Simian Oeconomicus

Economics is usually thought of as a specifically human science. However, there are no reasons to restrict its application to humans. After all, animals have goals and preferences exactly like humans. And exactly like humans, they face the sad reality that resources are scarse (be it sex, power or food). These basic observations plead in favour of more integration between economics and ecology. This is exactly what Ronald Noe (with Peter Hammerstein) has advocated for many years by promoting the idea of market in biology. In a forthcoming issue of PNAS, Cécile Fruteau, Bernhard Voelkl, Eric van Damme, and Ronald Noe¨ offer an empirical demonstration of the existence of market among primate societies. They show that "Supply and demand determine the market value of food providers in wild vervet monkeys" (the full article is freely available here).

Previous studies showed that primates pay more when commodities become scarcer: subordinates groomed dominants longer before being tolerated at food sites in periods of shortage; females groomed mothers longer before obtaining permission to handle their infants when there were fewer newborns and males groomed fertile females longer before obtaining their compliance when fewer such females were present.

Freteau et al. have demonstrated experimentally how supply and demand determine the value of food providers:



"We further substantiated these results by conducting a 2-step experiment in 2 groups of
free-ranging vervet monkeys in the Loskop Dam Nature Reserve, South Africa. We first allowed a single low-ranking female to repeatedly provide food to her entire group by triggering the opening of a container and measured grooming bouts involving this female in the hour after she made the reward available. We then measured the shifts in grooming patterns after we added a second food container that could be opened by another low-ranking female, the second provider. All 4 providers received more grooming, relative to the amount of grooming they provided themselves."






Mean estimates of the grooming ratios for the food provider. Schematic representation of the 3 experimental phases. Probability density (PD) functions are shown of mean estimates for grooming ratios of nonproviders (blue), first provider (orange), and second provider (red) in phase 0 with no provider (E and H, the two providers have negative ratios because they are low-ranking), phase1 with one provider (F and I: the first provider is much more groomed by others), and phase 2 with 2 providers (G and J: the value of the first provider decrease as the supply of food increase via the second provider).

They found that grooming ratios shifted to the advantage of female vervets that produced food bonanzas. The crucial result, however, lies in the quantitative shifts of the exchange rate between grooming and providing food: the positive effect on the grooming ratios of the first providers was roughly half as strong after we added a second provider in each study group, confirming a central tenet of biological market theory.

In others words, an "invisible hand" had changed the value of the food!


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