Even more extreme fertility insurance and the sex ratios of protozoan blood parasites.

Theory developed for malaria and other protozoan parasites predicts that the evolutionarily stable gametocyte sex ratio (z*; proportion of gametocytes that are male) should be related to the inbreeding rate (f) by the equation z*=(1-f)/2. Although this equation has been applied with some success, it has been suggested that in some cases a less female biased sex ratio can be favoured to ensure female gametes are fertilized. Such fertility insurance can arise in response to two factors: (i) low numbers of gametes produced per gametocyte and (ii) the gametes of only a limited number of gametocytes being able to interact. However, previous theoretical studies have considered the influence of these two forms of fertility insurance separately. We use a stochastic analytical model to address this problem, and examine the consequences of when these two types of fertility insurance are allowed to occur simultaneously. Our results show that interactions between the two types of fertility insurance reduce the extent of female bias predicted in the sex ratio, suggesting that fertility insurance may be more important than has previously been assumed.