The National Academy of Science published a new paper in the September issue of the Proceedings of the National Academy of Sciences (PNAS). The paper observes the supply and demand in the food distribution among vervet monkeys. See excerpt:
Supply and demand determine the market value of food providers in wild vervet monkeys
Cecile Fruteau, Bernhard Voelkl, Eric van Dammea, and Ronald Noe
Center for Economic Research, University of Tilburg, 5000 LE Tilburg, The Netherlands; Ethologie des Primates, Departement Ecologie, Physiologie et Ethologie, Institut Pluridisciplinaire Hubert Curien, Centre National de la Recherche cientifique Unite Mixte de Recherche 7178, 67087 Strasbourg Cedex 2, France; Tilburg Law and Economics Center, University of Tilburg, 5000 LE Tilburg, The Netherlands; and Applied Behavioural Ecology Research Unit, School of Environmental Sciences, University of South Africa, Private Bag X6, Florida 1710, South Africa
Edited by Frans B. M. de Waal, Emory University, Atlanta, GA, and approved June 1, 2009 (received for review December 3, 2008)
Animals neither negotiate verbally nor conclude binding contracts, but nevertheless regularly exchange goods and services without overt coercion and manage to arrive at agreements over exchange rates. Biological market theory predicts that such exchange rates fluctuate according to the law of supply and demand. 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.
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 lowranking female, the second provider. All 4 providers received more grooming, relative to the amount of grooming they provided themselves. As biological market theory predicts, the initial gain of first providers was partially lost again after the introduction of a second provider in both groups. We conclude that grooming was fine-tuned to changes in the value of these females as social partners.
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