Delayed gratification

Natural History, Feb, 2006 by Nick W. Atkinson

Given the choice, many animals prefer a small, immediate reward to a larger one in the future. Both common marmosets and cotton-top tamarins, two species of South American monkey, fit that profile--though a marmoset will wait quite a bit longer than a tamarin will. Such behavior actually makes good sense in the wild: a monkey that waits too long risks losing its reward altogether. The food could be snatched up by a competitor, spoiled in the heat, or blown away by the wind.

Now a team of Harvard University primatologists, led by Jeffrey R. Stevens, has discovered a twist to this tale. If the rewards are separated by distance instead of time, cotton-top tamarins switch their preference. Tamarins faced with either a small food reward nearby or a larger one farther away, choose to make the longer trip, even though it takes more time. Common marmosets, however, are less willing to travel for food--they will wait, but they won't walk.

Stevens and his team suggest the reason for the unexpected reversal arises from differences between the two species' foraging habits: tamarins range over great distances to find insects, whereas marmosets rely on more predictable, localized food sources, such as tree sap. (Current Biology 15:1855-60, 2005)