Chimpanzees (Pan troglodytes) tolerate some degree of inequity while cooperating but refuse to donate effort for nothing

Abstract

In cooperative hunting, a carcass cannot be divided equally, and hunts may be unsuccessful. We studied how chimpanzees respond to these two variables, working for unequal rewards and no rewards, which have been rarely included in experimental cooperative tasks. We presented chimpanzees with a task requiring three chimpanzees to work together and varied the reward structure in two separate experiments. In Experiment 1, two individuals received more rewards than the third, making the outcome unequal. We wanted to know if cooperation would continue or break down, and what mechanisms might maintain performance. Experiment 2 used equal rewards, but this time one or more locations were left unbaited on a proportion of trials. Thus, there was a chance of individuals working to receive nothing. In Experiment 1, the chimpanzees worked at a high rate, tolerating the unequal outcomes, with rank appearing to determine who got access to the higher-value locations. However, equal outcomes (used as a control) enhanced cooperative performance, most likely through motivational processes rather than the absence of inequity aversion. In Experiment 2, performance dropped off dramatically when the chimpanzees were not rewarded on every trial. Their strategy was irrational as donating effort would have led to more rewards in the long run for each individual. Our results lead to a hierarchy of performances by condition with equity > inequity > donating effort. Chimpanzees therefore tolerate mild inequity, but cannot tolerate receiving nothing when others are rewarded.

Keywords: chimpanzees; cooperation; effort; inequity; motivation.

Figure 1

The apparatus required two individuals to remove the barriers at locations A and C by pulling the rods before another individual could pull in the whole tray at location B. When the tray moved all the way in rewards were dispensed to each individual from the food cups.

Figure 2

Successes per hour over the course of experiment 1. The 30 sessions were grouped into 5 session bins for analysis. The inequity sessions were the first 4 bins, or sessions 1-20. The last 2 bins (sessions 21-30) were the equity sessions in which all locations were baited with the same number of grapes. Performance followed a significant cubic trend over the course of the experiment. Error bars represent 95% confidence intervals.

Figure 3

Time of last success (in minutes) in the phase 2 inequity sessions vs. phase 3 equity sessions. Box plots show the median (solid horizontal line), inter-quartile range (IQR; hinges), values within 1.5×IQR (whiskers), and outliers (solid circles).

Figure 4

Cooperation index in the phase 2 inequity sessions vs. the phase 3 equity sessions. Box plots show the median (solid horizontal line), inter-quartile range (IQR; hinges), values within 1.5×IQR (whiskers), and outliers (solid circles).

Figure 5

Mean successes and trials for each condition, in order of testing. Error bars represent 95% confidence intervals.

Figure 6

Mean efficiency for each reward rate. Errors bars represent 95% confidence intervals.

Figure 7

Canonical discriminant function at the group centroids by reward rate. Note that the group centroids for the 0% and 20% conditions overlap.

Figure 8

Mean bout length by reward rate for continuous successes and non-successes. Errors bars represent 95% confidence intervals.

Figure 9

Mean number of approaches and leaves by reward rate. Error bars represent 95% confidence intervals.