Figure 4.

Threshold values of vertical transmission and parasite virulence allowing the evolution of parasite manipulation in the (a) absence or (b) presence of sexual transmission between males and females. Values of vertical transmission and virulence rate for which a mutant parasite manipulating the IHs, ζ^m > 1, invades a wild-type population without behavioural manipulation at endemic equilibrium, ζ^wt = 1 (below the dashed line in a), and a mutant parasite manipulating the behaviour of female IHs, ζ_F^m > 1, invades a wild-type population without behavioural manipulation in both male and female IHs at equilibrium, ζ_M^wt = ζ_F^wt = 1, (hatched area in a). In this latter case, behavioural manipulation is always selected in males and is represented by the shaded area in (a). Allowing sexual transmission from male to female in IHs (b) decreases the conditions for the evolution of manipulation in males (shaded area) and in females (hatched area). Note that these latter curves are obtained assuming no manipulation of the IH behaviour in both resident and mutant, i.e. ζ_M^wt = ζ_F^wt = ζ_M^m = ζ_F^m = 1.