The deal is straightforward. In return for some careful grooming, a female baboon is allowed to cuddle another's infant until the mother asks for its return, writes Steve Farrar.
The exchange rate varies and when infants are scarce, the amount of grooming required to secure the deal rises accordingly. Furthermore, a chance to hold the infant of a high-ranking baboon commands a higher price than that of a lower-ranked animal.
The baboons seem to know the rules and keep a careful eye on the fluctuating values of their services.
The primologists who have observed this behaviour have interpreted it as solid evidence for a biological market, where animals exchange commodities to the mutual benefit of both parties.
The concept of biological markets was first proposed as a way for animals to cooperate without the risk of being cheated by the French zoologist Ronald No in 1994. It arises when animals choose their trading partners and can vary the "price" they are prepared to pay for a commodity.
Peter Henzi, professor of zoology at the Bolton Institute of Higher Education, and Louise Barrett, a lecturer in biology at Liverpool University, have studied two troops of baboons in South Africa to find evidence to support this hypothesis.
Female baboons find young infants very attractive. Yet mothers are reluctant to part with their offspring as they might fall into the hands of a male baboon and be killed.
"The females have to pay to get hold of these infants," Professor Henzi said.
Grooming is the ideal commodity to exchange, but as it is time consuming, females pay only as much as they need to and adjust their effort to match infant availability.
"The fact that mothers tolerate these fluctuations in grooming time suggests they are sensitive to the price their infants fetch in the marketplace," Professor Henzi said.
The findings are published in the journal Animal Behaviour .