Biomes Of Australia
April 16, 2003
Adaptations of Mammals to Arid Australian Environments
High temperatures and low rainfall characterize much of Australia. It is hard to believe that animals can survive in arid conditions, much less prosper. Yet there is a whole range of mammals, among other life forms, that do. These animals survive because they have adaptations that allow them to live in the hot, dry conditions. The function of the adaptations is to balance thermoregulation with water gain and loss. For instance, many mammals that live in the desert obtain much or all of their water from the food they consume. The reduced water intake is partially balanced through concentrated urine and dry faeces. Evaporative cooling helps to regulate temperature. To limit the water lost through evaporative cooling, mammals are nocturnal, have light coloration and other body features to help dissipate heat, and use microenvironments to reduce heat gain. This is only a short list of the many amazing adaptations Australian mammals have to survive the harsh arid conditions; however, it serves to illustrate the balance between thermoregulation and water regulation.
Seventy percent of Australia is considered arid or semi-arid (Climate averages, 2000). Arid regions are characterized by receiving 100-250 millimeters of rain a year, and semi-arid regions are characterized by receiving 250-500 mm of rain per year (Costa, 1995). The average rainfall in Australia is only 165 mm per year (Climate averages, 2002). Not only are these regions faced with sparse rainfall, but they are also confronted with extreme temperatures. During the summer months of January and February, temperatures regularly exceed 40°C in the arid regions (Climate averages, 2002). The temperatures are even more scorching for small animals that live close to the ground because the soil becomes much hotter than the air (Walsberg, 2000). Therefore, Australian mammals must have adaptations to cope with the heat and lack of available water. Additionally, they must have adaptations to balance thermoregulation with water regulation. This paper will address these adaptations and explain how they work.
Mammals can be broadly classified as homeotherms, which means their body temperature is relatively independent of the external environmental temperature (Ricklefs, 2001). Mammals and birds keep their body temperatures between 37°C and 38°C, however there are exceptions (Walsberg, 2000). Maintaining a constant internal temperature requires animals to have mechanisms to regulate their body temperature. This is challenging for desert animals due to the extreme heat and lack of water. Many of the cooling techniques mammals use involve evaporation. However, the animals also have unique adaptations to counter the evaporative water loss.
When the surroundings of a mammal are cooler than its body temperature, conduction and radiation are the main ways an animal will dissipate heat (Schmidt-Nielsen, 1964). However, the air temperature is often higher than mammalian body temperatures in Australia, so the only physiological thermoregulatory mechanism available is evaporation (Farid, 1989). Mammals use three evaporative cooling techniques that include sweating, panting, and saliva spreading.
Most small mammals do not sweat because they would lose too much body mass if they did. For example, in a hot desert the amount of water a mouse would use to maintain a constant body temperature would be more than 20% of its body weight per hour (Schmidt-Nielsen, 1954). This is a lethal amount; therefore, smaller animals must find other ways to regulate their body temperature. Camels do not visibly sweat because the sweat is forming and evaporating from under the fur on the surface of the skin (Schmidt-Nielsen, 1964). Evaporation from the skin maximizes the amount of heat transported from the body and minimizes water lost through sweating, as compared to evaporation from the fur of an animal. When sweat evaporates from the fur, less cooling occurs, and the animal has to sweat more to maintain a constant body temperature.
Panting is rapid, shallow respiration that cools an animal by increased evaporation from the respiratory surfaces. It is a common technique that small animals make use of. According to Bligh (1972), as body size increases the effectiveness of panting decreases. Mammals smaller than 100 kg employ panting as the primary cooling mechanism, while mammals larger than this use sweating (Schmidt-Nielsen, 1972). For example, kangaroos and rodents employ panting as the major source of heat loss (Dawson, 1972).
Saliva spreading is a means of thermoregulation that marsupials use. During heat stress, saliva will drip from a kangaroo’s mouth and is then wiped on its fore and hind legs (Dawson, 1972). This technique has two disadvantages. The animal is wetting its fur instead of the skin, and this reduces the effectiveness of evaporative cooling (Dawson, 1972). Also, the animal cannot spread saliva when it is moving, so other techniques of evaporative cooling must be used in these situations (Schmidt-Nielsen, 1964).
Evaporative cooling techniques use water, a resource that is scarce in arid environments. Therefore, adaptations that reduce the amount of water lost through evaporation are important. One such adaptation is the nasal counterflow system, which reduces respiratory evaporation (Walsberg, 2000). This system functions by lowering the temperature of exhaled air. If a kangaroo rat inhales dry air at 28°C, the air will be exhaled at 24°C (Schmidt-Nielsen, 1972). Water loss can be reduced up to 85% by exhaling at a lower temperature (Schmidt-Nielsen, 1972).
