Grooming reciprocation in Himalayan tahr and the cognitive constraint hypothesis

The cognitive constraint hypothesis maintains reciprocity is rare among animals because it requires cognitive capabilities that are rare, if not absent, in animals. In particular, it suggests that temporal discounting, limited memory, and limited capability of complex calculations make long-term reciprocation essentially impossible for animals. The cognitive constraint hypothesis therefore predicts that, if reciprocity ever occurs in animals, it should always be immediate. In this study, we tested for long-term reciprocation in Himalayan tahr (Hemitragus jemlahicus), a species that is not notable for its advanced cognitive capabilities. We showed that tahr are able to reciprocate grooming over extended time periods, that is, that they groom preferentially those individuals from which they receive more grooming even when all cases of immediate reciprocation are excluded from analysis. These results do not support the cognitive constraint hypothesis. We note the cognitive constraint hypothesis makes two silent assumptions: that all reciprocation is necessarily calculated and that it is based on partner control processes. We suggest these assumptions are unwarranted, and that most group living animals base their ability to reciprocate on emotionally based partner choice.