How do you measure pleasure? A discussion about intrinsic costs and benefits in primate allogrooming

Social grooming is an important element of social life in terrestrial primates, inducing the putative benefits of β-endorphin stimulation and group harmony and cohesion. Implicit in many analyses of grooming (e.g. biological markets) are the assumptions of costs and benefits to grooming behaviour. Here, in a review of literature, we investigate the proximate costs and benefits of grooming, as a potentially useful explanatory substrate to the well-documented ultimate (functional) explanations. We find that the hedonic benefits of grooming are well documented. However, we did not find convincing evidence for costs. If proximate costs do exist, they might consist of energetic, cognitive, opportunity costs, or some combination of all of these. Nonetheless, there remains the possibility that grooming costs are negligible, or even that the provision of allogrooming is rewarding in itself. We suggest empirical research to resolve this issue.