicit favourable treatment.

Coercion of favours through the threat of violence is a common feature of human societies, but is rarely observed in the animal kingdom, where aggression within groups tends to occur when animals are competing for food or mating opportunities.

The study by Dr Richard McFarland and Dr Bonaventura Majolo from the School of Psychology at the University of Lincoln, UK, is one of the first to provide evidence of coercion in animal societies.

Dr McFarland said: "Aggression is commonplace in macaque societies, as group members often compete over food and mating partners. However, in our study we also found that dominant individuals use aggression as a direct means to gain grooming. Grooming is an important behaviour in primates. It helps to clean the fur from parasites and dirt and also has a stress-releasing effect. Therefore, the capacity to use social tactics to access grooming is likely to be strongly selected for."

The Barbary macaque is a rare and threatened species of primate native to the Middle Atlas Mountains of North Africa. An international team of academics, led by Dr Majolo, has been gathering data on the social behaviour and ecology of two wild groups of monkeys from a field site near the Moroccan city of Azrou since 2008.

Their latest findings are published in the online biology journal PLoS ONE.

The research team observed dominant macaques would attack subordinates, and if their victims failed to groom the aggressor immediately, they would attack again.

The greater the difference in dominance between individual macaques, the greater the occurrence of coercion. While receiving aggression could be costly for the subordinate victim - resulting in heightened anxiety, risk of renewed aggression and relationship damage - there appeared to be no costs for the dominant aggressor.

The researchers also observed that when grooming was successfully coerced, subordinates were less at risk of receiving further aggression. They concluded that grooming coercion could be considered a trading of social services, as both animals obtained some benefits: grooming for the aggressor and reduced risk of aggression for the victim.

Dr Majolo said: "Conclusive evidence for coercion in animal societies is scarce, whereas it is a well-documented phenomenon in humans. Our findings demonstrate that monkeys employ social strategies, including use of aggression, to coerce favourable services. This type of comparative data on non-humans is essential to further understanding of the evolutionary origin of inequalities, coercion and punishment in human societies."

The paper 'Grooming coercion and the post-conflict trading of social services in wild Barbary macaques' by McFarland R and Majolo B (2011) was published in PLoS ONE 6(10): e26893. doi:10.1371/journal.pone.0026893. To view the paper in full, visit: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0026893