Source Jim Devine
Date 10/10/18/14:14

Monkeys Follow Economic Rules Of Supply And Demand

ScienceDaily (Sep. 8, 2009) — A monkey that has acquired the sole
power to hand out apples is generously rewarded with grooming sessions
by the other monkeys in its group. But as soon as another monkey can
hand out apples as well, the market value of the first monkey is

The monkeys therefore unerringly obey the law of supply and demand.
Dutch-sponsored researchers Ronald Noë, Cécile Fruteau and Eric van
Damme demonstrated this in their article that was published online on
7 July by the journal PNAS (Proceedings of the National Academy of

Cécile Fruteau placed food containers with highly-desired pieces of
apple in two groups of South African vervet monkeys. For the monkeys
there was just one problem: only one in each group could open the food
container. This monkey had a low position in the rank order and was
therefore scarcely groomed. However, as soon as she acquired the power
to hand out apples she was valued more and was groomed a lot by the
rest of the group. Yet she could only enjoy that privilege briefly;
the researchers placed a second food container that could be opened by
another low-ranking female. From that moment onwards the market value
of the first monkey was halved, and she was therefore groomed half as

Long-term planning

The experiments revealed that the female monkeys that could open the
food containers were groomed more than when they exerted no power over
the food production. The females concerned also did not have to groom
the other monkeys as long. They were therefore paid for their services
as food suppliers. Biological market theory predicts that the market
value of these female monkeys should vary according to the law of
supply and demand. The fact that the grooming time of the first monkey
was halved as soon as the second monkey gained the power to distribute
apples, confirms this idea; the price of goods - in this case the
female monkeys who could open the containers - was instantaneously
adjusted to the market.

Immediately after the opening of the food containers, the researchers
registered how long the females were groomed for. The next occasion on
which the females could open a container was, however, several days
later. The fact that the females were still groomed more indicates
that the vervet monkeys apply a strategy that works in the long term.
The choice of partners is also influenced by long-term attitudes; the
monkeys can value one monkey relatively more than the others.

A change in price - grooming for less long if there is another monkey
that supplies apples - is only possible if a negotiation process takes
place. Many economists assume that such negotiations can only take
place if they are concluded with a contract. However, the vervet
monkeys do not have the possibility to conclude such binding contracts
and yet they still succeed in agreeing to a change in price for a

The research of Eric van Damme (Tilburg University) and Ronald Noë
(University of Strasbourg) is part of the Evolution and Behaviour
research programme of the NWO Division for Social Sciences.

Story Source:
The above story is reprinted (with editorial adaptations by
ScienceDaily staff) from materials provided by NWO (Netherlands
Organization for Scientific Research).Journal Reference:
Fruteau et al. Supply and demand determine the market value of food
providers in wild vervet monkeys. Proceedings of the National Academy
of Sciences, 2009; 106 (29): 12007 DOI: 10.1073/pnas.0812280106

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