What are the proximate causes of the behavior? How do these causes function mechanistically to regulate the behavior?

Primates are thought to be motivated to perform allogrooming partially by reciprocity; they "expect" some benefit in return from those that they groom and/or the larger population (Ventura et al., 2006).  Frequently this is simply being groomed in return, but there are many other possible social benefits(for more on reciprocity, see adaptive value). 

Giving grooming is also thought to be a means of strengthening social bonds and hence increasing the giver's social standing.  In this description, an individual is motivated to groom others because it will increase its social clout in conflicts.  Social bonding and interaction may also be desirable, in and of themselves, to these animals and this desire for social interaction may be a component of the mechanism motivating the behavior. 

Click on the video below to see Japanese macaques' grooming behavior (from 00:53 to 02:30)

Challenge to "Bonding" Explanation

However, some argue that explanations in terms of social bonding are not well supported, and the focus should be on the direct observable benefits an animal reaps from performing an act of allogrooming.  This might include reciprocation of grooming, or tolerance from higher-ranked individuals; when a lower-rank primate grooms a higher rank primate, this generally results in better treatment and less aggression from the higher-ranked primate (Ventura et al., 2006).

Physical and Chemical Motivations

The specific physical and chemical mechanisms which motivate primates to seek better social position and/or social interactions and bonding are complex.  But it is clear that all of the social benefits of giving allogrooming are generally desired from an adaptive viewpoint, and seem to be consistently pursued by primates in all contexts.  It is also hard to say what mechanism governs reciprocity; it is intuitive to think of it in terms of a thought process based on expectation, but this does not add to our mechanistic understanding unless we understand the thought processes involved and how they are built. 

On the other hand, primates are known to be motivated to receive allogrooming by opioids (Martel, 1995).  Being groomed prompts a release of beta-endorphins, which cause pleasure and relaxation (Keverne EB, 1989).  This is the mechanism of desire to be groomed and explains, combined with the expectation (and usually, fulfillment) of reciprocal grooming, the motivation primates have to groom one another.



Figure 7: Beta-Endorphin acts on opioid receptors and is an analgesic. Keverne EB' tested the effects of adding Naltrexone to infant and adult monkeys and found that increased level of b-endorphin stimulated more social interactions in their study. (