Grooming relationships between adult male chimpanzees are often reciprocal, i.e. individuals receive grooming from those they groom. Grooming may be reciprocated at the same time it is received (mutual grooming), or later within the same grooming session. \ud Alternatively, it can be reciprocated at a much later stage, in another session. An analysis of individual grooming sessions at the dyadic level was used to investigate how chimpanzees reciprocate grooming within these sessions.\ud This study describes the grooming and reciprocation of grooming by male chimpanzees, living in a multi-male, multi-female group at the Edinburgh Zoo, Scotland. A method for the analysis of dyadic grooming relationships was based on the presence or absence of mutual and unilateral grooming in a session, which allows seven types of grooming session to be distinguished. Grooming session was defined empirically, and the duration of the bout criterion interval (BCI) depended on the presence or absence of oestrous females. For comparison, however, the same BCI was used throughout.\ud Without oestrous females, grooming was primarily reciprocated in sessions with mutual grooming and unilateral grooming by both participants. This kind of session proved highly cooperative and each male adjusted the duration of his unilateral grooming to that of mutual grooming, rather than to the duration of unilateral grooming by the other male. Mutual grooming was less important to dyads which had a strong grooming relationship. It is suggested that mutual grooming serves as an indication of the motivation to groom unilaterally. There was no indication that males reciprocated on the basis of TIT-FOR-TAT within these sessions, or between sessions in general.\ud Alternative hypotheses of mutual grooming were only partly confirmed in that some dyads used mutual grooming to reduce the (already very short) time they spent in grooming. However, mutual grooming did not arise from the accidental overlap in the grooming of two partners.\ud In the presence of oestrous females, grooming cooperation between the males broke down, and this was the result of heightened aggression as well as the presence of oestrous females itself. The balance in grooming given and received shifted in the direction of dominants\ud (i.e. dominants received more) under the influence of oestrous females, but in the opposite direction under the influence of aggression. Feeding had no effect on the reciprocity of grooming. \ud There was considerable dyadic variation. Some dyads groomed more when there were oestrous females, others groomed less. Some dyads had proportionally less mutual grooming with increasing numbers of oestrous females, others had more. There were generally no clear patterns of grooming reciprocation over longer time-spans than the session, but the overall degree of reciprocity of a dyad was frequently reached at the end of each day. Tracing the degree of reciprocation over a few weeks indicated that some dyads' grooming was governed by dominance, whereas that of others by cooperatio
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