Eurytherm

A eurytherm is an organism capable of functioning across a wide range of ambient temperatures. This classification includes all life stages, from larvae to adults. Most mammals, including humans, are considered eurytherms. This ability is a major evolutionary advantage. It allows species to inhabit diverse geographic areas. It also helps them survive environmental crises like ice ages. This capacity is often linked to the tolerance of the organism's proteins. In contrast, stenothermic organisms can only operate within a very narrow temperature range.

Eurythermic organisms use different thermal coping mechanisms to survive. They can be categorized as either regulators or conformers. Regulators, such as endotherms, maintain a consistent internal body temperature regardless of the environment. Conformers, like tardigrades, allow their internal physiology to vary with the external temperature. Regulators often use behavior to assist with homeostasis. For example, reptiles use basking or burrowing to manage heat. Humans also use behavior, such as wearing clothing or using air conditioning.

Humans demonstrate complex internal regulation through thermogenesis. When cold, humans use shivering to create heat through involuntary muscle contractions. This process signals the body to produce irisin, a hormone. Irisin helps convert white fat into brown fat. This brown fat contains a protein called thermogenin. Thermogenin performs non-shivering thermogenesis by uncoupling the electron transport in the mitochondrion. This process causes energy to be lost as heat instead of being stored as ATP. To cool down, humans rely on sweat evaporation. Eccrine sweat glands produce a watery fluid that evaporates to cool the blood.

Tardigrades are extreme examples of eurythermal conformers. They can enter an anhydrobiotic state known as a tun. In this state, they reduce their bodily water to just 1–3% weight. This allows them to survive temperatures from –273 °C to 150 °C. Their survival in extreme cold is a form of cryptobiosis called cryobiosis. Unlike some organisms that use antifreeze proteins, tardigrades tolerate ice formation. They use ice nucleating proteins to manage this process in their extracellular water. Scientists speculate they could theoretically survive the temperatures found on Mars.

Many animals show the success of eurythermy through their wide ranges. The green crab is a common littoral species. It lives from Iceland and Norway to South Africa and Australia. It can survive water temperatures between 8 °C and 35 °C. The desert pupfish is another example found in places like the Colorado River Delta. This fish functions in waters ranging from 8 °C to 42 °C. Killer whales are considered a cosmopolitan species. They inhabit nearly every latitude of the ocean. They can withstand water temperatures from 0 °C to 35 °C.

Plants also exhibit eurythermal traits, particularly in boreal forests. Boreal deciduous conifers, such as the black spruce, are cold-eurythermic. The black spruce grows from Indiana to the Arctic Circle. It can endure temperatures as low as –85 °C and as warm as 20 °C.

This is very different from stenothermic plants like the banyan tree. Tropical plants often have a narrow tolerable range, such as 10 °C to 25 °C. Eurythermy provides the mobility and agility necessary for survival in changing climates.

The biological cost of eurythermy is often seen in the struggle of stenotherms. Coral reefs provide a notable example of this vulnerability. Most coral species are stenothermic and sensitive to heat. Rising ocean temperatures cause corals to expel their zooxanthellae algae. This algae provides the coral with food and color. When expelled, the coral undergoes bleaching. This has caused a 50% mortality rate in corals off Cape York, Australia. Globally, there is a 12% bleaching rate in observed reefs. The ability to handle thermal shifts is a primary factor in evolutionary fitness.