Female monkeys rule the roost in groups with more males

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Washington, July 16 : Female monkeys are more dominant in groups with relatively more males, says a new research by researchers at the University of Groningen.

The study, which will be published in the open-access journal PLoS ONE, suggests that such behaviour is caused by self-organisation.

Researchers used a computer model for conducting the study.

Many animals living in groups have a social hierarchy, a so-called 'pecking order'. Monkeys, too, have a social hierarchy. Highest in the pecking order is the most dominant monkey, who consistently wins aggressive interactions (such as biting) with other group members, the researchers said.

At the bottom of the hierarchy is the lowest-ranking monkey, who consistently loses interactions with other members of the group. Monkeys have to fight for their place in this hierarchy every day, they added.

The scientists said that the position of females in the hierarchy varies among different monkey species. In most species females are ranking below the males. This is no wonder, because they are usually much smaller than males.

However, in the case of the Lemur species of Madagascar the females are dominant, in bonobos, males and females roughly equal each other in dominance, and among a lot of other species (macaques and the grivet, for instance) females are weakly dominant, they added.

"This means that the most dominant females rank above approximately a third of the males," says Charlotte Hemelrijk, theoretical biologist at the University of Groningen and the first author of the article.

The computer model predicted females to be more dominant in a group with a relatively large number of males.

To verify this prediction, the researchers analyzed data of aggression of a large amount of literature in which primate behaviour is described in order to calculate for the first time female-dominance among many different groups and monkey species.

Their analysis showed the predictions of the computer model to be accurate.

As for the reason behind such kind of a behaviour, Hemelrijk said: "Male aggression is more intense than that of females. In groups with more males, males are more often defeated by other males.

"Consequently, high-ranking females may be victorious over these losers. Furthermore, the presence of more males in the group leads to more interactions between males and females, causing more chance winnings by females. Through a self-reinforcing effect, these females will go on to win more frequently in later interactions and grow more dominant."

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