What Makes An Animal Endothermic?

Woman adjusting thermostat
For human beings, the well-known room temperature range of 68 to 72 degrees Fahrenheit is optimal for allowing us to keep our temperature at 98.6 degrees.   Tetra Images/Getty Images

Endothermic animals are those that must generate their own heat to maintain an optimal body temperature. In ordinary language, these animals are commonly referred to as "warm-blooded." The term endotherm comes from the Greek endon, meaning within, and thermos, which means heat. An animal that is endothermic is categorized as an endotherm, a group that includes primarily birds and mammals. The other largest group of animals are ectotherms—the so-called "cold-blooded" animals with bodies that adapt to whatever temperature is present in their surroundings. This group is also very large, including fish, reptiles, amphibians, and invertebrates such as insects. 

Seeking to Maintain an Ideal Temperature

For endotherms, most of the heat they generate originates in the internal organs. For example, humans generate about two-thirds of their heat in the thorax (the midsection) with about fifteen percent generated by the brain. Endotherms have a higher rate of metabolism than ectotherms, which requires that they consume more fats and sugars to create the heat they need to survive in cool temperatures. It also means that in cold temperatures they must find means of guarding against heat loss in those portion of their bodies that are primary heat sources. There is a reason why parents scold their children to bundle up with coats and hats in the winter. 

All endotherms have an ideal body temperature at which they thrive, and they need to evolve or create various means of maintaining that body temperature. For human beings, the well-known room temperature range of 68 to 72 degrees Fahrenheit is optimal for allowing us to actively work and keep our internal body temperature at or near the normal 98.6 degrees. This slightly lower temperature allows us to work and play without exceeding our ideal body temperature. This is the reason why very hot summer weather makes us sluggish—it is the body's natural means of preventing us from overheating.

Adaptations for Keeping Warm

There are hundreds of adaptations that have evolved in endotherms to allow various species to survive in a variety of climate conditions. Most endotherms generally have evolved into creatures covered with some kind of hair or fur to protect against heat loss in cold weather. Or, in the case of humans, they have learned how to create clothing or burn fuels in order to stay warm in cold conditions. 

Unique to endotherms is the ability to shiver when cold. This rapid and rhythmic contraction of skeletal muscles creates its own source of heat by the physics of muscles burning energy. Some endotherms that live in cold climates, like polar bears, have developed a complex set of arteries and veins that are lie close to each other. This adaptation allows the warm blood flowing outward from the heart to preheat the colder blood flowing back toward the heart from the extremities. Deep-sea creatures have evolved thick layers of blubber to guard against heat loss.  

Tiny birds can survive frigid conditions through the remarkable insulating properties of lightweight feathers and down, and by specialized heat-exchange mechanisms in their bare legs. 

Adaptations for Cooling the Body

Most endothermic animals also have means of cooling themselves to keep their body temperatures at optimal levels in hot conditions. Some animals naturally shed much of their thick hair or fur during seasonal warm periods. Many creatures instinctively migrate to cooler regions in summer.

In order to cool down when too warm, endotherms may pant, causing the water to evaporate—resulting in a cooling effect through the thermal physics of water evaporating into vapor. This chemical process results in the release of stored heat energy. The same chemistry is at work when humans and other short-haired mammals sweat—this also cools us through the thermodynamics of evaporation. One theory is that the wings on birds originally developed as organs to dissipate excess heat for early species, which only gradually discovered the advantages of flight made possible by these feathered fans.  

Humans, of course, also have technological means of lowering temperatures to meet their endothermic needs. In fact, a large percentage of our technology over the centuries was developed out of the very basic needs of our endothermic natures. 

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Kennedy, Jennifer. "What Makes An Animal Endothermic?" ThoughtCo, Aug. 27, 2020, thoughtco.com/endothermic-definition-2291712. Kennedy, Jennifer. (2020, August 27). What Makes An Animal Endothermic? Retrieved from https://www.thoughtco.com/endothermic-definition-2291712 Kennedy, Jennifer. "What Makes An Animal Endothermic?" ThoughtCo. https://www.thoughtco.com/endothermic-definition-2291712 (accessed March 31, 2023).