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[Josh Greenman <josh.greenman AT gmail.com>]

http://www.npr.org/blogs/money/2009/10/podcast_economics_for_monkeys.html?ft=1&f=93559255

Podcast: Economics For Monkeys

On Planet Money:

Human beings aren't the only creatures who make economic decisions. It
turns out that monkeys do it, too. Scientists have observed our
primate kin exchanging goods and services and adjusting prices.

Ronald Noe, a professor of primate ethology at the University of
Strasbourg, says the vervet monkey of southern and eastern Africa uses
grooming as a kind of currency. They determine the value of food
providers and divide their attention according to the law of supply
and demand.