Caudal Vertebral Fragilities Related to Loss of a Tail Part in Two Species of the Japanese Field Mice

Abstract. In the Japanese field mice, Apodemus argenteus and A. speciosus, loss of a tail part has often been observed under natural conditions. Such loss is considered a way of avoiding damage during physical accidents and attacks. Here, we studied the types of losses, types of caudal breakages, and breakage positions along the tail. The losses mainly consisted of intervertebral breakage (inter-VB) and intravertebral breakage (intra-VB). VB occurred widely along the tail in both Apodemus species and showed no specific tendency for the inter-VB positions. Intra-VB was confirmed at caudal vertebrae of the middle portion of the tail, consisting of vertebrae with lower relative width (minimum width/maximum length in each caudal vertebra) ranges showing a mean of approximately 0.15–0.28 in both Apodemus species. Previous studies in lizards and rodents found autotomic inter-VB and intra-VB to be positioned in a specific small portion of the tail. However, we found that in field mice, inter-VB and intra-VB were widely distributed along the tail without a regular position. Therefore, it is suggested that both inter-VB and intra-VB as self-defense ways in Apodemus appear to be mere torn-off breakages rather than autotomies, due to certain fragilities in intra- and intervertebral structures.