Parasite removal affects foraging behavior of the Formosan field mouse (Apodemus semotus)

Foraging behavior can be influenced by many factors, including risk and individual condition. While predation risk has been shown to influence a forager’s patch use, the effects of other types of risk, such as parasitism risk, are rarely explored. Recent evidence suggests that hosts may reduce the amount of time spent foraging in a patch contaminated with feces, potentially to avoid parasite infection. In this study, I examined whether wild Formosan field mice, Apodemus semotus, alter their foraging behavior in response to parasite removal treatment using both field and laboratory experiments. I used ivermectin to remove helminth parasites from A. semotus. In the field experiment, I live-trapped the rodents and marked them with passive integrated transponder (PIT) tags. I set up food stations, each consisting of 3 food trays: (1) food contaminated with feces that have high numbers of parasite eggs (high risk patch); (2) food contaminated with feces that have low numbers of parasite eggs (low risk patch); and (3) food not contaminated with feces (no risk patch). A data logger and a PIT antenna were attached to each food tray to record the time an individual spent in that patch. I found that A. semotus treated with ivermectin spent less time in risky patches (high- and low-risk patches) than no-risk patches. In the laboratory experiment, I measured and compared energy intake, body mass, hematological and serum biochemical parameters of ivermectin-treated and control groups. Ivermectin-treated males had higher energy intake than the control whereas ivermectin-treated females showed the opposite pattern. However, I did not find parasite removal effects on body mass, immune function (hematological parameters) or nutritional status (serum biochemical parameters). This study provided empirical evidence that parasite removal can influence risk sensitivity in a wild rodent. Thus, parasites may play a role in hosts’ foraging behavior and by doing so, benefit from improved transmission.