Foraging host individuals can defend against fecal–orally transmitted parasites by avoiding feces‐contaminated patches, which has been widely documented among ungulates. However, it remains unclear whether smaller‐sized hosts (e.g., mice), with their high metabolism and constant needs for energy acquisition, can afford the same behavioral strategy. In this study, we used laboratory and field experiments to test whether feces‐contaminated patches are avoided by the Taiwan field mice Apodemus semotus. In the laboratory experiment, wild‐caught mice whose parasitic infection was not manipulated were given two options to forage from feces‐contaminated and uncontaminated patches. These naturally infected mice spent less time in feces‐contaminated than uncontaminated patches. In the field experiment, we reduced gastrointestinal parasite load of randomly chosen mice via anthelmintic treatment. Whereas the untreated mice did not discriminate among food patches with different levels of parasitism risk (i.e., high‐ or low‐risk patches containing conspecific feces of high or low parasite egg counts, no‐feces patches containing no feces), the treated mice spent less time in feces‐contaminated patches than in no‐feces patches. Similar to the larger‐sized ungulates, we demonstrated here that small mammals can also exhibit fecal‐avoidance foraging. Furthermore, such behavior may be influenced by both environmental parasitism risk and host infection status, which has implications in host–parasite transmission dynamics, namely the selective use of uncontaminated patches by the less‐infected (treated) mice may drive parasites to aggregate within the infected portion of a host population.

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 fecal 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 count, percent granulocyte count, and percent lymphocyte count through hematological 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.