Sex-specific effects of parasites on host body mass and survival: a case study of helminth parasites and rodent host Apodemus semotus in Taiwan

Background/Question/Methods

Parasites can have a wide range of effects on host physiology, behavior and life history. Previous studies demonstrated that helminth parasites suppressed population density and summer reproduction of their rodent hosts. However, other studies suggest that helminth effects on host survival and body condition are not always detectable. In this study, we used a field manipulative experiment to test the effects of helminth parasites on the survival and body conditions (i.e. body mass) of the Taiwan field mouse (Apodemus semotus). We conducted mark-capture-recapture survey of the rodents from August 2011 to May 2012 in old-field and forest habitats in subtropical Taiwan. Parasite removal treatment via oral administration of Ivermectin was assigned to half of the females and half of the males randomly chosen from among all captured individuals. We quantified parasite intensity using fecal egg count and host survival using Cormack-Jolly-Seber models. We used model selection approach to determine the effects of parasite removal on male and female survival, and tested if the individuals receiving the parasite removal treatment gain more body mass than those receiving the control treatment using ANOVA.     

Results/Conclusions

Our treatment was effective in reducing the fecal egg counts of nematode. However, the affected taxa were different between male and female rodents: Physaloptera sp. was reduced in males, and Ancylostoma sp. and Strongyloides sp. was reduced in females. Survival from August through May did not differ between the treatment and control groups, nor did it differ between sexes. But for body mass, females in forest habitats that received Ivermectin were heavier than females that did not received Ivermectin. In contrast, male body mass did not differ between the treatment and control groups. These results indicate that nematode could reduce female body condition even though it did not affect host survival. The sex-specific effects of nematode on female A. semotus illustrate the complex nature of parasite effects on host population demography.