April 17, 2002
Adaptations of Australian Animals to Desert Conditions
Australian desert animals are exposed to such conditions as scarcity of food, increased body temperature, and dehydration. However, through behavioral, physiological, and anatomical adaptations, they can survive in the harsh outback. What specific functions allow desert animals to conserve water and reduce heat gain while maintaining homeostasis? How is metabolism affected? For many Australian animals, enzymes or cells are altered and hormones adjusted. Australian Western chestnut mice exhibit a specific physiological adaptation of recent discovery. These mice are able to regain glycogen through endogenous carbon sources after periods of exercise, thereby making up for scarce food resources. Behaviorally, poikilotherms adapt to harsh desert conditions through quiescence, or inactivity during the day, and panting or licking for evaporative cooling. What other seemingly ordinary ways have Australian animals allowed for their survival? Research explains how Australian animals have adapted, such that their physiology and lifestyles prevent susceptibility to harsh desert conditions.
What exactly are the conditions to which Australian desert animals have adapted? In the summer, air temperatures can reach more than 110° F, plus intense sunlight can increase the effective temperature to at least 140° F. Also, rainfall is low during this time of year. As a comparison, under these conditions a human would need more than ten quarts of water a day, but red kangaroos adapted to water shortages may only need two to three quarts each week or two. (Dawson, "Red Kangaroos," 44) Also, due to extreme heat, plants sources of food are scarce. (Barboza, 29) How have animals behaviorally and physiologically adapted to the harsh conditions of the Australian outback? What physical characteristics separate them from animals of other climates? Particularly, how are the body systems affected by environmental stresses? Is there a noticeable change in metabolism rate or in excretion processes? What cooling processes and conservation habits do Australian animals exhibit? All adaptations, whether behavioral or physiological, are in effort to maintain homeostasis, the idea that the cells and organs of the body must keep a steady state to function. It is the scope of this paper to reveal some answers to the questions proposed as they relate to homeostasis.
It is important to understand the basics of cell functions and homeostasis before delving into the more complex topic of adaptation. (Phillips, 5) When external or internal conditions change, the body responds by counteracting the change. (Phillips, 6) Physiologically, cells are the principal factors in maintaining equilibrium. The steady state maintained by the body is a dynamic rather than static process. (Phillips, 9) Cells are constantly involved in regulation. Pertinent to the discussion of adaptations, cell homeostatic mechanisms maintain ion, metabolite, and water levels, and also serve to control enzyme functioning. (Phillips, 10)
The movement of water and electrolytes or ions is important in maintaining a concentration gradient within cells. This gradient affects flow of substances into and out of the cell. All organisms have two major organic ions, Na+ and K+. Normally, Na+ is concentrated outside the cell, while K+ is mainly inside the cell. (Phillips, 10) These ions equilibrate across the permeable cell membrane by diffusion, which is dependent on concentration gradients between the inside and outside of the cell. (Phillips, 11) When the levels of Na+ and K+ are not in equilibrium, the cell works to move the ions against the concentration gradient by a process known as active transport. Specifically, this is called the Na+, K+ pump, located within the cell membrane. ATP is the fuel required to drive active transport. Simple diffusion of ions or small molecules such as water across the cell membrane and along the concentration gradient does not require energy. Facilitated diffusion, which involves the use of transport carriers to permit ions against a concentration gradient, also does not require any ATP energy source. (Phillips, 13)
Maintaining homeostasis can also involve fluctuating enzyme levels, to control the rate of metabolism for example. Enzymes may be controlled by changing the concentration of substrates and products of the enzyme or by changing the concentration and catalytic activity of the enzyme itself. (Phillips, 20) Many metabolites can inhibit certain enzymes by binding directly to them. Another form of control is feedback inhibition. As the product of one enzyme inhibits an enzyme in an earlier enzyme pathway, negative control occurs. (Phillips, 21)
Poikilotherms vs. Homeotherms
Some basic behavioral adaptations can begin to explain how Australian desert animals have adapted to harsh conditions in an effort to maintain homeostasis. Poikilotherms can only regulate temperature through behavioral modifications because their body temperatures vary with environmental temperature. Common poikilotherms are reptiles, such as snakes and lizards. While poikilotherms can only change their behavior in response to environmental stresses, homeotherms have the ability to regulate body temperature at the expense of a certain amount of energy. The energy expended is minimal however. Processes involving energy expenditure gain heat, so homeotherms must be efficient in order to have a net heat loss. Mammals and birds are examples of homeotherms. (Phillips, 84)
Temporal Niches & Lifestyles
