Environmental Parasitism Risk and Host Infection Status Affect Patch Use in Foraging Wild Mice

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.