How much energy should manipulative parasites leave to their hosts to ensure altered behaviours?

Although host manipulation is likely to be costly for parasites, we still have a poor understanding of the energetic aspects underlying this strategy. It is traditionally assumed that physiological costs are inevitably associated with mechanisms evolved by parasites to induce the required changes in host behaviours. While most energetic expenditures of parasites relate primarily to bringing about the altered behaviours, manipulative parasites also have to consider the condition of their host during the manipulation. Here, we suggest that because of this trade-off, the energy required to accomplish parasite-induced behaviours may represent a key energetic constraint for parasites. Depending on the energetic expenditures specific to each type of manipulation, parasites should undergo selection to secure resources for their host to allow them to perform manipulated behaviours.