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Sexual dimorphism of Apodemus speciosus in wild populations

Sexual size dimorphism was found in many Apodemus species, and males were dominant in many cases (review, Montgomery 1989). However, it was not found in wild populations of A. speciosus (Imaizumi 1962; Miyao and Mori 1967). Imaizumi (1962) did not find any sexual difference in external morphology (head and body length, ear length, and hind foot length) in the Izu, Miura, and Izuoshima populations. Imaizumi (1962) determined the age class of each sample by checking molar wear, and showed that males and females belonging to the same age class were not different in these external traits. However, Oh and Mori (1998) showed sexual difference in growth of body weight and external morphology of A. speciosus under captivity. There was no sexual difference at birth for all characters, but significant differences were found at nearly 40 days of age in body weight, head and body length, and tail length. Thus, there is inconsistency in sexual dimorphism between wild and captive A. speciosus. It is therefore, necessary to clarify the sexual dimorphism in wild populations of A. speciosus.