Forum Ideal flea constraints on group living: unwanted public goods and the emergence of cooperation

Long ago, Hamilton (1971) proposed that the ‘‘selfish herd’’ effect, while primarily thought to reduce predation risk, might also apply to avoiding parasites. Solitary individuals suffer higher ectoparasite burdens if they lack conspecifics either to absorb collateral damage from the local ectoparasite population or to remove ectoparasites by allogrooming. By grouping, therefore, animals may reduce their individual risk of exposure to parasites (Mooring and Hart, 1992). This is important because there are significant fitness costs associated with ectoparasite loads. More ectoparasites take more blood, cause more irritation, increase the probability of infection, and decrease the time available for other activities, because grooming becomes a higher priority. These costs vary with group size because a greater number of hosts and shared den sites means ectoparasites are more likely to survive stochastic variation. The dynamics of parasite control might therefore present crucial constraints on group size and a novel origin for sociality itself. We present a model suggesting that these constraints may lead to an egalitarian system among social host species, who need to cooperate to get rid of their ectoparasites. Mobile ectoparasites are moderated by two mechanisms: (1) the dilution of ectoparasites toward an ideal free distribution (IFD) among hosts, and (2) the removal of ectoparasites by cooperative host allogrooming. Host interactions represent a ‘‘biological market,’’ in which the benefit of cooperation varies with the relative amount of the tradable commodity (the ectoparasites) each individual has. This is not, however, a normal market as the goods are unwanted. Nevertheless, the dilution effect means that all individuals in a group have a common stake in reducing the mean parasite burden, which may preclude or reduce any advantage of cheating. Predictions of the model were tested with empirical data from a longterm study of badgers, Meles meles, and their flea burdens in Wytham Woods, Oxford. A basic tenet in models of population dynamics (Hanski, 1999; Sutherland, 1996) is IFD, a general explanation for fluctuating distributions of animals as they move to find sites where the rewards are highest and individuals optimize their fitness (Fretwell, 1972; Fretwell and Lucas, 1970). Assuming individuals really are free (i.e., no resource defense and perfect information), this represents a process whereby consumers become diluted among all available sites. In short, the ratio between the numbers of individuals in sites A and B should be proportional to the relative availability of resources Behavioral Ecology Vol. 15 No. 1: 181–186 DOI: 10.1093/beheco/arg093