Cold-blooded animals are often just as warm as warm-blooded ones
Calling reptiles and fish cold-blooded is misleading. Their bodies often sit in the same temperature range as mammals. The real difference, which John Hunter pinned down through many observations, is steadiness: birds and mammals hold an almost constant internal temperature, while most other animals drift with their surroundings.
Thermoregulation is an organism's ability to keep its body temperature within limits even when the outside world is much hotter or colder. Animals that do not bother, called thermoconformers, simply match their environment. The rest sit on a spectrum. Endotherms generate most of their heat through metabolism, and their cells carry more mitochondria so they can burn fats and sugars faster when it turns cold. Ectotherms rely on outside heat, basking in sun or retreating into shade, and a lizard warming itself on a hot rock is soaking up both radiation and conduction.
Keeping cool takes ingenuity. Sweating works well for animals with sweat glands, but furry animals rely mainly on panting. Cats, dogs and pigs have sweat glands only on their foot pads and snouts, where the moisture mostly improves grip. Birds flutter the skin of their throats rapidly. Desert camels even use dense coats to block incoming heat.
Holding warmth relies on insulation and thrift. Down feathers trap air much as fur does, and many mammals have a layer of fat under the skin, called blubber in whales and polar bears. Some fish make antifreeze proteins that stop ice crystals from forming in their tissues. Other animals deliberately let their temperature fall to save energy, as in bear hibernation, bat torpor and the short overnight torpor many creatures use to survive frosty nights.
In both endotherms and ectotherms, the control centre is the preoptic area at the front of the hypothalamus. Work in rats shows that nerve cells there send a steady brake signal to circuits driving heat production, strengthening it in hot conditions to open skin blood vessels and relaxing it in the cold. Human limits are real: normal core temperature is about 37 degrees Celsius, and hypothermia begins below 35.
Source: Thermoregulation
