On the basis of body temperatures, all the living beings have been classified into two categories: the cold-blooded and the warm-blooded. Cold-blooded creatures are those whose body temperature does not remain constant. Their body temperature changes according to the surroundings. It becomes high during the summer and low during the winter. Interestingly it changes even during the day – being less in the cool morning than in the hot moon. Frogs, tortoises, snakes, skunks, and some variety of fishes are the cold-blooded creatures. During the winter their temperature falls down to such a proportion that in order to survive they go into their holes inside the earth. If they do not do so, their blood would freeze and they would die. During winter they keep on sleeping in their holes inside the earth. This is called the ‘hibernation’ period. They do not take food during this period. Even their physical processes get slowed down. When they come up on the earth’s surface after this period, they remain very weak.
On the other hand warm-blooded creatures are those whose body temperature remains almost constant in winter or summer. The surroundings do not affect their body temperature. Man, bat, dog, cat, cock, cow, etc., come under this category. For example, the normal body temperature of man is 98.4°F. And of some other creatures it is as follows: bat, 104°F; dog, 102°F; cow, 101°F; cat, 101.6°F; cock, 101°F. They need more energy to maintain a constant body temperature. That is why the warm-blooded creatures need to consume more food. Their physical processes continue normally all round the year. There are special organs like hypothalamus in their bodies that control the temperature and do not allow it to go beyond or below a certain level. In these animals there is a balance between the heat lost and the heat produced.