Will travel for food: spatial discounting in two new world monkeys

Abstract

Nonhuman animals steeply discount the future, showing a preference for small, immediate over large, delayed rewards. Currently unclear is whether discounting functions depend on context. Here, we examine the effects of spatial context on discounting in cotton-top tamarins (Saguinus oedipus) and common marmosets (Callithrix jacchus), species known to differ in temporal discounting. We presented subjects with a choice between small, nearby rewards and large, distant rewards. Tamarins traveled farther for the large reward than marmosets, attending to the ratio of reward differences rather than their absolute values. This species difference contrasts with performance on a temporal task in which marmosets waited longer than tamarins for the large reward. These comparative data indicate that context influences choice behavior, with the strongest effect seen in marmosets who discounted more steeply over space than over time. These findings parallel details of each species' feeding ecology. Tamarins range over large distances and feed primarily on insects, which requires using quick, impulsive action. Marmosets range over shorter distances than tamarins and feed primarily on tree exudates, a clumped resource that requires patience to wait for sap to exude. These results show that discounting functions are context specific, shaped by a history of ecological pressures.

Figure 1:. Experimental apparatus

(A) Both tamarins and marmosets traveled to receive their rewards in a Plexiglas enclosure. (B) The food rewards were lined up in an array on a ledge in the box, each piece approximately 1 cm apart. (C) Food boxes were placed at one of seven distances (35–245 cm) from the front of the enclosure. A wall was placed behind the far box.

Figure 2:. Effect of species and distance on choice

Tamarins maintained their preference for the large reward across all distances, whereas marmosets reduced their preference for large as the distance to large increased. Error bars represent standard error of the mean.

Figure 3:. Effect of species and distance on running time

Tamarins and marmosets took the same amount of time to travel to the short distance (increment 1; 35 cm). Tamarins, however, traveled to the farthest distance (increment 7; 245 cm) significantly faster than marmosets. Error bars represent standard error of the mean.

Figure 4:. Effect of species and reward magnitude on choice

Neither tamarins nor marmosets altered their preferences for the larger reward when choosing between one and three pellets or between two and six pellets. Therefore, they maintained their preferences for the same ratio of rewards but ignored absolute magnitude. Error bars represent standard error of the mean.