Cold-blooded animals - Hen-on-dobutsu

An animal whose body temperature changes depending on the outside temperature, a term used in contrast to warm-blooded animals. They are also commonly called cold-blooded animals. All animals (invertebrates, fish, amphibians, and reptiles) except for mammals and birds belong to this category. The body temperature of small aquatic animals is almost equal to the water temperature, but the body temperature of large animals changes according to the outside temperature and is not always the same as the outside temperature. Fish often have a body temperature several degrees higher than the surrounding water temperature, and animals with moist skin such as frogs are said to have a body temperature slightly lower than the air temperature when at rest because they lose heat by evaporation. When the outside temperature is low, honeybees gather in groups to prevent heat loss and generate heat through muscle movement to maintain a body temperature several degrees higher than the outside air. When the body temperature of lizards drops, they diffuse the melanophores in their skin to absorb radiant heat and raise their body temperature, but conversely, when their body temperature rises too much, they increase their breathing rate to radiate heat and aggregate the melanophores to avoid absorbing heat. However, because cold-blooded animals cannot regulate their body temperature perfectly, when the external temperature drops below a certain level, their regulation limit is exceeded, and their internal metabolism slows down, causing them to lose activity. In areas with significant seasonal changes, cold-blooded animals hibernate during the cold season or overwinter as eggs or larvae. Generally, it is easier for cold-blooded animals to live in areas with high temperatures.

[Seiichiro Kawashima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

外界の温度によって体温が変化する動物のことで、恒温動物に対する語。俗に冷血動物ともいう。哺乳(ほにゅう)類と鳥類を除いたすべての動物（無脊椎(むせきつい)動物・魚類・両生類・爬虫(はちゅう)類）がこれに属する。小形の水生動物の体温は水温にほぼ等しいが、大形の動物の体温は外温のままに変化してつねに外温と同じになるとは限らない。魚類は周囲の水温より数度高い体温をもつことが多く、カエルのように皮膚の湿っている動物は、安静状態での体温は蒸発熱を奪われるために気温よりやや低いといわれる。ミツバチは外気温が低いときに、集団となって熱の発散を防ぐとともに、筋肉運動によって発熱し外気よりも数度近く高い体温を維持している。また、トカゲ類は体温が下がると皮膚にある黒色素胞を拡散し、放射熱を吸収して体温を上げ、逆に体温が上がりすぎると呼吸数を増して放熱し、黒色素胞を凝集して吸熱を避けている。しかし、変温動物の体温調節は不完全であるから、外温がある程度以下になると調節の限界を超え、体内の物質代謝が低下して活動力を失う。季節的変化の著しい地方では、変温動物は寒期に冬眠したり、卵や幼虫の状態で越冬したりする。一般に変温動物にとっては気温の高い地方がすみやすい。

［川島誠一郎］

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