Across the animal kingdom, individuals face choices between patience and impulsivity. A classic case, confronted by all animals--humans included--is that between a small, immediate food reward and a delayed, but larger, reward. In such cases, impulsivity typically trumps patience as individuals fail to delay gratification. But what factors influence these decisions? Researchers have gained new insight into this question by showing that the particular ways in which animals exhibit patience and impulsivity differ from one context to another and may be closely related to the animals' ecological niches and their everyday interactions with the natural world.
Comparing two monkey species with very different food-gathering strategies, the researchers show that the species exhibit differing propensities toward patience and impulsivity, depending on the context of the choice being made--for example, whether the trade-off for reward is temporal (waiting for a reward) or spatial (traveling for a reward). The work is reported by Jeffrey Stevens, Marc Hauser, and colleagues at Harvard University.
Earlier work by others had demonstrated that two species of New World monkeys--cotton-top tamarins (Saguinus oedipus) and common marmosets (Callithrix jacchus)--behave impulsively, only waiting between eight and twenty seconds to receive a threefold increase in food reward. Those results also showed a significant species difference: Marmosets waited almost twice as long as tamarins for the larger reward. This pattern is consistent with each species' foraging adaptations: Marmosets primarily feed on tree exudates, a food that requires the patience to wait for sap to exude, whereas tamarins feed on insects, a food that requires quick, impulsive action. Yet it had been unclear whether this impulsivity depended on context.
In the new work, the researchers tested whether these same two monkey species act impulsively "over space" as well as over time. The scientists found that when faced with a choice between a smaller, nearby reward and a larger, more distant reward, tamarins were willing to travel farther than marmosets--therefore, tamarins act more impulsively over time, but marmosets act more impulsively over space. Like the temporal impulsivity data, the new findings parallel details of each species' ecology. Tamarins range over large distances to feed on insects, whereas marmosets range over shorter distances to feed on tree exudates, a clumped resource. These results show that impulsivity is context specific, shaped by a history of ecological pressures.
The researchers included Jeffrey R. Stevens, Alexandra G. Rosati, Kathryn R. Ross and Marc D. Hauser of the Primate Cognitive Neuroscience Laboratory at Harvard University in Cambridge, Massachusetts. This research was supported by a National Research Service Award of the NIH, the Harvard College Research Program and an NSF-ROLE grant.
Stevens et al.: "Will travel for food: spatial discounting in two New World monkeys." Publishing in Current Biology Vol 15, pages 1855-1860, October 25, 2005, DOI 10.1016/j.cub.2005.09.016 www.current-biology.com
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