According to current evolutionary dogma, multiple infections generally increase a parasite's virulence (i.e. reduce the host's reproductive success). The basic idea is that the competitive interactions among strains of parasites developing within a single host select individual parasites to exploit their host more rapidly than their competitors (thereby causing an increase in virulence) to ensure their transmission. Although experimental evidence is scarce, it often contradicts the theoretical expectation by suggesting that multiple infections lead to decreased virulence. Here, we present a theoretical model to explain this contradiction and show that the evolutionary outcome of multiple infections depends on the characteristics of the interaction between the host and its parasite. If we assume, as current models do, that parasites have only lethal effects on their host, multiple infections indeed increase virulence. By contrast, if parasites have sub-lethal effects on their host (such as reduced growth) and, in particular, if these effects feed back onto the parasites to reduce their rate of development, then multiplicity of infection generally leads to lower virulence.

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Summary1. Many animals choose to breed in sites where they have previously been successful. Such ®delity could arise from the predictability of high quality breeding sites in a temporally stable environment. The quality of a site may be indicated by factors other than an individual's own success, because it may fail as a result of a random event that is unrelated to the intrinsic quality of the site. In particular, prior experience (familiarity) with the breeding area and the performance of neighbours could give complementary information about the quality of the site. 2. We present results from a long-term study of colonial great cormorants (Phalacrocorax carbo sinensis), where movement and reproductive success of individually marked birds within a colony was known. 3. Individuals were more likely to return to the same breeding site if they had been successful the previous season. 4. Fidelity of both males and females increased with increasing level of familiarity with the breeding area. Males were more likely to breed again in the same area, and their ®delity was more dependent on familiarity with the area than female ®de-lity. 5. The success of breeding sites within the colony was spatially autocorrelated, and cueing on neighbour performance should thus be advantageous. Female ®delity increased with increasing success of neighbouring birds, while male ®delity was unaected by neighbour success. 6. We suggest that the dierence between the sexes in the criteria determining ®de-lity arises because it is mainly the males that are involved in territorial disputes. They may therefore bene®t more than females from knowing their neighbours, and this could override the importance of the intrinsic quality of the breeding area (i.e. reproductive success of neighbours). 7. The con¯ict over preferred breeding sites that arises within breeding pairs because of this sex dierence may be an explanation for the high rate of mate change between years (92.5%) observed in this species.

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