All the organisms which live in the Arctic environment must deal with variable and, at times, extremely harsh living conditions. Under normal circumstances the environmental temperature limits for animal activity range from slightly below 0° C (32° F) when body fluids freeze, to 45° or 50° C (113° to 122° F) when proteins coagulate and dissolved albuminoids break down. The optimum temperature for life processes is often close to the maximum temperature an animal can tolerate. Climatic fluctuations demand constant adaptation, and in both aquatic and terrestrial habitats animal diversity decreases where conditions approach limiting values, such as in polar areas, deserts, and high mountains.
With regard to metabolism, terrestrial Arctic animals can be classified as homoeothermic (the internal body temperature remains relatively constant – independent from the ambient temperature) or poikilothermic (the internal body temperature varies according to the ambient temperature). The Arctic homoeothermic animals, also referred to as warm-blooded animals, include the birds and mammals, and they have a definite advantage in that they maintain the optimum temperature inside their bodies regardless of the ambient temperature. Living at the optimum temperature means their life processes such as nerve impulse transmission, muscle contraction, digestion, etc., operate at their most efficient rates. The poikilothermic, or cold-blooded animals of the Arctic (such as insects, spiders, mites, and a few amphibians and reptiles), are affected directly by the ambient temperature insofar as their metabolic processes are concerned. In other words: lower ambient temperatures produce a reduced body temperature and metabolic rate. This also means poikilothermic animals run the risk of freezing.
Homoeotherms produce internal heat within their bodies through two mechanisms. First, heat is a major byproduct of the metabolization of food, accounting for nearly 90% of the food's caloric potential. Second, heat energy is produced during the use, i.e. contraction, of muscles (shivering is a body's attempt to produce internal heat quickly by causing rapid contractions of various muscles). Body temperature is also affected by a variety of physiological and behavioral processes, including nervous control of the blood vessels near the body surface (dilating or constricting skin capillaries can significantly affect the rate at which heat is radiated through the skin), physically adjusting the insulative qualities of fur or feathers (by molting during the warmer months, or simply fluffing and aerating the coat), through the evaporation of water from the body (water has excellent heat absorbing abilities and can remove a significant amount of heat though the evaporation of sweat, and loss of body moisture through the lungs and mouth), and simple behavior (such as exposing oneself directly to the Sun's radiant energy, sitting in the shade, getting in cool water, etc.). And, of course, many animals migrate in order to be in the most hospitable areas during different times of the year.
Homoeothermic animals also must somehow insulate themselves from the cold. Air is a very poor heat conductor and is readily available as an effective insulator. The birds take full advantage of this by using feathers to retain an encircling layer of air around their bodies. Coverts and contour feathers cover and protect fluffy down feathers which hold the air close to the body. Birds can also hold their folded wings close to the body and receive even more protection from wind and low temperatures. Birds lack highly vascularized exposed structures such as ears and tails, their legs are tendonous, their beaks are nonvascular and horny, and many species have large internal air chambers which act as heat sinks. It is very important for birds to keep their feathers from getting waterlogged, because this condition tends to nullify the effect of air insulation. Water has a thermoconductivity value about 25 times greater than air, so it very quickly absorbs heat away from a warmer object. Most Arctic birds have an oil gland near the base of the tail which produces a secretion that the birds rub all over their plumage in order to make it water resistant.
Arctic poikilotherms endure tremendous variations in temperature and have developed strategies to survive freezing. They must prevent ice from forming inside their cells, and at the same time induce ice formation slowly within the rest of the body, including the contents of the gut, the blood, and the spaces between the cells. This applies to invertebrates and vertebrates alike. Arctic frogs actually freeze solid for several months of the year, but thaw out in Summer with no obvious deleterious effects. Some species appear to become dehydrated when exposed to low temperatures, and this causes the salts, sugars, and other constituents to concentrate in tissues, thereby reducing the freezing point temperature. If the cells are not ruptured during freezing the animal stands a good chance of surviving. Invertebrates produce cryoprotectant chemicals, such as glycerol, which allow body tissues to survive freezing by reducing the proportion of body water locked up in ice.
By contrast, the marine Arctic animals live in a much more uniform environment, because the water temperatures vary only slightly throughout the year. However, that temperature is close to or below the freezing point of fresh water. The cetaceans (whales, dolphins, and porpoises) protect themselves from heat loss with a thick layer of oilrich, subcutaneous fat, or blubber. Fat serves dual purposes since not only is it an excellent insulator, it also provides storedfood energy to allow the animals to survive when food is scarce. Unlike most mammals, the cetaceans have no hair and therefore cannot use air for insulation. The reason for this may be they lost their fur long ago through natural selection as a useless covering since they are unable to come out of the water in order to preen, clean, and aerate their fur.
The Arctic pinnipeds (fur seals, sea lions, walruses, and true seals) have a thick layer of insulative fat, like cetaceans, but, except for the walrus, they also have fur as an added protection against the cold. As much as 50% of the body weight of some seals is skin and fat. In fact, seals have such efficient protection against heat loss they cause little or no visible melting on ice even after having lain in one spot for several hours, and they will retain a high internal body temperature many hours after death. Like feathers, fur needs to hold air within the hairs in order to properly insulate the body from the cold surrounding waters. The fur of pinnipeds is made up of two types of hairs...guard hairs and wooly under fur. Each guard hair has a bundle of wooly under fur hairs surrounding it. The general rule is the more wooly under fur hairs per guard hair an animal possesses, the better its insulation, water repellency, etc. The density of fur varies considerably among the pinniped species, thereby giving widely different insulative values to different species. The dense, luxurious coat of fur seals is by far the most highly developed and efficient insulative pelt among the pinnipeds, and has long been highly regarded as a very valuable commercial commodity.
Many marine invertebrates deal with this situation by accumulating salts and organic compounds, such as glucose and amino acids, which lower the freezing point temperatures of the body fluids. Arctic fish, like all marine fishes, maintain a body salinity considerably lower than that of the sea water in which they live. Theoretically, they should freeze at a slightly higher temperature than does sea water. Some Arctic fish, however, can actually lower their freezing point temperature by accumulating an abundance of sodium, potassium, or chloride ions, or urea, in their body tissues. The enzyme systems of these fish are so efficient they are able to maintain a high level of activity even in these extremely cold waters. The content of dissolved oxygen is very high in the cold Arctic waters, and many fish are able to survive with fewer red blood cells. This gives them a white, or nearly colorless appearance. It is interesting to note that if these fish come into sustained contact with sea ice, their tissues will freeze and death results.
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