Warm-blooded

The term warm-blooded describes animals that maintain a body temperature higher than their surroundings.

Scientists often use more precise terms to explain how this works. They study thermoregulation, which is the way animals control their body temperature. While many people use the labels warm-blooded and cold-blooded, scientists now find these terms too simple. Animals actually use many different methods to manage their heat. This creates a wide spectrum of temperature control across the animal kingdom.

There are three main categories used to describe these processes. Endothermy is the ability to control body temperature through internal means. This can include things like shivering muscles to create heat. The opposite of this is ectothermy. Homeothermy refers to maintaining a stable internal temperature regardless of the outside environment. This stable temperature is often higher than the air or water around the animal. The opposite of homeothermy is poikilothermy. Finally, tachymetabolism describes a high resting metabolism. This means the animal's biological processes are almost always active. Creatures with slow metabolisms are called bradymetabolic.

Most birds and mammals exhibit all three of these traits at once. They are endothermic, homeothermic, and tachymetabolic. However, some animals show different patterns through a process called heterothermy. For example, some small birds and bats change their state during sleep. They may become poikilothermic and bradymetabolic when they rest. Other animals also show unique ways to manage heat. The opah is a fish that shows warm-blooded characteristics. Swordfish and certain sharks use circulatory mechanisms to keep their brains and eyes warm. This helps them detect and react to prey more effectively. Tunas and some sharks also keep their muscles warm to improve stamina while swimming.

Heat is generated through a biological process called metabolism. This involves chemical reactions in cells that break down glucose. This process produces adenosine triphosphate, or ATP, which is a high-energy compound. ATP powers many different cellular processes. During these energy conversions, metabolism is somewhat inefficient. About 60% of the available energy is converted into heat rather than ATP. In most organisms, this heat simply escapes into the environment. Endothermic homeotherms are different because they produce more heat and retain it better.

These animals use several tools to stay warm. They often have insulation like fur and blubber in mammals, or feathers in birds. If their temperature drops, they may use shivering to create more heat. Shivering involves rapid muscle contractions that use up ATP. This stimulates cellular metabolism to produce more heat. Almost all eutherian mammals, except for swine, have brown adipose tissue. This is a type of fat with mitochondria that can perform non-shivering thermogenesis. This process uses an uncoupling protein to release heat directly.

When animals get too hot, they must shed the excess energy. They often use evaporative cooling to lower their temperature. Some mammals do this by sweating. Many mammals and all birds use panting to stay cool. These methods are generally not found in poikilotherms. Maintaining this temperature requires a lot of food. Because tachymetabolic creatures have such high energy needs, they struggle when food is scarce. Their bodies are essentially "on" all the time, requiring constant fuel.

Some scientists believe warm-bloodedness evolved for protection. One hypothesis is that it serves as a defense against fungal infections. Very few fungi can survive at the high body temperatures of birds and mammals. In contrast, insects, reptiles, and amphibians often deal with fungal issues. High internal temperatures may act as a barrier against pathogens from the environment. This helps these animals stay healthy in various habitats. The ability to regulate heat is a complex and vital part of animal life.