House Musk Shrews (Suncus murinus) Do Not Copulate in the Light Period when First Paired in the Dark

House musk shrews (Suncus murinus) are known to have an alimentary canal more structurally similar to that of human beings than rats or mice (Kurohmaru, 1985), to be sensitive to motion sickness (Kaji et al. 1990), and to be more resistant to Xrays than C3Hf/He mice (Tsubouchi et al., 1984). They have been widely used in studies on disease models for motion sickness, gastric cancer, and hematopoietic disorders. They are also expected to be models for hormone-dependent tumors because of their spontaneous, androgen-dependent pilosebaceous tumor (Itami & Takayasu 1983, 1984). Knowledge of their reproductive efficiency is needed for such research. It is considered that matured female house musk shrews with reflex ovulators are ready to mate with males at any time. Recently Hashimoto et al. (2004) investigated whether a male house musk shrew could impregnate more than one female within 24 h. In the present study, we focused our attention on the appearance of ejaculation during 24 h in male house musk shrews when first paired in the light or dark. Materials and Methods Animals Ten sexually experienced male and female 1-yearold house musk shrews of the Jic: SUN strain were supplied from the Central Institute for Experimental Animals (Kawasaki, Japan). The animals were housed in polycarbonate cages (220 deep x 380 wide x 200 high mm) with wood shavings as bedding in an animal room kept at 24± 2 ̊ C and 55± 10% humidity with a 12:12h light:dark cycle with lighting on at 07:00. They were given CLEA305 diet (CLEA Japan, Inc., Tokyo, Japan) and tap water ad libitum. All experiments were performed under the Guidelines for Animal Experimentation of the Japanese Association for Laboratory Animal Science.