Endotherm

An endotherm (from Greek ἔνδον endon "within" and θέρμη thermē "heat") is an organism that keeps its body at a comfortable temperature by producing heat inside its body, instead of just depending on the temperature around it. This internal heat usually comes as a by-product of the animal's normal activities, like eating and moving. But when it gets very cold or the animal isn’t active, it can use special tricks to make more heat, like shaking its muscles rapidly (shivering) or using special tissues (brown adipose tissue).

Only birds and mammals are considered true endotherms. However, some other animals, like the Argentine black and white tegu, leatherback sea turtles, certain sharks and fish such as lamnid sharks, tuna and billfishes, cicadas, and winter moths, also show some degree of internal heat control, known as being mesothermic. Some reptiles, especially certain types of python and tegu, can only maintain their body heat during their reproductive season.

Endotherms are often called "warm blooded" in everyday language. The opposite of an endotherm is an ectotherm, which relies on external sources of heat to regulate its body temperature, but the difference between the two isn’t always clear-cut.

Origin

Endothermy, the ability to keep a stable body temperature, was thought to have started near the end of the Permian Period. Some scientists believe it began in a group called Mammaliamorpha during the Late Triassic period, around 233 million years ago. Others think it appeared later, during the Middle Jurassic time, in early true mammals.

Signs of endothermy have been discovered in many ancient creatures, including early synapsids like "pelycosaurs", pareiasaurs, ichthyosaurs, plesiosaurs, mosasaurs, and some early archosauromorphs. It’s even possible that the very first amniotes were endotherms.

Mechanisms

Many endotherms, such as mammals and birds, have more mitochondria in their cells. This helps them create heat by burning fats and sugars faster. Because of this, they need to eat much more food than animals that cannot control their own temperature.

To save energy, some animals let their body temperature drop when they are resting. This can happen for short periods each day, like in hummingbirds, or for long periods during winter, like in animals that hibernate. Even humans get a little colder during sleep.

Smaller animals lose heat quickly, so they need ways to keep warm, like fur or feathers. Animals that live in water, like seals, have thick layers of blubber. Some birds, like penguins, have special feathers and blood vessel arrangements to keep heat in.

In hot weather, animals also have ways to stay cool. They might pant, sweat, or use body parts like ears to release heat.

Pros and cons of an endothermic metabolism

Endotherms, or warm-blooded animals, can keep their body temperature steady even when the outside temperature changes. This helps their bodies work best, no matter where they live. They have special ways to warm up or cool down as needed, giving them tight control over their temperature.

Endotherms need more energy and food to keep their temperature constant. They can stay active more often throughout the day and year compared to cold-blooded animals, which rely on the sun or other outside sources to heat up. This constant warmth may also help protect them from certain infections and allow them to reproduce in more places. However, it also means they must eat more to keep up their energy levels.

Facultative endothermy

Many insects can keep their bodies warmer than the surrounding air by moving their muscles. For example, honey bees can warm up by moving their flight muscles without actually flying. This helps them stay active in cooler weather, but only works when it's not too cold.

Some snakes, like the Python molurus and Morelia spilota, also use this method to keep their eggs warm. The mother snake shakes her body to generate heat, helping the eggs incubate properly.

Regional endothermy

Some ectotherms, like certain fish and reptiles, can use regional endothermy. This means that their muscle activity keeps specific parts of their bodies warmer than other areas. This helps them move better and use their senses more effectively in cold places.

Contrast between thermodynamic and biological terminology

Students might get confused because the words used in physics and biology mean different things. In physics, "exothermic" means giving out heat, and "endothermic" means taking in heat. But in biology, it’s the opposite in a way. Animals called "ectotherms" need outside heat to stay warm, while "endotherms" make their own heat inside to keep their bodies at the right temperature.