Parasitized mates increase infection risk for partners

Individuals can gain fitness benefits and costs through their mates. However, studies on sexual selection have tended to focus on genetic benefits. A potentially widespread cost of pairing with a parasitized mate is that doing so will increase an individual’s parasite abundance. Such a cost has been overlooked in systems in which parasites are indirectly transmitted.We manipulated the abundance of the nematode parasite Trichostrongylus tenuis, an indirectly transmitted parasite, within pairs of wild red grouse Lagopus lagopus scoticus in spring. Parasite levels were correlated within pairs before the experiment. We removed parasites from males, females, or both members of the pair and evaluated individual parasite uptake over the subsequent breeding period. At the end of the breeding season, an individual’s parasite abundance was greater when its mate had not been initially purged of parasites. This cost appeared to be greater for males. We discuss the implications of our results in relation to the costs that parasites may have on sexual selection processes.This study was funded by a Natural Environment Research Council grant (NE/D014352/1). J.M.-P. was funded by the Junta de Ampliación de Estudios (JAE-doc) from the Ministerio de Educación y Ciencia (Spain) during the writing of the article. P.V. was supported by a postdoctoral contract from the Ministerio de Educación y Ciencia and by an Intra-European Marie Curie Fellowship (European Union 7th Framework Programme, FP7/2007-2013, grant agreement 252499). F.M. was supported by the Junta de Comunidades de Castilla-La Mancha in Spain (grant PAII1C09-0227-0104).Peer reviewe