Animals with constant body temperature are called homoiotherms. They have insulating coat to check the loss of body heat. This coat consists of hair in most mammals, blubber (subcutaneous fat) in whales and seals and feathers in birds. Shivering warms up the body and perspiration cools down the body of these animals when required. These are also termed endotherms as they regulate their body temperature by physiological means and maintain more or less constant internal temperature. Poikilotherms are cold-blooded animals which are unable to regulate their body temperature which changes with change in temperature of environment e.g., fish, frog, lizards. They are also called as ectotherms.