The price of grooming in redfronted lemurs

At first blush grooming among primates might seem to strictly serve hygienic purposes. After all, primates are furry little mammals crawling with ectoparasites like lice, ticks, and other icky arthropods. Despite this, primates seem to allocate more time to grooming than necessary for basic hygiene. Primates groom each other to strengthen social bonds and reduce tension. But do they trade grooming with each other for mating opportunities, protection, or aid in fights?

Biological market theory predicts that grooming is a commodity that may be exchanged among partners for other commodities–protection against predators, coalitionary support, reciprocal grooming, tolerance, food, or mating opportunities to name a few [Noë & Hammerstein 1994, 1995]. Individuals belong to different ‘trader classes’ and members of the same class compete with each other to offer commodities to another class [Noë & Hammerstein 1994, 1995]. If grooming is being exchanged for grooming, then two individuals might match each other’s service with respect to time during a grooming bout. In other words, A might groom  B for 45 seconds and then B might reciprocate and groom A for about 45 seconds. However, if  A happens to be a dominant animal (and member of a different class) and can offer B coalitionary support, then B might have to offer more grooming to trade for this commodity. This sets up competition within a trader class for the commodity offered by another trader class. Think of it this way, if a dominant animal can offer protection to a subordinate animal at some price of X amount of grooming, then subordinate animals (members of the same class) will try to outbid each other in order to acquire this protection.

Nonsense you say. Well, grooming behavior among redfronted lemurs (Eulemur fulvus rufus) can be explained within a biological market framework [Port et al., 2009]. Redfronted lemurs are  Malagasy strepsirrhines  that live in multimale-multifemale groups of 6-15 individuals [Port et al., 2009]. One male (the central male) is dominant to all others with no clear linear dominance relationships existing among subordinates. Thus, central and subordinate males can be considered as members of different trader classes. In accordance with biological market theory, the asymmetry in grooming relationships between central and subordinate males increases as the number of subordinate males increases [Port et al., 2009]. This suggests that subordinates are trying to outbid each other in order to acquire some commodity from dominants. This commodity might be tolerance as dominants have been observed to evict subordinate animals from the social group [Port et al., 2009].  Addtionally, grooming relationships among members of the same trader class (high ranking females or subordinate males) were characterized by generalized reciprocity in time spent grooming [Port et al. 2009]. This suggests members of the same trader class match the price of grooming since they are both offering the same commodity.

While biological market theory has been around for over 15 years, it has not quite caught on as much as previous ‘big ideas’ in sociobiology (i.e. inclusive fitness theory). This is likely due to the simple reason that most predictions derived from biological market theory can be explained using older, more traditional frameworks (sexual selection, mutualism, kin selection, etc.). However, biological market theory is particular useful in explaining interspecific cooperation (mutualism) as well as non-kin cooperation within a species. Personally, I think it offers a unique perspective of looking at the proximate costs and benefits of social interactions among conspecifics, especially when those individuals are not likely to be kin, i.e. the subjects of my dissertation research.

Noë, R., & Hammerstein, P. (1994). Biological markets: supply and demand determine the effect of partner choice in cooperation, mutualism and mating Behavioral Ecology and Sociobiology, 35 (1), 1-11 DOI: 10.1007/s002650050063

Noë, R. (1995). Biological markets Trends in Ecology & Evolution, 10 (8), 336-339 DOI: 10.1016/S0169-5347(00)89123-5

Port, M., Clough, D., & Kappeler, P. (2009). Market effects offset the reciprocation of grooming in free-ranging redfronted lemurs, Eulemur fulvus rufus Animal Behaviour, 77 (1), 29-36 DOI: 10.1016/j.anbehav.2008.08.032