Effects of the tapeworm Hymenolepis diminuta on maternal investment in rats

Parasites can influence the optimal allocation of energy between present and future reproduction by changing the value of current offspring relative to that of future ones. We examined the effect of the tapeworm parasite Hymenolepis diminuta on maternal investment in rats. Litter sizes and average pup body mass did not differ between parasitised and nonparasitised mothers, either at birth or at weaning. The two groups of mothers also did not differ with respect to pup growth or pup mortality during lactation. Mass change during lactation was similar between parasitised and nonparasitised mothers. However, a pup-retrieval experiment showed that parasitised mothers were quicker at retrieving their pups to safety than nonparasitised mothers, and spent less time checking for potential sources of danger. These differences were particularly pronounced toward the end of the lactation period. It would appear that the parasite increases the relative value of the current litter and current levels of maternal investment, possibly because it reduces future reproductive success.