Differences in temporal niches and habitats exist among Australian desert animals. An obvious way of behaviorally overcoming the heat is to be nocturnal, sleeping during the day and becoming active at night. Crepuscular animals take advantage of the cooler parts of the day by being active during mornings and evenings. However, diurnal animals are active during the day to avoid predators at night and to optimize prey availability. Fossorial animals live underground where the temperature is cooler and moisture levels are higher. (Costa, 53) These burrowers can be either vertebrates or invertebrates. (Costa, 56) Snakes and wombats are fossorial animals. Larger animals such as the Euro kangaroo often spend hot summer days in caves and under rock ledges to escape the sun. (Dawson, "Thermoregulation," 143) Birds can somewhat escape the heat by soaring high in the air where it is cooler. (Schmidt-Nielson, 378)
During the hot part of the day, some Australian birds and mammals enter a state of torpor in which their metabolic rate lowers to conserve energy. (Phillips, 84) Torpor can occur daily according to an animals circadian rhythm. Small rodents especially demonstrate torpor. Many animals exhibit dormancy, which is a prolonged period of torpor. During this time, the animals metabolic rate, oxygen consumption, and body temperature are reduced to a minimum. In the summer, this period is called aestivation and can be seen in ectotherm and endotherm vertebrates. (Costa, 52)
Because food is scarce in deserts, it is beneficial for animals to be generalist feeders, meaning they do not just have a few different types of prey or food, but eat a variety. (Costa, 101) Strict herbivores often have a food shortage problem since their food supply is dependent on rainfall. Carnivores as well, may have fewer resources since their herbivore prey are scarce. Therefore, generalist feeders fare best. Some animals such as wombats have become accustomed to eating foods of poor quality, since their basic nutritional needs are low in the first place. In many cases, minimal food and water needs mean survival for Australian animals. (Barboza, 28)
Feeding before dawn is a strategy that enables animals to receive extra water that has accumulated on plants from the humid air. (Nagy, 55) Certain desert rats have taken advantage of the underground humidity. By storing food in underground burrows, seeds and plants can absorb moisture from the air, maximizing hydration. (Phillips, 52) Predators also affect animals feeding habits. Because water holes may be in an area known by predators, kangaroos risk the danger only about once a week for water (Dawson, "Red Kangaroos," 41).
Body Cooling Techniques
Cooling techniques include thermal panting in small mammals, reptiles, and birds, or gular fluttering exclusively in birds. Panting is the breathing movement of the thorax and abdomen, while gular fluttering is the rapid movement of the thin floor of the mouth and upper area of the throat. (Dawson, "Thermoregulation," 378) The incoming air cools the respiratory passage. Animals that pant are often of smaller size. The shorter and wider the animals throat is, the more effective it is at evaporative cooling. Because respiratory evaporation involves the release of water vapor, some desert animals exhale at temperatures below their normal body temperature to reduce evaporation. For example, kangaroo rats breathe in dry air at 82° F and breath out at a temperature below 75° F. This reduces water loss by 85%. (Schmidt-Nielson, 376)
While panting or gular fluttering relies on an animals own respiratory airflow for cooling, the moistening of skin and sweating rely on external airflow or wind for convection. Techniques of evaporative cooling allow for life under the hot sun. Kangaroos and quokkas, both marsupials, display this behavior as they lick their legs and other body parts exposed to the sun. Kangaroos spread saliva along their forearms over a superficial network of blood vessels, expediting the cooling of the rest of the body. (Dawson, "Red Kangaroos," 44) Some animals even use their urine as a means for moistening fur or skin. Animals that moisten their skin in this manner usually have a large surface area for which this technique works well. (Dawson, "Thermoregulation," 375)
Australian desert animals have specific body characteristics that aid survival in the heat. The epidermis is specialized in many cases. Many animals have adapted to the heat by having short, thin hair. Hair allows protection from the sun by preventing the inward flow of heat. (Costa, 49) Birds molt their feathers in the summer to reduce heat-retaining insulation. Molting is onset by environmental cues of the season such as the length of days. (Phillips, 41) Snakes have a thick skin of several cell layers to prevent water loss and therefore do not have to stay in wet areas. (Phillips, 4) A light coloration, more often in mammals, helps reflect sunlight. (Costa, 50)
Big ears of kangaroos and Australian rabbit bandicoots maximize body cooling due to the network of tiny superficial blood vessels, which act to radiate heat away from the body. Another characteristic enabling kangaroos to stay cool are long legs, which allow them to hold the core of their body away from the hot earth. (Costa, 50) Kangaroos have a strong tail for use as an extra leg to aid in hopping, reducing body stress and heat gain. Initially it takes a lot of energy to hop, but once they start going, it takes less energy in the long run. Perhaps, the spring of the tail stores energy for constant reuse for each hop. (Dawson, "Red Kangaroos," 41) Also to lessen heat load, internal organs may be reduced in size, as they are in wombats. (Barboza, 28) Each of these body characteristics functions to decrease heat gain or conserve water.
