Temperature fluctuations:
They can occupy a place in the environment where the temperature remains constant and compatible with their physiological processes. Physiological processes may have adapted to the range of temperature in which the animals are capable of living or they can generate and trap heat internally to maintain constant body temperature, despite fluctuation in the temperature of external environment.
Ectotherms:
They derive most of their body heat from the environment rather than from their own metabolism. They have low rates of metabolism and are poorly insulated. In general, reptiles, amphibians, fishes and invertebrates are ectotherms, although few reptiles, insects and fishes can raise their internal temperature. Eclotherms tend to move about the environment and find places that minimize heat or cold stress to their bodies.
Endotherms:
Birds and Mammals are endotherms because they obtain their body heat from cellular processes. A constant source of internal heat allows them to maintain nearly constant core temperature, despite the fluctuating environmental temperature. Core means body’s internal temperature as opposed to the temperature near its surface. Most endotherms have bodies insulated by fur of feathers or hairs and large amount of fat. This insulation enables them to retain heat more efficiently and to maintain high core temperature. Endothermy allows animals to stabilize their core temperature so that biochemical processes and nervous system functions can proceed at steady high levels. Endothermy allows some animals to colonize habitats derived to ectotherms.
Homeotherms and heterotherms:
Most endotherms are homeotherms (maintain constant body temperature) and most ectoderms are heterotherms (have variable body temperature) there are many exceptions. Some endotherms vary their body temperatures seasonally (e.g. hibernation), others vary it on daily basis. For example some birds (e.g. humming birds) and mammals (e.g. Shrews) can only maintain high body temperature for a body mass so small that they cannot generate enough heat to compensate for the heat lost across their large surface area.
Humming birds must devote much of the day to locating and sipping nector (a very high calorie food source) as a constant energy source for metabolism. When not feeding, humming birds rapidly run out of energy unless their metabolic rates decrease considerably. At night humming birds enter a sleep like state called daily torpor and their body temperature approaches that of cooler surroundings, some bats also undergo daily torpor to conserve energy. Some ectotherms like some reptiles that can maintain fairly constant body temperatures by changing position and location during the day to equalize heat gain and loss.
Thermoconformers:
Many invertebrates have low metabolic rates and have no thermoregulatory mechanisms, thus, they passively confirm to the temperature of their external environment. These invertebrates are termed thermoconformers. Zoologists know that many arthropods such as insects, crustaceans and horse shoe crab (limulus) can sense thermal variation e.g. ticks of warm blooded vertebrates can sense the ‘warmth of a nearby meal’ and drop on the vertebrate most.
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