Food and parasitism can have complex effects on small mammal reproduction. In this study, we tested the effects of sex, food and parasitism on reproductive performance of the Taiwan field mouse (Apodemus semotus). In a field experiment, we increased food availability for a portion of the mice in the population by providing sorghum seeds to a set of food stations. We reduced parasite intensity of randomly-chosen mice through ivermectin treatment. We determined the number and quality of offspring for the mice using paternity analysis. We quantified seed consumption with stable carbon isotope values of mouse plasma and parasite intensity with faecal egg counts of intestinal nematodes and cestodes (FEC). In a laboratory experiment, we reduced parasite intensity of randomly-chosen mice through ivermectin treatment. We quantified their immune functions by total white blood cell, percent granulocyte count, and percent lymphocyte count through haematological analyses. We measured the FEC and energy intake of the mice. From the field experiment, the number of offspring in A. semotus increased with increasing seed consumption. Due to the trade-off between number and quality of offspring, the offspring quality decreased with increasing seed consumption for the females. The ivermectin treatment did not affect offspring number or quality. However, the FEC was positively correlated with number of offspring. In the laboratory experiment, the percent lymphocyte/granulocyte count changed with parasite intensity at low energy intake, which was relaxed at high energy intake. This study demonstrated positive effects of food availability and neutral effects of parasitism on A. semotus reproduction. However, the benefits of food availability for the females need to take into account the offspring number-quality trade-off, and at high infection intensity, parasitism might negatively affect offspring quality for the males. We suggest that food availability could mediate the relationships between parasite intensity and immune responses.

Field and laboratory data on Apodemus semotusThis Excel file contains two worksheets, 'field data' and 'lab data'. The 'field data' worksheet has the data from the field experiment, including the body mass, parasite intensity (i.e. faecal egg count), seed consumption (i.e. stable carbon isotope value), sex, ivermectin treatment, offspring number and offspring quality (i.e. mean body mass of the offspring) of Apodemus semotus. The 'lab data' worksheet has the data from the laboratory experiment, including ivermectin treatment, sex, parasite intensity (i.e. faecal egg count), energy consumption and three immune metrics (i.e. white blood cell count, percent granulocyte count and percent lymphocyte count of Apodemus semotus.Apodemus_semotus_Shaner2018EcoEvo.xlsx