Markets Explain Cooperation in the Animal World

A biological theory suggests that the reason animals don't always kill each is economics.

Cooperation was common in nature—not just between animals of the same species but also between different species (for example, a plant and its pollinator). But the origins of cooperation were a mystery. How could two animals work together when Darwin’s theory of evolution taught about survival of the fittest? Shouldn’t natural selection always favor ruthless self-interest?

“It was one of the early questions in behavioral biology,” says Hammerstein. “Why do animals not always kill each other? Why is aggression limited?”...

In 1994, Noë and Hammerstein laid out their new theory of biological markets in the journal Behavioral Ecology & Socialbiology. The paper fused the biologists’ different styles: Hammerstein developed the mathematical models, while Noë dug through the scientific literature for evidence from the field. Examples turned up across the animal kingdom. Male scorpion flies offer females a “nuptial gift” of prey before mating. In some species of bird, such as the purple martin, a male will allow another male to occupy part of his territory in exchange for help raising his young. Lycaenid butterfly caterpillars produce a sweet “nectar” whose only purpose is to attract ants, which eat the nectar and protect the caterpillars from predators.

In each example the “exchange rate” is not fixed but rather contingent on the supply of available partners. “It is essentially a supply-demand theory,” says Frans de Waal, the eminent primatologist from Emory University and a former mentor of Noë. The more male scorpion flies available on the market, the larger the nuptial gift the female will demand. The male purple martin chooses the most juvenile-looking and least threatening tenant. And the caterpillars adjust the amount of nectar they produce to the number of ants in the vicinity.

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