Permalink : http://hdl.handle.net/2115/89098

JaLCDOI : 10.14943/lowtemsci.81.173

KEYWORDS : クマ;冬眠;精子形成;着床遅延;歩く冬眠;bear;hibernation;spermatogenesis;delayed implantation;walking hibernation

クマ類の冬眠は，体温の降下度が小さい，中途覚醒がない，筋肉や骨の退行がない，インスリン抵抗性になる，などの特徴を有する．オスでは，冬眠中（2～3月）に精子形成が再開し，メスよりも早く冬眠から覚める．メスは，初夏の交尾後に着床遅延を維持するが，冬眠導入期（11月下旬～12月上旬）に着床する．その後約2ヶ月で胎子発育を完了し，冬眠中間期の1月下旬～2月上旬に出産する．さらに冬眠後半期に新生子を哺育するが，母グマのみ冬眠を継続する．ヒグマやツキノワグマと違ってホッキョクグマでは，メスだけが出産・哺育のために冬眠するが，雌雄共に夏～秋にはほぼ飢餓状態になるため“歩く冬眠”と呼ばれる冬眠様生理状態に切り替える．
Hibernation in bears is characterized by slight drop in body temperature, no periodic arousal, no muscle or bone regression, and insulin resistance. Males resume spermatogenesis during hibernation (February-March) and emerge from hibernation earlier than females. Females maintain delayed implantation after mating in early summer and implant embryos at the time of hibernation induction (late November-early December). Female bears then complete fetal development in about two months and give birth in the mid-hibernation period (late January to early February), followed by nurturing their neonates in the late hibernation period, when only the mother bear continues to hibernate but not neonates do. Unlike brown bears and black bears, only females of polar bears hibernate to give birth and nurture their young, but both male and female polar bears switch to a hibernation-like physiological state called "walking hibernation" from summer to autumn, when they get starvation.