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Friday, November 19, 2010

Friendship Among Macaques

From the
Hanging Out, Grooming, Fighting Foes: Friendship Among Macaques

The human tendency to form close bonds with people other than kin may have primal roots. Researchers from Germany report in the journal Current Biology that male macaques exhibit a social bonding behavior similar to human friendship.

Macaque monkeys live in groups of 50 to 60, but “every male in the group has a few other males he interacts with more than others,” said Oliver Schülke, the study’s lead author and an evolutionary biologist at the University of Göttingen.

Dr. Schülke and his colleagues studied male Assamese macaques in Thailand over a period of five years and monitored their behavior. Macaques that spent a lot of time within 1.5 meters of each other were considered friends, since it is easy to attack another macaque at this distance. Males that groomed each other’s bodies frequently and for excessive periods of time were also considered friends. Often, they groomed areas that an individual could groom himself. “The grooming seems to work to foster these bonds,” Dr. Schülke said. “The hygiene aspect was only one part of it.” The bonds can lead to the forming of coalitions, where a group of males might fight another male to improve rank and social status, the researchers found. “The interesting thing is that these coalitions can help pull up low-ranking individuals and help high-ranking males stay where they are,” Dr. Schülke said. “Both things are going on at the same time.”

It appears that, just as in humans, some friendships were long lasting whereas others broke up after a short time. Why this happens is still unclear.

It was previously known that female macaques form strong social bonds, but these bonds tend to be with kin. Females prefer to form close relationships with their mothers, sisters and daughters.

Schülke O, Bhagavatula J, Vigilant L, Ostner J (2010)Social Bonds Enhance Reproductive Success in Male Macaques. Current Biology. doi: 10.1016/j.cub.2010.10.058

  • Males act politically when manipulating their own and others' rank
  • High- and low-ranking coalition partners benefit by maintaining and enhancing status
  • The strength, rather than the number, of social ties affects males' siring success
For animals living in mixed-sex social groups, females who form strong social bonds with other females live longer and have higher offspring survival [1,2,3]. These bonds are highly nepotistic, but sometimes strong bonds may also occur between unrelated females if kin are rare [2,3] and even among postdispersal unrelated females in chimpanzees and horses [4,5]. Because of fundamental differences between the resources that limit reproductive success in females (food and safety) and males (fertilizations), it has been predicted that bonding among males should be rare and found only for kin and among philopatric males [6] like chimpanzees [7,8,9]. We studied social bonds among dispersing male Assamese macaques (Macaca assamensis) to see whether males in multimale groups form differentiated social bonds and whether and how males derive fitness benefits from close bonds. We found that strong bonds were linked to coalition formation, which in turn predicted future social dominance, which influenced paternity success. The strength of males' social bonds was directly linked to the number of offspring they sired. Our results show that differentiated social relationships exert an important influence on the breeding success of both sexes that transcends contrasts in relatedness.

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