Abstract
Allogrooming is a key aspect of chimpanzee sociality and many studies have investigated the role of reciprocity in a biological market. One theoretical form of reciprocity is time-matching, where payback consists of an equal duration of effort (e.g. twenty seconds of grooming repaid with twenty seconds of grooming). Here, we report a study of allogrooming in a group of twenty-six captive chimpanzees (Chester Zoo, UK), based on more than 150 hours of data. For analysis, we introduce a methodological innovation called the “Delta scale”, which unidimensionally measures the accuracy of time-matching according to the extent of delay after the cessation of grooming. Delta is positive when reciprocation occurs after any non-zero delay (e.g. A grooms B and then B grooms A after a five second break) and it is negative when reciprocation begins whilst the original grooming has not yet ceased. Using a generalized linear mixed-method, we found evidence for time-matched reciprocation. However, this was true only for immediate reciprocation (Delta less than zero). If there was a temporal break in grooming between two members of a dyad, then there was no evidence that chimpanzees were using new bouts to retroactively correct for time-matching imbalances from previous bouts. Our results have implications for some of the cognitive constraints that differentiate real-life reciprocation from abstract theoretical models. Furthermore, we suggest that some apparent patterns of time-matched reciprocity may arise merely due to the law of large numbers, and we introduce a statistical test which takes this into account when aggregating grooming durations over a window of time.
Highlights
What is reciprocity? In the human sense, it refers to “a set of motivationally interrelated twoway gifts”: the actors in a dyad are psychologically motivated to repay a gift with a commensurate gift
Some [7], [16], [17] have argued that contingent cooperation is an unlikely explanation for observed patterns of reciprocity —arguing instead that cooperation in the animal kingdom consists predominantly of cases of shorter-term mutualism: where there is no pattern of “loss and delayed payback” and, instead, mutualism entails immediate benefits that flow to both parties [6], [16]
For comparison with other studies, we examined scatter plots of time invested against grooming received to see whether there is any symmetry in grooming durations, and using the lme4 library [59], we performed linear regressions of the mixed-model Y = X + (1|d), where d is the dyad number: i.e. we regress grooming reciprocated with grooming received as a fixed effect, and the dyad as a random effect on the intercept
Summary
What is reciprocity? In the human sense, it refers to “a set of motivationally interrelated twoway gifts” (ref. [1], p. 137): the actors in a dyad are psychologically motivated to repay a gift with a commensurate gift (the second being contingent on the first). 137): the actors in a dyad are psychologically motivated to repay a gift with a commensurate gift (the second being contingent on the first). Time-matching in chimpanzee allogrooming [8], [9], [10], [11], [12], [13], one version, for example, being the idea of “reciprocal altruism” where the cost of providing an altruistic gift (“cost” measured as reduced reproductive output) is offset by the receipt of a later gift [14]. Among non-human primates, recorded examples of reciprocity-like patterns are numerous and well known [5], [6], [15]. Some [7], [16], [17] have argued that contingent cooperation (e.g. where A grooms B because B groomed A, as required in reciprocal altruism) is an unlikely explanation for observed patterns of reciprocity —arguing instead that cooperation in the animal kingdom consists predominantly of cases of shorter-term mutualism (pseudo-reciprocity): where there is no pattern of “loss and delayed payback” and, instead, mutualism entails immediate benefits that flow to both parties [6], [16].
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