Endothermy and ectothermy relate to a widely accepted classification scheme based on the source of heat stored in the body.
- Endotherms: animals that can generate their own heat
- Ectotherms: animals that rely on environmental sources of heat
Endotherms, sometimes called ‘warm blooded’ animals, generate body heat as a by-product of metabolism. They often elevate their body temperatures to above ambient temperature and most produce metabolic heat at a high rate. Most endotherms have fur, feathers or fat as a form of insulation and this allows them to conserve heat.
Endothermic Animals And Their Ability To Live In Cold Environments
Endotherms have been able to survive in habitats that are too cold for most ectotherms, due to their ability to maintain body temperature above ambient temperatures in cool climates. This does however mean it is at great metabolic cost for an endotherm. At rest an endotherm's metabolic rate is at least 5 times higher then that of an equally sized ectotherm.
Endotherms use a variety of mechanisms to survive in cold environments. Endotherms, which include almost all mammals and birds, use a variety of physiological and behavioural mechanisms to produce and maintain body heat. These include:
- Thermogenesis
- Countercurrent heat exchange
- Body insulation
How Endotherms Use Thermogenesis, Countercurrent Heat Exchange And Body Insulation To Deal With Cold Environments
There are two primary means of heat production or thermogenesis: shivering and nonshivering thermogenesis. Shivering means the use of muscle contraction to generate heat. Most endothermic vertebrates (animals with backbone, vertebrae), as well as some insects use shivering thermogenesis.
The nervous system activates groups of skeletal muscles, which start the shivering. There is little net muscle movement and the muscle activation causes ATP (adenosine triphosphate) to provide energy for the contraction. The muscle contractions produce no useful physical work, so chemical energy released appears as heat.
Nonshivering thermogenesis is when enzyme systems for fat metabolism are activated and fats are broken down to produce heat.
Endothermic animals don’t want their limbs hindered by layers of insulation as this could affect movement. Instead these animals have well vascularised limbs that are thin and have large surface areas to dissipate body heat. Heat loss from the periphery (external surface of body), can be reduced by countercurrent heat exchange.
Warm arterial blood that flows into a limb will pass through arteries that lie beside veins that carry blood back from extremities. As arterial blood passes venous blood, the arterial blood gives up heat to the returning blood in veins and thus the blood in arteries heading to the periphery has been cooled within a few degrees of ambient temperature, leaving little heat to lose.
Endotherms will fluff up fur or feathers and move to warmer microenvironments to avoid heat loss in a cold windy environment. Other examples of the use of insulation are in Arctic animals with thick pelage (wool or other soft covering), plumage or a layer of subcutaneous (under the skin) fat. These layers show acclimatization and change with season to match the endotherm's needs.
In warmer seasons these animals shed fur or feathers or grow new body coverings, thereby producing insulation during winter, but preventing overheating during summer.
Endothermic Animals and Their Ability to Live in Hot Environments
Endotherms living in hot environments need to get rid of excess heat and conserve body fluid. Camels, for example, deal well in hot climates. They have a large body mass and thick pelage that actually insulates them from external heat. The large body mass allows for slow loss of heat during the cold nights. Smaller animals that cannot stay in the heat for long periods of time will spend much of the day inside burrows, coming out in the cooler evenings. Other ways endotherms deal with the heat are:
- Heat windows
- Evaporative cooling
How Endotherms Use Heat Windows And Evaporative Cooling To Deal With Hot Environments
Body surface temperature is an important factor in heat loss. The closer the body surface temperature is to the core temperature, the higher the rate of heat loss. In endotherms this heat loss is regulated by blood flow to the surface vessels. This is referred to as ‘heat windows’.
An example would be the horns of various mammals such as goats and cattle, which are highly vascularised, or the large, lightly furred ears of the rabbit, also well vascularised. These heat windows allow heat dissipation by regulation of blood flow to these areas.
Some vertebrates sweat or pant to produce evaporative cooling. In sweating mammals, sweat glands in the skin eliminate water through pours onto surface of the skin. Sweating is under autonomic control.
Not all mammals have sweat glands. These mammals use the respiratory system to lose heat by evaporative cooling. Highly vascularised nasal passages play a role in retaining body heat, so animals will pant, breathing rapidly through their mouths, to dissipate heat.
Resource and further reading:
Silverthorn, D.U, 2007, ‘Energetic Costs of Meeting Environmental challenges’, in Human Physiology, ed. Benjamin Cummings, San Francisco, chap. 17.
Endothermic Goats
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