Circulatory Adaptations & Body Temperature
Behavioral adaptations aside, homeotherms have further adapted through the physiology of their body systems. The circulatory system is involved in thermoregulation. Minimizing heat, brain temperature can be kept lower than the central arterial blood temperature. Blood to the brain flows through the arteries that run through the cavernous sinus, which is filled with cool venous blood draining from the nasal region where evaporation occurs. In this way, the arterial blood to the brain is already cooled. (Schmidt-Nielson, 373)
Body temperature varies among Australian animals. Eutherian mammals keep their body temperature at 100° F, while marsupials can keep a normal body temperature between 95 and 97° F. Monotremes, such as desert living echidnas, are even more efficient, maintaining a temperature around 86° F. (Schmidt-Nielson, 373) Although it is effective if the animal can preserve a lower body temperature, a high normal body temperature is a benefit for those animals that do not have efficient means of keeping cool and hydrated. A higher normal body temperature contrasts less with extreme heats from the environment. (Schmidt-Nielson, 377)
The excretory system is a site of waste elimination and water and salt reabsorption for the maintenance of homeostasis. (Phillips, 43) Sweat, produced only by mammals, is both a cooling technique and an elimination process. It provides a layer of moisture on naked skin (areas like foot pads) for evaporative cooling. (Phillips, 41) Kangaroos are unique in that they stop sweating the instant they cease exercising. They pant instead to save water. (Dawson, "Red Kangaroos," 44) An interesting excretory adaptation in birds is the salt gland near their eyes. These salt glands excrete salt without water through the external nares. (Phillips, 54)
Many other intricate excretory functions prevent water loss. Certain desert rats drink very little, producing little urine. When not enough salt or water is taken in, the urine is reduced by a third to conserve these necessities. (Phillips, 51) The more water taken in, the more salts that are excreted along with the excess water. The desert rats efficiently prevent this problem. They can acquire enough hydration from their food intake. The presence of an extra long loop of Henle in rats helps to concentrate the urine and reduce water loss. (Phillips, 52) Even though the rats produce very little urine, excretion is effective. The urine has a high concentration of urea (about 24%), the waste product of protein metabolism. Some camels and kangaroos even have the capability of transforming urea into a new protein beneficial for the body. With less urea to be excreted, less water loss results. (Phillips, 53)
Similar to the absorption capabilities of the loop of Henle, an elongated large intestine and rectum in red kangaroos reabsorbs water as wastes pass down the digestive tract. Therefore, kangaroos produce fairly dry feces. (Dawson, "Red Kangaroos," 42) Insects, reptiles, and birds are also efficient in conserving water through their excretory system. Water is easily reabsorbed from the uric acid in their feces because this compound is not water-soluble. (Costa, 50)
As animals with few food resources, it is quite beneficial for red kangaroos to have microorganisms in their digestive tract to help break down foodstuffs. Plants composed of cellulose and lignin, structural carbohydrates, are incompatible with normal enzymatic digestion. Microorganisms such as bacteria, fungi, and protozoa live in the foregut of the kangaroos digestive tracts and ferment the structural carbohydrates that kangaroos cannot do alone. This symbiotic relationship is similar to that found in ruminants. (Dawson, "Red Kangaroos," 40) The small fatty acids that the microorganisms do not use are available for absorption by the kangaroo. The microorganisms are also useful in converting normally wasted nitrogen compounds into agreeable proteins for the kangaroo. (Dawson, "Red Kangaroos, 41) Wombats are even more efficient at this carbohydrate breakdown because microorganisms ferment food before it even reaches the small intestine. (Barboza, 27) Like kangaroos, wombats are able to reuse nitrogen from urea, at an amount of 42%. This reduces the amount of protein needed in the wombats diet. (Barboza, 28)
It is possible for some greatly efficient mammals to never drink their whole life if they can get their water requirements through food or from the breakdown of food into hydrogen molecules that can bond to inspired oxygen internally. Fat molecules contain many hydrogen atoms and are used for this purpose. Herbivorous animals show the adaptation of storing fat in various place of the body so that they can produce water when the resource is scarce. (Costa, 51)
