Mating rate

The second sphere in which sexual transmission parasites have been suggested to impact on host behaviour is in the evolution of mating rate. In systems with an STI, it is suggested, selection should favour modifiers that reduce mating rate. However, it is very unlikely to select for monogamy. As for the case of mate choice, the advantage gained by less promiscuous individuals is intrinsically frequency-dependent (Thrall et al., 1997, 2000; Boots and Knell, 2002). If there are benefits to promiscuity to either sex, then we would expect either intermediate levels of promiscuity across the population, or a polymorphism with some individuals remaining promiscuous, and others less so.

Knell and Webberley (2004) note that testing for the effect of STIs on promiscuity levels is very difficult, because STI prevalence and promiscuity are intrinsically linked epidemiologically: very promiscuous species are likely to have higher parasite prevalence. This association between promiscuity and prevalence is observed in the field (Webberley et al., 2004), and is likely to be a strong effect that confounds the effect of selection in any comparative analysis. Knell and Webberley (2004) suggest that a selection experiment approach may be the best method to detect whether or not these parasites can select for reduced promiscuity in their hosts.

In a final twist, it is notable that host promiscuity is, of course, beneficial to the parasite, and thus promiscuity increases could potentially occur due to parasite manipulation of the host. In mammalian systems, parasite-induced sterility will of course increase female mating rate (a female will stop mating when she falls pregnant). In insects, a similar effect is possible, if the parasite can divert resources from fertilization and fecundity to increased promiscuity. Definitive experiments need to be undertaken to examine whether this occurs. Knell and Webberley (2004) note some evidence consistent with such effects, although we are a long way from establishing proof.

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