Body size variation in tapeworms (Cestoda): adaptation to intestinal gradients?

We describe patterns of variation in the body size of tapeworms (Cestoda) parasitizing rodents and shrews. Tapeworms display considerable interspecific variation in body size. Tapeworms also occur only in certain, species-specific parts of intestine. Empirical data on tapeworms suggest that these two phenomena are related: the biggest tapeworm species tend to occur in midgut. We put forward a hypothesis that the observed variation in body size and age-at-maturity reflects adaptation to specific levels of mortality and amount of nutrients in intestinal environment. We construct a simple energy allocation model in which we assume that optimal life-histories maximize the expected reproductive success (R,).Under realistic intestinal gradients of mortality and amount of nutrients, the predicted patterns of variation in body size resemble those observed in tapeworms of small mammals.