First of all we need to understand what ectothermic and endothermic animals are. Animals differ in their abilities to regulate body temperature (thermoregulation). We sometimes use the terms "cold-blooded" or "warm-blooded." Most reptiles feel cold to the touch, while mammals and birds often feel warm.
Somewhat more precise descriptions can be made by using the terms poikilothermic and homoiothermic. The body temperature of poikllotherms is relatively variable, while that of homeotherms is relatively constant.
Even more useful terms are Ectothermic or Endothermic, which suggest two different mechanisms of thermoregulation. Ectotherms generally obtain heat from their external surroundings. Their body temperature varies, corresponding at any time with the temperature of their external environment.
Endothermic animals, on the other hand, have relatively constant body temperatures. Their body temperature is independent of that of their external environment. Monkeys and walruses, for example, both have body temperatures of about 38„aC, despite living in very different habitats.
However if body temperature rises above its optimum level (usually around 40„aC in mammals) then the enzyme rate inside the body will go into sharp decline. This is because enzymes are proteins, and become denatured. One of the first organs to be affected is the brain. Since the brain controls breathing and the circulation, the rise in body temperature disrupts the normal functioning of these important systems.
If the body temperature decreases dramatically (hypothermia) then this will slow metabolic activity and impairs brain function.
Here is a graph to show the relationship between the body temperature and environmental temperature for a cat (endotherm) and a lizard (ectotherm)
Also we need to clarify what is meant by an extreme climate. In this investigation I will be using two different climates, The Desert and The Arctic
When an endotherm is subjected to severe cold it is liable to lose heat energy but this can be counteracted in a number of ways;
„h It could raise the hairs into a more vertical position by the contraction of the erector pili muscles. Air can then be trapped in the spaces between the hairs and, being a poor conductor of heat, it serves as an insulatory layer round the animal. This is an involuntary response brought about by the nervous system.
„h The arterioles leading to the superficial capillaries constrict and as a result blood flow to the surface of the skin is greatly reduced (vasoconstriction). This is brought about by the sympathetic nervous system and is useful in uncovered regions such as the ears where the surface-volume ratio is particularly large and so particularly susceptible to cold.
„h The metabolic rate can be increased therefore heating the inside of the body. A general increase in the metabolic rate is brought about by the hormones adrenaline and thyroxin which are produced in large amount during cold conditions. There is then a general increase in muscle tone, which is then followed by spasmodic contractions (shivering)
The response to high temperature is basically the reverse of the above processes; i.e „h Hairs lie flat against the body
„h The arterioles dialate (vasodialation) allowing blood to flow back to the surface of the skin so heat can be lost by diffusion.
„h Sweating occurs and the evaporation of the water from the skin cools the skin and blood.
„h Panting occurs. In some animals there are no sweat glands except in the pads of the paws. Thus they pant which speeds up evaporation from the lungs, pharynx and other moist areas helping to cool blood.
„h Metabolic rate decreases, so less energy is generated by the body
Arctic Polar Bears (Thalarctos maritimus)
Polar bears have successfully adapted to one of the world's most inhospitable climates, and remain active throughout the harsh winter. They are found on the sea ice of the Arctic throughout the North Polar basin.
The creamy white fur of the polar bear is especially dense and each of the individual hairs is hollow and contain insulating air spaces. This heavy fur and a large fat layer helps to retain body heat even while the animal is swimming in the coldest arctic weather. The fur does not absorb water so once the animal has climbed out of the water it merely has to shake itself dry. It has plantigrade feet (heel and sole touching the ground), with five sharp claws for gripping the ice. Long hair between the pads protects the bear's feet from the cold and provides traction on the ice.
Another adaption is in the thickness of the sub-cutaneous fat. Animals living in cold climates, polar bears and seals for example have a particularly thick layer of sub-cutaneous fat. In the seal and other marine mammals such as whales, the fat is called blubber.
In all Endotherms, including polar bears, thermoregulation is controlled by the tuber cinereum of the hypothalamus. If this small downward projection at the base of the brain were to be removed the animal would lose the ability to regulate its temperature and essentially becomes 'cold blooded.'
The tuber cinereum is stimulated either by nerve impulses from the cold receptors in the skin or by direct cooling. It is sensitive and will react to temperature changes of only a few tenths of a degree. If the body temperature is too cold, the thermoregulator stimulates a thyroid secretion, which increases the rate of metabolism. If the body temperature continues to drop, the work of internal organs like the liver may be increased and muscle activity in the form of shivering may begin, for muscular activity is the main source of heat available to animals.
Camels are large native animals in the desert regions of Asia and northern Africa. There are two types; the Dromedary with one hump and the Bacterian camel that has two humps. The humps are stores of flesh and fat that are absorbed as nutrition when food is scarce.
What makes the camel survive so well in the desert is its ability to be able to go for many days without water. There is however no evidence that the camels store water in their stomachs. Desert vegetation can provide a camel with enough water for several months, and a thirsty camel can consume as much as 30 gallons of water at a time to replenish its reserves. In addition the camels metabolism can alow them to drink the salt water that is available in their habitat.
A camel's body temperature can rise by as much as 6„aC to 8„aC to minimize its sweating further aiding in water conservation. Thick, broad sole pads and thick callosities on the joints of the legs and on the chest, upon which it rests in a kneeling position, enable it to withstand the heat of the desert sand. Moreover, its nostrils may be closed against flying dust, and its eyes are shielded by very long eyelashes and a third eyelid.
Horned toad (Phrynosoma cornutum)
These are short tailed, short-legged lizards in the iguana family. Found mostly in the dry regions of the western United States and Mexico. They can control their temperature by varying their heartbeat and the rate of the bodies' metabolism.
Although reptiles are considered as ectotherms, they do regulate their own body temperatures to a surprising extent. This thermoregulation is behavioral rather than metabolic as in endotherms, however.
It is usually assumed that reptiles have a body temperature roughly the same as the air around them. but it has been shown that the temperature of reptiles is usually higher than that of the surrounding air. In fact, the body temperatures of lizards may be as much as 40„aC or 50„aC higher than the air temperature. For example a lizard lying on a black rock on a cool but sunny day will take on the temperature of the rock rather than that of the air.
If its body temperature is too low, the lizard may flatten itself against the rock, absorbing heat from the sun and more heat from the rock. If it gets too hot, it may leave the rock or stand up slightly to allow some air to pass underneath it. Snakes can regulate their temperature by placing different proportions of their bodies in sun or in shade. The variations are numerous, but the end result is the same ¡V some degree of temperature regulation in reptiles.
In conclusion the animals which exist in the extreme climates of the world have adapted to be able to live well in these habitats and will probably stay living in these habitats for a long time to come.
Books: Biology, Principles and processes
By Michael Roberts, Michael Reiss, Grace Monger
Life on earth and Life in the freezer
By David Attenborough
Video: BBC Biological science-Endotherms and Ectotherms
Multimedia: Encarta 97 Encyclopedia