A correlated response of a parasite's virulence and life cycle to selection on its host's life history

Since the observation that snails exposed to trematode parasites bring forward their maturity and increase their fecundity to lessen the parasite's impact on their reproductive success (Minchella & Loverde, 1981; Minchella, 1985), several studies have considered modi®cations of a host's life history as its adaptive response to parasite pressure (Michalakis & Hochberg, 1994). The marine snail Cerithidea californica matures at smaller sizes in populations where parasite prevalence is high (Lafferty, 1993); great tits, Parus major, increase their parental care and thus their reproductive effort, when nestlings are infected with the haematophagous ̄ea Ceratophyllus gallinae (Perrin et al., 1996); the lizard Lacerta vivipara increases reproductive effort when infected with an ectoparasitic mite (Sorci & Clobert, 1995), which leads to offspring with higher quality from infected than from uninfected mothers. Such observations have been complemented with theoretical studies predicting the optimal evolutionary responses of age at maturity (Hochberg et al., 1992) or reproductive effort (Forbes, 1993; Perrin et al., 1996) to parasitism. While the host's adaptive response to parasite exposure and infection has been investigated, the opposite in ̄uence ± that of a host's life history on its parasite's transmission and life cycle ± remains largely unexplored. There are, in principle, three ways in which a host's life history can in ̄uence its parasite's life cycle. First, many