Generalized reciprocity in rats

Abstract

The evolution of cooperation among nonrelatives has been explained by direct, indirect, and strong reciprocity. Animals should base the decision to help others on expected future help, which they may judge from past behavior of their partner. Although many examples of cooperative behavior exist in nature where reciprocity may be involved, experimental evidence for strategies predicted by direct reciprocity models remains controversial; and indirect and strong reciprocity have been found only in humans so far. Here we show experimentally that cooperative behavior of female rats is influenced by prior receipt of help, irrespective of the identity of the partner. Rats that were trained in an instrumental cooperative task (pulling a stick in order to produce food for a partner) pulled more often for an unknown partner after they were helped than if they had not received help before. This alternative mechanism, called generalized reciprocity, requires no specific knowledge about the partner and may promote the evolution of cooperation among unfamiliar nonrelatives.

Figure 1. Rats Cooperate in an Instrumental Task

(A) Experimental set-up: two rats in the test cage are separated by a wire mesh. By pulling a stick fixed to a baited tray one rat produces food (an oat flake) for the partner rat but is not rewarded herself for this behavior. (B) Experience phase: the focal rat (black) either experienced help by three different partner rats (A, B, or C) that pulled (test treatment; help is indicated by an arrow), or it experienced no help by three different partner rats (D, E, or F) that did not pull (control treatment; no help is indicated by a crossed arrow). (C) Test phase: the pulling behavior of the focal rat was tested against a new partner (rat G). (D) Solo pulling control: the pulling behavior of the focal rats was tested when alone in the cage. The time schedule of the experiment is also shown. Each focal rat was exposed to both test and control treatments in a randomized sequence. There was a four-day interval between the first experimental test (day 6) and the start of the second experience phase (on day 11).

Figure 2. Experience Influences Cooperation

Rats pulled more often after help experience (filled dots) than after receiving no help (open triangles). On average the rats' propensity to pull was 21% higher in the test treatment. Numbers with the same first digit denote individuals from the same family (e.g., 10, 11, and 12 were siblings).