Hormonal & Metabolic Adaptations
Hormonal regulation is affected by environmental stresses.Sweating involves a loss of water and ions. As Na+ ions are lost through the sweat glands, cells must counteract the electrolyte imbalance to maintain the ions needed for the chemical gradient. In response to changes in salt and water concentrations, hormones are secreted. Aldosterone is released from the adrenal cortex and antidiuretic hormone (ADH) produced in the hypothalamus, is released from the pituitary gland. Many desert rats have a high concentration of antidiuretic hormone in the blood to help reabsorb more water molecules for each Na+ ion pumped. (Phillips 42, 51)
ADH and aldosterone work directly to conserve water and salts, while the thyroid and adrenal glands decrease activity reduce heat load on the body. (Hadley, 194) Wombats exhibit extremely low levels of thyroid hormones for this reason. (Barboza, 28) Thyroidal release rates have also been shown to decrease during torpidity to reduce heat. (Hadley, 197) Likewise, metabolic levels are influenced by the endocrine system to slow down. (Hadley, 196) Desert animals overall have lower metabolic rates than their non-desert counterparts. (Schmidt-Nielson, 371) Marsupials have even lower metabolisms than Eutherian (placental) mammals. A low metabolic rate decreases the heat load of the body while allowing the animal to live on a certain amount of food for a longer time. (Schmidt-Nielson, 372)
In an environment of relatively few sources of nutrition, some rodents in Australia have adapted by internally restoring carbohydrate levels following activity. The Western chestnut mouse, along with over 50 other native rodents, displays this mechanism in which muscle glycogen is resynthesized from endogenous carbon sources. Fish, frogs, snakes and lizards also exhibit glycogen repletion. In an experiment conducted by Lambert Brau, et al., adult, male Western chestnut mice caught from Barrow Island were fasted for 40 hours before experimentation. This time was long enough for their digestive tracts to deplete most of the hepatic glycogen but not their stores of muscle glycogen. The mice were chased around a circular track for 3 minutes and then allowed to recover during a 50 minute time period. Under anesthesia, the mice were killed and tests on samples of their muscle were carried out. (Brau, 272)
Results showed that during the recovery period, the glycogen content of the muscle returned to previous levels even though no food was ingested. Exercise caused an increase in muscle and plasma lactate concentrations, which also returned to normal levels during recovery. After exercise, endogenous carbon sources caused the decrease of lactate along with the increase of glycogen. The synthesis of glycogen involved the lactate as its precursor. (Brau, 273) Although further details are yet to be discovered, the significance of the experiment is that Western chestnut mice are not strictly dependent on diet for complete resynthesis of muscle glycogen following exercise. They can restore glycogen from endogenous carbon sources alone. The mechanism could be a crucial adaptation in desert-living animals to conserve carbohydrate supplies. (Brau 275)
Desert animals have effectively adapted to the scarcity of food and water resources and the hot and dry conditions of the Australian outback. The primary physiological response in homeotherms occurs at the cellular level to maintain homeostasis. Cells and enzymes regulate body mechanisms in such a way to reduce heat load produced by the bodys own internal processes and that caused by environmental factors. In homeotherms, hormones shut metabolism down during times of heat or stress to reduce heat gain. The excretory system is probably the most specialized system in conservation of water and electrolytes. It serves to reabsorb these substances while effectively eliminating wastes. The lack of prey or nutritional plant matter makes the function of glycogen repletion a particularly valuable adaptive mechanism. Behaviorally, poikilotherms and homeotherms reduce heat and conserve water through evaporative cooling techniques, such as licking, panting, and gular fluttering, allow thermoregulation. Temporal niches and torpor are also factors in behavioral homeostasis. Even basic body anatomy plays a role in survival. All these amazing behavioral and physiological adaptations suit Australian animals for life in the outback.
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