Chimpanzees have at least one thing in common with the late US president Richard Nixon: tracking those who do them favours and putting those who don鈥檛 come up to scratch on an 鈥渆nemies list鈥�.
Nearly 3000 hours of observations of wild chimpanzees show that they keep tabs on which of the troop has groomed them the most 鈥� returning the favour to frequent groomers, while freezing out the selfish ones.
In a way, grooming works like currency in chimpanzees, says , a behavioural ecologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. 鈥淚f you don鈥檛 have a set price, then you鈥檙e susceptible to being cheated and cooperation would probably break down.鈥�
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Among chimpanzees, grooming seems to be a hygienic practice to pluck parasites off fur, as well as a social glue between related and unrelated apes, she says.
Payback time
Gomes spent several years recording the daily behaviour of 44 chimps living in Ta茂 National Park in C么te D鈥橧voire. Each day, she tracked an individual chimpanzee, recording whom it groomed, who groomed it, as well as the length of each session on a handheld computer. She eventually accrued 87 hours of grooming.
More than two-thirds of the sessions were not immediately reciprocal, with animals foregoing a 鈥測ou scratch my back, I鈥檒l scratch yours approach鈥�. Even within a single day, many debts remained unsettled, Gomes found.
However, when she and colleagues Roger Mundry and took a longer view of their data 鈥� at least a week, but sometimes much longer 鈥� a tit-for-tat payback system emerged. 鈥淢ost of their reciprocity is going on over a longer period of time,鈥� she says.
Her team applied a statistical model to rule out fluke effects that could be due to sex, hierarchy, age, and friendship. 鈥淓verything I could come up with, I included,鈥� Gomes says. She says the only way to explain the symmetry of grooming exchanges between pairs over time is through reciprocity.
Bonds of friendship
However, this is not to say that chimpanzees keep a mental balance sheet of grooms received and tendered. 鈥淚t does not necessarily have to be a cognitive process,鈥� she says, 鈥渋t could emotional.鈥�
Hormones, she argues, are a likelier explanation for the behaviour of chimpanzees 鈥� and the same could apply to humans. By finely tweaking the levels of endorphins, primates could learn to associate generosity with some animals and meanness with others.
鈥淚 think that this is the basis of human friendships,鈥� she adds.
Take a situation familiar to many: buying a round in the bar. A show of generosity from the first person to buy a round could instil trust and bonhomie in others. 鈥淢ost people are thinking, I鈥檓 going to buy a round for everybody because it鈥檚 a nice thing to do,鈥� Gomes says.
, a primatologist at the University of California, Los Angeles, who has observed long-term reciprocity in female baboons, says the new study cast doubt on market-based models of primate reciprocity that ignore long-term payoffs.
She also hopes others will take on the task of determining whether such generosity is emotional, as Gomes suspects, or more calculated, as in Richard Nixon鈥檚 case.
Journal reference: (DOI: 10.1098/rspb.2008.1324)
