Foraging activity is an essential behavior for animals. Animals can get energy from food, but also pay costs during the activity. Therefore, what food they select and how they forage it are important. Wood mice feed on acorns of Fagaceae trees. Acorns have various ecological trait, one of which is insect infestation; there are several categories such as insect species and the presence or absence of its larvae. However, whether the mice examine the insect-infestation status is unclear.
We investigated the response of large Japanese field mouse (Apodemus speciosus) and small Japanese field mouse (A. argenteus) to acorns of Japanese chestnut (Castanea crenata) with special attention to insect infestation. By five-year survey on conditions of the acorns on the forest floor, we found that the majority were sound (no insect-infestation), infested by moth larvae with exit hole (hereinafter, EH), and infested by weevil larvae still inside acorn (hereinafter, SI). As manipulation experiment in the field, we placed the acorns with combinations of sound vs. EH and sound vs. SI on the forest floor in 2022 and 2024. Consequently, removal rates of sound and insect-infested acorns by the mice were similar in the overall value, being over 80% in both years. Based on the removal order, in the combination of sound vs. EH, the mice removed sound acorns earlier, whereas there was no significant difference between sound and SI acorns, in the removal order. Regarding behavioral patterns, we observed removal after examining (sniffing and clutching) acorns and removal without the examining, in both acorn combinations and both mouse species. Interestingly, in the combination of sound vs. SI, A. speciosus spined the acorns with its forelegs to examine it. Furthermore, we compared the removal rate of sound acorns between the cases with and without the examining, and found that in the combination of sound vs. EH, the former (with-examining) was significantly higher than the latter (without-examining) in 2022, and the same trend was found in 2024. On the other hand, in the combination of sound vs. SI, there was no significant difference in the rate between them in both years. From the above results, we discuss foraging strategies of A. speciosus and A. argenteus for insect-infested chestnut acorns.