Biased sex ratios and parasite mating probabilities

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Abstract
An earlier paper (May, 1977) developed a theoretical framework for exploring the consequences of dioecy for the population dynamics of schistosomes, assuming an unbiased sex ratio. This paper extends the analysis to biased sex ratios, as have been reported in practice. We consider the relationships of the mean number and distribution of worms among hosts, the sex ratio, and the mating system (monogamous or polygamous) to: (i) the female mating probability, Φ the prevalence of mated female worms. Ω: and (iii) the mean number of mated female worms per host, ξ. Among other results, we show how high values of Φ are associated with male-biased sex ratios and polygamous mating; that Ω is independent of the mating system and is relatively unaffected by the sex ratio; and that ξ is maximal for unbiased sex ratios given monogamous mating, and for female-biased sex ratios if mating is polygamous. These results, together with the confounding effects of the mean number and distribution of worms, are described in detail in the main body of the paper.