Abstract
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.
Highlights
Allogrooming, the grooming of one animal by another of the same species by licking or carefully nibbling, is common in animals ranging from arthropods to apes[1]
Allogrooming often reflects a reciprocal exchange of given and received grooming bouts between social partners (e.g.: impala[16], herb-field mouse[17], chimpanzee[18]; reviewed in ref. 19): by alternating the roles between donor and recipient, the payoffs of social partners can correlate among one another and reciprocal cooperation becomes stable over time[19, 20]
Allogrooming rates provided by focal rats did not significantly correlate with allogrooming rates received from their respective partners (GLMM, ß = 0.01 ± 0.01, X2 = 0.86, p = 0.35)
Summary
Allogrooming, the grooming of one animal by another of the same species by licking or carefully nibbling, is common in animals ranging from arthropods to apes[1]. Allogrooming often reflects a reciprocal exchange of given and received grooming bouts between social partners Badgers that have a pronounced hierarchy groom partners based on short-term reciprocal exchanges, regardless of dominance relationships[31]. Whereas spots that are difficult to reach are allopreened reciprocally independent of rank, allopreening of the rest of the body is directed more often to dominants, and allopreening initiated by dominants is more often reciprocated than if it is initiated by subordinates These non-primate examples illustrate that reciprocal exchange of hygienic care may be highly specific to the context and the hierarchical structure of a social system
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