Chimpanzees help each other upon request

Abstract

Background: The evolution of altruism has been explained mainly from ultimate perspectives. However, it remains to be investigated from a proximate point of view how and in which situations such social propensity is achieved. We investigated chimpanzees' targeted helping in a tool transfer paradigm, and discuss the similarities and differences in altruism between humans and chimpanzees. Previously it has been suggested that chimpanzees help human experimenters by retrieving an object which the experimenter is trying to reach. In the present study, we investigated the importance of communicative interactions between chimpanzees themselves and the influence of conspecific partner's request on chimpanzees' targeted helping.

Methodology/principal findings: We presented two tool-use situations (a stick-use situation and a straw-use situation) in two adjacent booths, and supplied non-corresponding tools to paired chimpanzees in the two booths. For example, a chimpanzee in the stick-use situation was supplied with a straw, and the partner in the straw-use situation possessed a stick. Spontaneous tool transfer was observed between paired chimpanzees. The tool transfer events occurred predominantly following recipients' request. Even without any hope of reciprocation from the partner, the chimpanzees continued to help the partner as long as the partner required help.

Conclusions/significance: These results provide further evidence for altruistic helping in chimpanzees in the absence of direct personal gain or even immediate reciprocation. Our findings additionally highlight the importance of request as a proximate mechanism motivating prosocial behavior in chimpanzees whether between kin or non-kin individuals and the possible confounding effect of dominance on the symmetry of such interactions. Finally, in contrast to humans, our study suggests that chimpanzees rarely perform acts of voluntary altruism. Voluntary altruism in chimpanzees is not necessarily prompted by simple observation of another's struggle to attain a goal and therefore an accurate understanding of others' desires in the absence of communicative signals.

Figure 1. The apparatus and setup for experiment 1: view from outside the booths.

In this picture, the stick-use situation was set up in booth A, and the straw-use situation was set up in booth B. A straw was supplied to booth A, but could be effectively used only by the chimpanzee in booth B. A stick was supplied to booth B, but could be effectively used only by the chimpanzee in booth A. In the actual procedure, the tools (a straw and a stick) were supplied in the booths (not presented outside the booths as depicted here), and the partition of booth B was closed (not partially opened as depicted here).

Figure 2. Tool transfer upon recipient's request.

A chimpanzee (Mari) in the near-side booth picks up a stick and hands it over to her partner (Pendesa) in the far-side booth who requested the tool by poking her arm through the hole between the booths.

Figure 3. The percentage of recipients' success in receiving a tool from a partner.

“with request” is cases in trials in which the recipients exhibited request, and “without request” is cases in trials in which the recipients failed to exhibit request. The data from all the participants were pooled.