Parasitic castration: a perspective from a model of dynamic energy budgets

Models of the evolution of virulence have typically focused on increased mortality, one of two negative effects that parasites can inflict on their host. Those that consider the other effect, fecundity reduction, can predict that parasites should completely sterilize their hosts. Although this prediction seems extreme, sterilization features prominently in a fascinating strategy, parasitic castration. Such castration can be accompanied by gigantism (unusually large growth of infected hosts), long infectious periods, and fecundity compensation (where, before heavy parasite burdens ensue, newly infected hosts reproduce earlier/more than they would if not infected). Using a model of dynamic energy budgets (DEB), we show how these results readily emerge, assuming that parasites consume energy reserves of the host. The simple, but mechanistic, DEB model follows energy flow though hosts and parasites, starting with ingestion, and continuing with storage of assimilated energy, and use of those reserves for growth and reproduction, as allocated by the host according to the "κ-rule". Using this model, we compare and contrast two strategies for parasites. "Consumers" only steal energy from their hosts, thereby indirectly altering allocation of energy to growth and reproduction, reducing fecundity, and enhancing mortality. "Castrators" steal energy but also directly modify the scheme by which hosts allocate reserve energy, shunting resources from reproduction to growth. Not surprisingly, the model predicts that this strategy should promote gigantism, but it also forecasts longer infectious periods and fecundity compensation. Thus, commonly observed characteristics of parasitic castration readily emerge from a mechanistic model of energy flow using a minimal number of assumptions. Finally, the DEB model for both "consumers" and "castrators" highlight that variation in resources supplied to hosts promotes variation in virulence in a given host-parasite system, holding all else equal. Such predictions highlight the potential importance of resource ecology for virulence in disease systems.