Spite, xenophobia, and collaboration between hosts and parasites

Results of current computer simulations suggest that a pathogen-mediated ve rsion of Hamilton's spite hypothesis may play an unexpected role in the evo lution of host-parasite interactions. Theory predicts that hosts' ability t o influence parasite transmission to conspecifics is a neutral character pr ovided that transmission routes are random. There are two possible deviatio ns from this. Firstly, vertical transmission is biased in favour of kin, an d this selects hosts to decrease pathogen transfer. Secondly, horizontal tr ansmission is biased against kin and this selects hosts to increase pathoge n transfer to conspecifics. Unfortunately. we typically do not know transmi ssion statistics in relation to genetic kinship within a host population, t hus 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 ev en simple descriptive information on host and parasite life-histories may b e used to make rough estimations about transmission routes and selection pr essures. Interspecific predictions suggest that pelagic hosts, passively ae rial hosts, and the intermediate hosts of pathogens transmitted along the p redator-prey transmission routes are not selected to influence pathogen tra nsmission. Intraspecific predictions suggest that parasitism and disease sh ould facilitate emigration, and immigrants surrounded by non-related conspe cifics should adaptively enhance pathogen transmission to conspecifics. Thi s behaviour may select social animals, like humans, for an adaptive xenopho bia in the context of an intraspecific arms race.