Ectotherm

Pseudemys turtles (shown here basking for warmth) are ectothermic.

An ectotherm, from the Greek εκτός (ektós) "outside" and θερμός (thermós) "hot", refers to organisms that control body temperature through external means. As a result, organisms (such as frogs) are dependent on environmental heat sources and have relatively low metabolic rates. For example, many reptiles regulate their body temperature by basking in the sun. The opposite of ectothermy is endothermy, where heat is primarily generated as a result of internal metabolic processes. Many ectotherms are also poikilotherms, meaning their temperature varies over a wider range than homeotherms.

Ectotherms are animals that warm their bodies by absorbing heat from their surroundings. In most ectotherms, the body temperature fluctuates with changes in the surrounding temperature; these ectotherms are called poikilotherms. The body temperature of snakes, for example, cools in cold weather and warms up in hot weather. However, most marine fishes and invertebrates live in water that stays at constant temperature. Their body temperature, therefore, does not change, and these ectotherms are therefore considered homeotherms.

Adaptations

Certain ectotherm behaviors help regulate body temperature. To warm up, reptiles find sunny places, and stretch out for maximum exposure. If it gets too warm, lizards alternate between sun and shade. Amphibians warm up by moving into the sun or diving into warm water. They cool off by entering the shade. In cold weather, honey bees huddle together to retain heat. Butterflies and moths may orient their wings to maximize exposure to solar radiation in order to build up heat before takeoff. Many flying insects, such as honey bees and bumble bees, also raise their internal temperatures endothermically prior to flight, by contracting their flight muscles without moving their wings.

In addition to behaviors, physiological adaptations help ectotherms regulate temperature. Diving reptiles conserve heat because their blood circulates inward toward the body core during a dive. The skin of bullfrogs secretes more mucus when it is hot, allowing more cooling by evaporation. Many ectotherms exist at a lower temperature during torpor, a state of slowed metabolism. This helps them survive a food shortage. If the food supply increases, they come out of torpor in a few hours.

Advantages and disadvantages

Ectotherms gain most of their heat from external sources such as sunlight energy, therefore they have less dependence on respiration for generation of heat. As a result, ectotherms such as reptiles can survive on just a single large meal per week. The advantage of this to ectotherms is that they are less vulnerable to fluctuations in food supply, increasing their chances of survival.

On the other hand, tropical ectotherms may be particularly vulnerable to climate warming and are experiencing large increases in metabolic rate and will probably have an increased need for food. A study Looking at the effects of increased temperature on entire life cycle of tropical ectotherms suggested species reproduction may be greatly reduced by global changes in temperature predicted during the 21st century.

Contrast between thermodynamic and biological terminology

Note that because of historical accident, students encounter a source of possible confusion between the terminology of physics and biology. Whereas the thermodynamic terms "exothermic" and "endothermic" respectively refer to processes that give out heat energy and processes that absorb heat energy, in biology the sense is effectively inverted. The metabolic terms "ectotherm" and "endotherm" respectively refer to organisms that rely largely on external heat to achieve a full working temperature, and to organisms that produce heat from within as a major factor in controlling their bodily temperature.