Complex life cycle and body sizes at life-history transitions for macroparasites

Questions: Macroparasites often require multiple hosts. (1) What are the conditions under which a complex life cycle (using both definitive and intermediate hosts) is more advantageous than a simple life cycle (using definitive host only)? (2) What are the optimal body sizes at life-history transitions? (3) Can a complex life cycle co-exist with a simple life cycle in the same population? Mathematical method: We study the evolutionary dynamics of phenotypes that differ in their life cycles and in their body sizes (size at birth, at transition between hosts, at maturity, etc.) Key assumptions: Evolution of the life-cycle pattern is linked to the evolution of body sizes at life-history transitions. Mortality, growth rate, host finding success, and hatching success are given as functions of body size. Conclusions: (1) The results are usefully illustrated graphically. (2) Size at birth, size at transition between hosts, and size at maturity depend on the productivity and mortality of parasites within different hosts. (3) The co-existence of phenotypes with different life histories is possible if the density dependence works both in an intermediate host and in a definitive host.