The smell of cooperation: rats increase helpful behaviour when receiving odour cues of a conspecific performing a cooperative task

Abstract

Reciprocity can explain cooperative behaviour among non-kin, where individuals help others depending on their experience in previous interactions. Norway rats (Rattus norvegicus) cooperate reciprocally according to direct and generalized reciprocity. In a sequence of four consecutive experiments, we show that odour cues from a cooperating conspecific are sufficient to induce the altruistic help of rats in a food-exchange task. When rats were enabled to help a non-cooperative partner while receiving olfactory information from a rat helping a conspecific in a different room, they helped their non-cooperative partner as if it was a cooperative one. We further show that the cues inducing altruistic behaviour are released during the act of cooperation and do not depend on the identity of the cue provider. Remarkably, olfactory cues seem to be more important for cooperation decisions than experiencing a cooperative act per se. This suggests that rats may signal their cooperation propensity to social partners, which increases their chances to receive help in return.

Keywords: Norway rat; altruism; cooperation; mammals; olfactory signalling; reciprocity.

Figure 1.

Experimental setup. The focal rat had access to a wooden stick (S) in the test phase of the experiment and could thereby move a tray (T) towards the partner or stooge that could then reach a food reward (R). The partner rat and stooge were put in a sealed Plexiglas box that was connected to an exhauster (E). (a) Set-up for experiments 1 and 2, where olfactory information was either provided or discarded from the partner in the neighbouring compartment. (b) Set-up for experiments 3 and 4, where the focal rat was provided with olfactory information from a partner rat that either helped a social partner or not in a different room, while the focal rat experienced the behaviour, but no olfactory information, of a stooge in the neighbouring compartment of its cage that never provided help.

Figure 2.

Is olfactory information important for cooperative decisions (experiment 1)? Kaplan–Meier curves according to the experimental treatments, showing that rats provided food earlier to cooperators (light green) than to non-cooperator (dark blue) when olfactory information was present (solid lines) compared to when no olfactory information was available (dotted lines); shading represents the 95 confidence intervals. (a) Kaplan–Meier estimator for direct reciprocity and (b) for generalized reciprocity. Three asterisks indicate a significant difference (p < 0.001), whereas n.s. indicates a lack of a significant difference (non-significant). See electronic supplementary material, table S7 for all statistical comparisons. (Online version in colour.)

Figure 3.

Does the smell of cooperation suffice to induce cooperative behaviour (experiment 3)? Focal rats provided food to a stooge earlier (a, b) and more often (c, d) while having access to smell of a cooperative individual (light green lines and symbols) than they did to a stooge while receiving olfactory information of a non-cooperative individual (dark blue lines and symbols). This applies in the experience phase (a, c) and in the test phase (b,d). Asterisks indicate a significant difference (p < 0.05), n.s. indicates a lack of a significant difference, a dot indicates a non-significant trend (p = 0.057). (a,b) Kaplan–Meier curves with 95% confident intervals. (c,d) mean numbers of pulls ± one standard error and the raw data points. For better visibility, overlapping data points were randomly offset horizontally. See electronic supplementary material, table S5 for statistical comparisons. (Online version in colour.)

Figure 4.

Does the performance of cooperation trigger cooperative responses (experiment 4)? Focal rats pulled earlier (a) and more often (b) for a stooge if receiving the smell from a cooperating conspecific (light green) acting in a different room than if receiving the smell of the same individual when it was not cooperating (dark grey). (a) Kaplan–Meier curves with 95% confident intervals. (b) Raw data and the mean number of pulls ± one standard error. For better visibility, overlapping raw data points were randomly offset horizontally. One asterisk indicates a significant difference at p < 0.05, two asterisks at p < 0.01. See electronic supplementary material, table S6 for statistical comparisons. (Online version in colour.)