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Comparative Study
, 1 (2), 223-6

The Ecology and Evolution of Patience in Two New World Monkeys

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Comparative Study

The Ecology and Evolution of Patience in Two New World Monkeys

Jeffrey R Stevens et al. Biol Lett.

Abstract

Decision making often involves choosing between small, short-term rewards and large, long-term rewards. All animals, humans included, discount future rewards--the present value of delayed rewards is viewed as less than the value of immediate rewards. Despite its ubiquity, there exists considerable but unexplained variation between species in their capacity to wait for rewards--that is, to exert patience or self-control. Using two closely related primates--common marmosets (Callithrix jacchus) and cotton-top tamarins (Saguinus oedipus)--we uncover a variable that may explain differences in how species discount future rewards. Both species faced a self-control paradigm in which individuals chose between taking an immediate small reward and waiting a variable amount of time for a large reward. Under these conditions, marmosets waited significantly longer for food than tamarins. This difference cannot be explained by life history, social behaviour or brain size. It can, however, be explained by feeding ecology: marmosets rely on gum, a food product acquired by waiting for exudate to flow from trees, whereas tamarins feed on insects, a food product requiring impulsive action. Foraging ecology, therefore, may provide a selective pressure for the evolution of self-control.

Figures

Figure 1
Figure 1
Experimental apparatus for discounting procedure. Both marmosets and tamarins experience the same choices: pulling one of two tools. One tool yielded two food pellets after no delay, and the other tool yielded six food pellets after a longer delay. Transparent Plexiglas covers prevented access to the pellets until the delay expired.
Figure 2
Figure 2
Species differences in discounting levels. Marmosets and tamarins differed in their indifference points with marmosets waiting almost twice as long for the six pellets than tamarins. The tamarin indifference point does not differ from that expected, but marmosets exhibit more self-control than predicted by short-term rate maximization. Error bars represent standard error of the mean indifference points.

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