A warm-blooded (homeothermic) animal is one that can keep its core body temperature at a nearly constant level regardless of the temperature of the surrounding environment (that is, to maintain thermal homeostasis). This can involve not only the ability to generate heat, but also the ability to cool down if necessary. Also known as endotherms, warm-blooded animals control their body temperature by regulating their metabolic rates.
Endotherms include birds, mammals, and insects. The advantages of endothermy are increased enzyme activity and a constant body temperature, allowing these animals to be active in cold temperatures. On the other hand, the disadvantage is the need to maintain thermoregulation, even during inactivity, otherwise the organism will die. Other living creatures such as fish and reptiles are called ectothermic or cold blooded, meaning that they cannot control their internal temperature and so were assumed to have the same temperature as their surroundings.
In winter, there may not be enough food to enable an endotherm to keep its metabolic rate stable all day, so some organisms go into a controlled state of hypothermia called hibernation, or torpor. This deliberately lowers the body temperature to conserve energy. In hot weather, endotherms expend considerable energy to avoid overheating: they may pant, sweat, lick, or seek shelter or water.
Diverse mechanisms can come into play to regulate body temperature, such as shivering (to generate heat from muscle contractions), blanching (circulatory changes to direct less heat to the skin), flushing (circulatory changes to radiate more heat from the skin), panting or sweating (to increase heat loss through evaporation).
Biochemical processes are heat dependent. The rule of thumb is that they go faster when they are warm and slower when they are cold. The advantage of being warm blooded is that you can always maintain yourself near one optimum temperature and you will now have all your internal chemical reactions functioning at their best. This means that you can think, move, digest, etc with your best possible speed and efficiency.
Warm blooded animals warm themselves by digesting food. The disadvantage of being warm blooded is that you must always have large amounts of food. Once you get accustomed to being warm blooded you are committed to it. When the core temperature of a warm blooded animal does change, even by a few degrees, the animal will rapidly lose its ability to function.
The advantage of being cold blooded is that you need much less food. This means that you can survive famine, long ocean voyages, and shortage of prey when warm blooded creatures would surely die.
The disadvantage of being cold blooded is that you need to have multiple chemical pathways available to you, some of them for cooler temperature functioning, others for warm. You may also find yourself moving or thinking more slowly than normal, simply because you are cold today.
It has been a while since the original distinction was made between warm and cold blooded animals. Time has passed, science has advanced, the warm cold business has been studied in closer detail. It turns out that the cold blooded animals all use behavioral means to adjust their temperatures, sometimes quite effectively. There are also creatures that do not properly fall into either category.
Some examples of in between creatures include: