Chimp markets reveal evolution of friendship
Chimp markets reveal evolution of friendship
What was the basis for the earliest friendships? If wild chimps are any guide: support in a fight, borrowing a valued tool, and a bite to eat now and then. Quite similar to our friendships today, in fact. Indeed, some chimps are so modern they have relationships that we would classify as friends with benefits.
Primatologists are reassessing the complexity of chimpanzee society in the light of new findings that also suggest answers to a long-standing question: why share things with non-relatives?
For the first time wild chimps in Senegal have been observed taking plant foods and tools from other chimps, who don't react to the intrusion. The chimps donating their stuff don't get paid, but neither do they protest. Instead, the trade appears to help build social cohesion.
What's more, in another west African study, this time in Ivory Coast, a "market" has been described where chimps exchange commodities in the shape of both social behaviours including grooming and sex, and resources such as meat.
Christophe Boesch of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, says we have only recently begun to appreciate the time and energy chimps invest in reciprocal relationships, and he compares chimp relationships to friendship. "These findings have prompted primatologists to use some terms that have in the past been reserved for humans."
Jill Pruetz of Iowa State University in Ames and colleague Stacy Lindshield documented a kind of proto-trade in savannah chimps (Pan troglodytes verus) living in the Fongoli area of south-east Senegal. They witnessed 41 transfers of plant foods and tools. Chimps are known to share meat, but this is the first study to document them sharing other types of goods (Primates, DOI: 10.1007/s10329-011-0287-x).
Food or tools were transferred from males to females 27 times. In most cases this was the result of a female simply taking the item and the male doing nothing to stop her. In other chimp populations males may lash out in this situation but at Fongoli males outnumber females and have to be nice to them if they want to have sex later.
Pruetz suspects that item transfer is a social lubricant. "It seems like the ulterior motive is social group harmony on some level," she says.
If a male is transferring goods to another male, then Pruetz predicts that the male will expect support from the recipient of his largesse in any future aggressive encounter with other males. If a male shares with a female he is likely to expect sexual benefits from her. "But other age-sex classes were also involved," says Pruetz, "and I think this reflects the cohesive nature of this chimp community."
In the rainforest of Tai national park in Ivory Coast, Boesch and colleague Cristina Gomes have analysed enough data from their observations of 44 chimps over 22 months to conclude that the apes really are trading commodities. They conclude that male chimps do indeed swap meat in return for sex - something that has been disputed in the past - and for support in aggressive encounters. Grooming was perhaps the lowest value commodity, only exchanged for reciprocal grooming (Behavioural Ecology and Sociobiology, DOI: 10.1007/s00265-011-1227-x).
"Social interactions are not random in a chimpanzee society," says Boesch. "Much of it is based on memory and long-term relationships including different commodities."
Adrian Jaeggi of the University of California, Santa Barbara, has studied sharing between various primate species. "The observed exchanges resemble human friendships, in which we don't keep detailed track of what we owe each other," he says.
Unusual behaviours displayed by the Fongoli chimps, who shelter in caves and even hunt other animals with spears, can be attributed to their savannah mosaic environment, says Pruetz. Fongoli is made up of a patchy mix of woodland, grassland and bamboo habitats. To cope with temperatures that can climb over 40 °C the chimps like to retreat to the cool, shady caves.
"The fact that the chimps transport food for consumption in shady areas probably creates more opportunities for scrounging," says Jaeggi.
Not withstanding the trading that goes on in the rainforest environment of Tai, this behaviour suggests mosaic savannah - the kind of habitat where humans likely evolved - played an important role in driving the evolution of the ape social system.
"Early hominids living in savannah environments would most likely have had a similar social system characterised by stable long-term relationships and might thus also have profited from food transfers," says Jaeggi. When early humans hunted large prey it cost them little to share their bounty. Even the hungriest caveman couldn't eat a whole antelope on his own but sharing created a sense of obligation in the recipients of his bounty.