Fig. 5.

Patterns of conversion rate follow a continuous, non-linear “reaction norm” in response to how parasites’ circumstances change during infections. Parasites in good state (e.g. replicating exponentially) can afford to invest in gametocytes, whilst a moderate loss of state requires parasites to reduce conversion (reproductive restraint, RR) to facilitate within-host survival and future transmission. During infections, parasites adopt variable but generally low conversion rates as state repeatedly improves and declines in line with dynamic features of infections, including immune responses, drug treatment, and changes in the age structure and density of RBC. However, in response to a catastrophic loss of state that likely eliminates the infection or imminent host death, parasites should maximise conversion (terminal investment, TI) because prioritising short-term transmission provides the best chance of fitness returns.