Density-dependent prophylactic immunity reconsidered in the light of host group living and social behavior

According to the density-dependent hypothesis (DDP), hosts living at high densities suffer greater risk of disease and so invest more in immunity. Although there is much empirical support for this, especially from invertebrate systems, there are many exceptions, notably in social insects. We propose that (A) density is not always the most appropriate population parameter to use when considering the risks associated with disease and (B) behavioral defenses should be given a greater emphasis in considerations of a host's repertoire of immune defenses. We propose a complementary framework stressing the connectivity between and within populations as a starting point and emphasizing the costs represented by disease above the risk of disease per se. We consider the components of immune defense and propose that behaviors may represent lower-cost defenses than their physiological counterparts. As group-living and particularly social animals will have a greater behavioral repertoire, we conclude that with group living comes a greater capacity for behavioral immune defense, most particularly for social insects. This may escape our notice if we consider physiological parameters alone.