The earth offers a huge variety of possible environments to inhabit: the hot arid environments of the desert, the salty environment of the oceans, the darkness of the deep sea, low oxygen environments of mountain peaks, and the frigid environments of the Arctic and Antarctic poles. This diversity of living conditions is reflected in the intriguing physiological adaptations developed by animals that live in these environments.
Adaptions to Cold
Temperature has a widespread impact on design. Two basic approaches to dealing with the challenge of temperature are to either maintain a constant and relatively high body temperature independent of ambient temperature (endothermy) or to let body temperature fluctuate with environmental temperature (ectothermy).
Endothermy. Endotherms maintain a high internal temperature through metabolic heat generation. Most of this heat comes from metabolism in the gut and brain. In cold weather, increased muscular activity through shivering or simply exercising provides a mechanism to increase metabolic heat production.
Some endotherms, like the arctic fox, are cold-weather specialists. Their most obvious strategy against the cold is insulation provided by a thick layer of fur. Aquatic animals rely predominantly on blubber for insulation as fur loses much of its insulation value upon immersion in water.
Another cold weather strategy is to temporarily decrease metabolic rate and body temperature. This regulated decrease in body temperature decreases the temperature difference between the animal and the air and therefore minimizes heat loss. Furthermore, having a lower metabolic rate is less energetically expensive. Many animals survive cold frosty nights through torpor, a short-term temporary drop in body temperature. Other animals such as marmots take a much more drastic approach: They hibernate through the cold months, letting their body temperature fall to a few degrees above ambient temperature. Contrary to popular belief, bears are not true hibernators as they undergo only a slight drop in body temperature and this activity can only be considered a deep sleep.
Ectothermy. Ectotherms, which rely mostly on external sources of heat, adopt much different strategies to the cold. Ectotherms have little or no insulation. This is helpful to gain heat from the environment but ectotherms have difficulty coping with cold temperatures. Without the ability to prevent heat loss, cold-weather ectotherms either must be able to tolerate freezing or to be able to live in sub-freezing environments without ice formation in their bodies. Freeze-tolerant animals like the wood frog can survive the freezing (crystallization) of up to 65 percent of their body water. Freeze-intolerant animals, including many antarctic fish, avoid freezing by having antifreeze compounds in their plasma to lower the freezing and supercooling point of their tissues.
Adaptations to Heat and Dryness
A major challenge in hot and dry environments is the balance of water and temperature regulation. For endotherms, the main cooling mechanism is evaporation of water, either across respiratory surfaces or across the skin in those animals possessing sweat glands (mammals). Animals with a body covered by fur have limited ability to sweat, and rely heavily on panting to increase evaporation of water across the moist surface of the tongue and mouth. Birds have no sweat glands and therefore all birds pant. Animals adapted to hot and dry environments have mechanisms for minimizing water loss while surviving the heat. Interestingly, dense fur on desert inhabitants may also help to insulate the animal from heat gain.
Long loops of Henle of the kidney are another adaptation to arid environments. These long tubes are capable of super-concentrating urine, and enabling desert dwellers such as the kangaroo rat to conserve water. Big noses also help in the heat. A camel's elongated nose is an adaptation to minimize water loss across the respiratory surface of the nasal passages and even to keep the brain cool. Camels also are known to let their body temperature rise during the day and dissipate the extra heat load during the cool night through conduction (contact with a cool surface), which does not require water.
Small animals, with their high surface area-to-volume ratio, are in great danger of heat overload in hot environments. Most small animals therefore remain in burrows during the day and come out at night when the temperature is lower. (The nocturnal lifestyle of desert-adapted rodents explains why gerbils keep their owners up at night.)
Adaptations to Marine Environments
Marine environments pose a similar problem to an arid environment, the lack of fresh water. Bony fish osmoregulate (control salt regulations) in this high-salt environment by drinking seawater and eliminating salt through pumps in the gills. Similarly, marine birds drink seawater and eliminate salt through glands located in their eye orbit. Sharks have the curious arrangement of salt glands in the rectum.
The ocean floor provides the strange environment of high ambient pressure and little or no light. One adaptation to lack of light has been the loss of eyes and pigmentation in some deep-sea fish. Other organisms have adapted to low light levels by possessing bioluminescent systems, either by having luminous organs or carrying bioluminescent bacteria. Such a system is useful for species recognition, luring prey, startling predators and mating. Deep-sea life also requires an adaptation to the extremely high pressure found at depths. Barophilic, or pressure-loving, organisms have adapted ways to avoid problems caused by high pressure. One adaptation is the modification of the set of lipids in cell membranes, designed to maintain fluidity despite the high pressure. Relatively pressure-insensitive enzymes are also found in organisms that live at great depths.
Adaptations to Low Oxygen Concentration
Just as high pressure influences organismal design, the low barometric pressure (and thus low oxygen availability) of the skies also presents an evolutionary force on physiology. A dramatic example of high altitude adaptation is seen in the bar-headed goose, a bird whose migration path between India and Tibet requires flight over Mount Everest. Research suggests that these birds maintain a phenomenal blood supply to flight muscles, and their blood has a unique hemoglobin structure, which optimizes oxygen transport in high-altitude conditions. Warm, stagnant bodies of water also present a low-oxygen environment and fish inhabiting these waters survive by managing to breathe both air and water. Lungfish, as the name suggests, possess both gills to breathe water and lungs to breathe air. It is likely that an organism similar to this air-breathing fish gave rise to terrestrial vertebrates millions of years ago.
Every organism on Earth represents a successful path to adapting to a specific environment, which helps to explain the impressive biodiversity of life present today.
see also Adaptation; Bony Fish; Cartilaginous Fish; Kidney; Osmoregulation; Temperature Regulation
Maureen E. Basha
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