Under the vivid green canopies of the tropics, a young gorilla sneaks up behind another, yanks its hair, and dashes away with a toothy grin on its face. It may seem like harmless fun, but this game of tag has profound implications. In a new study, researchers say the behavior indicates that gorillas know the limits of their social status—and that they play tag to help even the score.
Other studies have shown that nonhumans can sense unfairness. In 2005, for example, a group led by psychologist Sarah Brosnan of Yerkes National Primate Research Center in Atlanta reported that capuchin monkeys refused to exchange tokens with an experimenter for a cucumber if they saw a fellow monkey receiving a more desirable grape for its "money." But do animals also sense unfairness in more natural settings?
To find out, behavioral biologist Marina Davila Ross of the University of Portsmouth in Hampshire, U.K., and colleagues watched videos collected over 3 years of gorillas at various zoos and reserves in Germany and Switzerland. Almost every afternoon, a couple of the gorillas would begin wrestling with each other. In some instances, gorillas hit their playmates and ran away. Most gorillas seemed to play this game of tag, though older mothers observed from the sidelines.
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Ross's team noticed a pattern in the play: Gorillas lower on the social ladder were usually the taggers. These gorillas were also twice as likely to instigate another round of the game, and they frequently bared their teeth—a possible indication that they were willing to bite the other gorilla.
The findings, Ross says, suggest that low-status gorillas use the game as a sort of ego boost. They can hit a high-status individual without repercussions, she says, and that gives them a feeling of superiority, even if it's only temporary. And that means that gorillas are aware of inequities in their society, Ross says, marking the first time that such cognition has been observed in gorillas in a nonexperimental setting. The researchers will report  their findings online tomorrow in Biology Letters.
Brosnan says the findings could shed light on the evolution of competitiveness in humans. Renowned primatologist Frans de Waal of Emory University in Atlanta agrees. "There's a real feeling that gorillas have [to get the upper hand]," he says, "and humans have it too."