Spite, xenophobia, and collaboration between hosts and parasites

Results of current computer simulations suggest that a pathogen-mediated version of Hamilton's spite hypothesis may play an unexpected role in the evolution of host-parasite interactions. Theory predicts that hosts' ability to influence parasite transmission to conspecifics is a neutral character provided that transmission routes are random. There are two possible deviations from this. Firstly, vertical transmission is biased in favour of kin, and this selects hosts to decrease pathogen transfer. Secondly, horizontal transmission is biased against kin and this selects hosts to increase pathogen transfer to conspecifics. Unfortunately. we typically do not know transmission statistics in relation to genetic kinship within a host population, thus cannot easily predict whether there is a selective pressure upon hosts either to transmit pathogens or not to transmit. Here I aim to show that even simple descriptive information on host and parasite life-histories may be used to make rough estimations about transmission routes and selection pressures. Interspecific predictions suggest that pelagic hosts, passively aerial hosts, and the intermediate hosts of pathogens transmitted along the predator-prey transmission routes are not selected to influence pathogen transmission. Intraspecific predictions suggest that parasitism and disease should facilitate emigration, and immigrants surrounded by non-related conspecifics should adaptively enhance pathogen transmission to conspecifics. This behaviour may select social animals, like humans, for an adaptive xenophobia in the context of an intraspecific arms race.