ECTOPARASITE VIRULENCE IS LINKED TO MODE OF TRANSMISSION

Theory suggests that the evolution of parasite virulence is Linked to the dynamics of parasite transmission. All else being equal, parasites transmitted vertically from parents to offspring should be less virul ent than parasites capable of horizontal transmission to unrelated hos ts. This is because the fitness of vertically transmitted parasites is tightly linked to the reproductive success of the host, whereas the f itness of horizontally transmitted parasites is relatively independent of host reproduction. The virulence-transmission relation has seldom been tested because of difficulties inherent in comparing virulences o f different parasite-host systems. We compared the virulence of lice a nd mites infesting a single group of captive rock doves (Columba livia ). Lice, which were vertically transmitted, had no detectable effect o n host fitness, whereas horizontally transmitted mites drove host repr oductive success to zero. These results, in conjunction with a survey of the Literature, support the hypothesis that ectoparasite virulence is linked to the mode of transmission.