Grooming is one of the few, if not the only, naturally occurring altruistic behaviour in primates that can easily be quantified. Louise Barrett and Peter Henzi were the first to systematically look at grooming from a biological market point of view. They realised that grooming can be traded, both for itself as well as for other commodities, such as tolerance near resources, access to young infants, support in conflicts and compliance during mating. Grooming thus has something in common with a currency that can be used to pay for different goods and services. The possibility of quantifying grooming opens the way to testing whether shifts in supply and demand lead to shifts in exchange rates of different commodities.
Some general discussions by Barrett & Henzi
Barrett, L. & Henzi, S. P. 2001. Grooming and family life. Exchanging services among female monkeys. In: In: Macdonald, D. (ed.) The New Encyclopedia of Mammals. Oxford Univ. Press, Oxford, pp. 306-307.
Barrett, L. & Henzi, S. P. 2001. The utility of grooming in baboon troops. In: In: Noë, R.; van Hooff, J.A.R.A.M. & Hammerstein, P. (eds.) Economics in Nature. Social Dilemmas, Mate Choice and Biological Markets. Cambridge Univ. Press. pp. 119-145.
Barrett, L. & Henzi, S. P. 2006. Monkeys, markets and minds: biological markets and primate sociality. In: Cooperation in Primates and Humans (Ed. by Kappeler, P. M. & van Schaik, C. P.), pp. 209-232. Berlin: Springer.
Henzi, S. P. & Barrett, L. 2007. Coexistence in Female-Bonded Primate Groups. Advances in the Study of Behavior, 37, 43-81.
Baby markets
A striking example is the so-called 'baby market': the fewer infants in a group the higher the price that has to be paid in the form of grooming to gain the mother's permission to inspect and handle it. This was first described for chacma baboons, but similar results have been found in other primate species, such as sooty mangabeys (Fruteau et al. in prep.), vervet monkeys (Fruteau et al. in prep.), longtailed macaques (Gumert 2007) and spider monkeys (Schaffner & Aureli 2005; Slater et al. 2007).
Henzi, S. P. & Barrett, L. 2002. Infants as a commodity in a baboon market. Animal Behaviour, 63, 915-921.
Schaffner, C. & Aureli, F. 2005. Embraces and grooming in captive spider monkeys. International Journal of Primatology, 26, 1093-1106.
Slater, K. Y., Schaffner, C. M. & Aureli, F. 2007. Embraces for infant handling in spider monkeys: evidence for a biological market? Animal Behaviour, 74, 455-461.
Gumert, M. 2007. Grooming and infant handling interchange in Macaca fascicularis : the relationship between infant supply and grooming payment. International Journal of Primatology, 28, 1059-1074.
Trading tolerance for grooming
Henzi and Barrett also pointed out that 'tolerance' is another important commodity that can be traded. This explains why the asymmetry in grooming between a dominant and a subordinate is strongest when resources are scarce and can be monopolised by a single animal.
Barrett, L., Henzi, S. P., Weingrill, T. & Hill, R. A. 1999. Market forces predict grooming reciprocity in female baboons. Proceedings of the Royal Society B-Biological Sciences, 266, 665-670.
Henzi, S. P. & Barrett, L. 1999. The value of grooming to female primates. Primates, 40, 47-59.
Leinfelder, I., de Vries, H., Deleu, R. & Nelissen, M. 2001. Rank and grooming reciprocity among females in a mixed-sex group of captive hamadryas baboons. American Journal of Primatology, 55, 25-42.
Barrett, L., Gaynor, D. & Henzi, S. P. 2002. A dynamic interaction between aggression and grooming reciprocity among female chacma baboons. Animal Behaviour, 63, 1047-1053.
Payne, H. F. P., Lawes, M. J. & Henzi , S. P. 2003. Competition and the exchange of grooming among female samango monkeys (Cercopithecus mitis erythrarchus). Behaviour, 140, 453-471.
Trading grooming for sex: from mice to macaques
The idea that sex can be traded for other commodities is much older than the biological market paradigm (see also 'Mating markets'). The idea that reproduction takes place in the context of a mating market also exists at least since Darwin set the theory of sexual selection in motion. We seem nevertheless to have inspired a few people to take a closer look at exchange rates between grooming and access to sexual partners
Stopka, P. & Macdonald, D. W. 1999. The market effect in the wood mouse, Apodemus sylvaticus: selling information on reproductive status. Ethology, 105, 969-982.
Polechova, J. & Stopka, P. 2002. Geometry of social relationships in the Old World wood mouse, Apodemus sylvaticus. Canadian Journal of Zoology - Revue Canadienne de Zoologie, 80, 1383-1388.
Gumert, M. D. 2007. Payment for sex in a macaque mating market. Animal Behaviour, 74, 1655-1667.
Grooming for support in conflicts.
The popularity of this idea seems inversely correlated to the quantity of data supporting it, but here's an interesting exception:
Watts, D. P. 2000. Grooming between male chimpanzees at Ngogo, Kibale National Park. II. Influence of male rank and possible competition for partners. International Journal of Primatology, 21, 211-238.
