Sex-specific effects of parasitism on survival and reproduction of a rodent host in a subtropical montane region.

Parasites can generate complex life history trade-offs in a host. In this study, we experimentally reduced the infection level of intestinal helminth parasites in the Taiwan field mouse (Apodemus semotus) to test (1) whether parasite richness and load are biased towards male or female mice (sex-biased parasitism) and (2) whether the effects of parasitism on the host's survival and reproduction are different between the sexes (sex-specific effects of parasitism). Our findings indicate that neither parasite richness (number of helminth taxa found in a fecal sample) nor parasite load (number of helminth eggs per gram of fecal material) was sexually biased in our A. semotus study population. These results are in agreement with those of previous studies on endoparasites in Apodemus spp., but are in contrast to those on ectoparasites in Apodemus spp. Parasite removal reduced the survival rate of reproducing females, possibly by allowing reproducing females to increase maternal investment in their current litters at the cost of their own future survival. Single-litter mothers with reduced parasitism had a higher body mass than the untreated single-litter mothers, suggesting an increased maternal investment. In addition, the reproductively more active A. semotus, particularly the females, carried higher parasite loads, suggesting a trade-off between reproduction and parasite defense. By demonstrating that parasites can affect life history trade-offs in A. semotus, our results highlight the importance of maintaining variation in life history traits under parasitism risks and illustrate the subtle demographic processes (e.g. reduced future survival among healthy reproducing females) that might be driven by parasitism.