If individuals help more those who have previously helped them, stable cooperation may ensue through alternation of roles between donors and recipients. Allogrooming, which is costly to donors and beneficial to recipients, is often exchanged between social partners. Arguably, allogrooming and allopreening are the most frequently exchanged social services and have been used as a standard model of reciprocal cooperation. However, evidence for the application of reciprocity rules among social partners allogrooming each other hitherto is merely correlational. Here, we tested whether female Norway rats (Rattus norvegicus) apply the decision rule characterising direct reciprocity: help someone who has helped you before, by experimentally manipulating both the need for allogrooming and the behavioural response. Furthermore, we checked whether trading of grooming services is influenced by the rank of the social partner. We show that rats groom social partners reciprocally and prefer to do so up the hierarchy, i.e. they groom dominant partners more often than subordinates, while reciprocating with both. This provides experimental evidence that animals render a costly social service by applying reciprocity decision rules when showing a natural hygienic behaviour. The fact that allogrooming is more readily shown up the hierarchy may suggest an appeasing function.

Reciprocity can generate stable levels of cooperation among unrelated social partners. If individuals interact repeatedly, costs of altruistic acts can be compensated through an exchange of donor and receiver roles. Frequent interactions are conducive to attaining evolutionarily stable reciprocal exchange. High interaction frequencies are typical for group members maintaining close relationships among one another, which may thereby facilitate reciprocity. Norway rats (Rattus norvegicus) are highly social animals that were experimentally shown to reciprocally exchange food donations and allogrooming. Here we tested experimentally the relationship between reciprocal cooperation and other social behaviours exchanged within dyads of wild-type Norway rats. In particular, we asked phase. This suggests that affiliative and aggressive behaviours affect the allogrooming propensity of rats. In particular, higher frequencies of received aggression decreased the propensity to reciprocate previously received cooperation. We provide experimental evidence that rats are more likely to groom partners that pulled a stick to deliver food to them. Reciprocal exchange of allogrooming depends apparently on experienced cooperation, but also on the quality of the social relationship. Significance Statement Close social relationships among individuals may enhance reciprocal exchange of services and thereby ensure long-term cooperation. Thus we tested whether in unrelated and previously unfamiliar Norway rats the quality of social interactions, that is, the amount of exchanged affiliative and aggressive behaviours, affects reciprocal cooperation, and whether received cooperation in turn predicts subsequent social behaviour. Our results show that focal individuals are generally more helpful to previously helpful partners, but that the quality of social interactions may modify their decision to cooperate. Received aggressive and affiliative behaviours affected the subsequent reciprocal exchange of hygienic behaviour. Moreover, received food provisioning affected the exchange of cooperative, affiliative and aggressive behaviours also outside of a food-provisioning context. These data reveal a close relationship between the exchange of social behaviours between individuals and their propensity to cooperate with one another.

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In humans, non-right-handedness is associated with a higher incidence of psychiatric disorders. Since serotonin seems to be involved in both, the development of psychiatric disorders and lateralization, the present study focuses on the effect of the serotonin transporter (5-HTT) gene on behavioral lateralization. For this, we used the 5-HTT knockout mouse model, a well-established animal model for the study of human depression and anxiety disorders. For female mice from all three 5-HTT genotypes (wild type, heterozygous, and homozygous knockout), we repeatedly observed the direction and strength of lateralization of the following four behaviors: grid climbing (GC), food-reaching in an artificial test situation (FRT), self-grooming (SG), and barrier crossing (BC), with the FRT being the standard test for assessing behavioral lateralization in mice. We found no association between behavioral lateralization and 5-HTT genotype. However, in accordance with previous findings, the strength and temporal consistency of lateralization differed between the four behaviors observed. In conclusion, since the 5-HTT genotype did not affect behavioral lateralization in mice, more research on other factors connected with behavioral lateralization and the development of symptoms of psychiatric disorders, such as environmental influences, is needed.

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