The counter-current exchange system works because inhaled air flows over moist mucous membranes, and this causes water to evaporate from them. The evaporation cools the membrane and reduces its temperature. When the warm, moist air from the lungs passes over the cool mucous membrane on the way out, the air is chilled and water condenses on the membrane (Walsberg, 2000). Mammals that are adapted to desert conditions are better able to use this technique. Schmidt-Nielsen (1964) found that the kangaroo rat evaporates approximately 0.54 milligrams of water per milliliter of oxygen. The albino mouse evaporates 0.85 mg H2O/ml O2, and man evaporates 0.84 mg H2O/ml O2 (Schmidt-Nielsen, 1964). The kangaroo rat is adapted to extreme temperatures and dry conditions, whereas the albino mouse and man are not.
Although mammals are homeotherms, some are able to raise their body temperature as a way to decrease the amount of water used for thermoregulation. Camels and gazelles have been noted to increase their body temperature by 5-7°C during the day (Walsberg, 2000). This occurs more often when the animal is dehydrated. A 500-kilogram camel that is hydrated will face a temperature increase of 2°C, whereas a dehydrated camel’s temperature will rise by 6°C (Farid, 1989). Hyperthermia serves two functions. First, the mammals are saving water by letting their body temperature increase instead of using evaporation to keep it at a constant temperature. Second, the mammals are also saving water through reduced evaporative cooling because the thermal gradient between the animal’s body temperature and the air temperature has decreased (Walsberg, 2000). Therefore, hyperthermia is an adaptation that mammals can use to conserve water.
Behavioral adaptations are used to reduce the amount of heat gained by animals, and, therefore, reduce the need for evaporative cooling. One basic behavioral adaptation is the timing of activity rhythms. Nocturnal animals are able to regulate their heat load by resting during the day, since nighttime temperatures can be 15-20°C lower than the daytime maxima (Walsberg, 2000). Examples of nocturnal animals include the quoll, bilby, and the spinifex hopping mouse. Fossorial animals, such as mulgaras, spent much of their time below ground eating stored food (Costa, 1995). Crepuscular animals take advantage of the slightly cooler mornings and evenings and are only active at those times (Costa, 1995). All these behaviors are an attempt to escape the daytime heat, and to evaporate less water.
The use of microenvironments is another type of behavioral adaptation. The Red kangaroo is found in the open plains, and the Euro inhabits rocky hill country (Dawson, 1972). Since kangaroos are diurnal, they rest in the shade during the day to escape the sun. The Euro rests in rocky outcroppings, and the Red kangaroo rests in the shade of trees. The rocky areas are able to shield more of the radiation from the sun; therefore, the Euros face a smaller heat load (Dawson, 1972). To make up for this, the Red kangaroos have fur that reflects solar radiation better. The red fur of the Red kangaroo reflects approximately 35% of solar radiation, while the dark gray-brown fur of the Euro reflects only 22% of solar radiation (Dawson, 1972).
Burrows are another type of microenvironment that is used by smaller mammals. In Arizona, burrow temperatures of a round-tailed ground squirrel were recorded. The air temperature was 40°C and the soil surface was 70°C, but the burrow temperature did not exceed 29°C (Schmidt-Nielsen, 1964). According to Schmidt-Nielsen (1964), many burrows are at depths where evaporative cooling is not needed because it does not get hot enough in the burrows to require this technique. Additionally, absolute humidity in burrows can be three to four times higher than the outside air, which reduces the amount of water evaporated from the respiratory tract (Schmidt-Nielsen, 1964 & Costa, 1995).
Walsberg (2000) challenges the findings of Schmidt-Nielsen and others about burrow temperature and humidity. He claims that the measurements for the burrow temperatures were not taken from the hottest deserts, and many mammals place their burrows at similar depths in the hotter deserts. Walsberg (2000) also believes that in order for burrow temperatures to stay below 30°C, the burrows would have to be over 2.5 meters deep. This would have a large effect on the animal’s heat budget and its thermoregulation because the animal would have to exert a great deal of energy to dig a hole this deep. Walsberg (2000) also considers the general belief that burrows have high humidity to be inaccurate. He states the measurements that support this belief were made in a part of Arizona that receives over 500 mm of rain, so this area is not considered to be semi-arid (Walsberg, 2000). Additionally, during the data collection period of May-June 1948, the soil was moister than usual due to greater than normal rainfall in the months proceeding the measurements (Walsberg, 2000). Normally dry sand and the porosity of it will make humidity levels in the burrows lower than previously thought (Walsberg, 2000). However, more studies will need to be done to resolve this matter.
Torpor and Metabolic Rate
Many small mammals, such as rodents and squirrels, will enter a period of torpor in response to severe heat. This is a period where metabolism decreases and the heart and respiratory system slows down based on a daily circadian rhythm (Costa, 1995). Torpor can be considered a water-conserving mechanism because the animal’s body temperature is lowered, and it does not have to rely as heavily on evaporation. If the period of torpor becomes longer, it is called aestivation or summer dormancy (Costa, 1995). Aestivation allows an animal to survive when there are high temperatures and a scarcity of water and/or food. An aestivating animal can live longer off its energy reserves due to lowered metabolism, and there is reduced water loss though lowered breathing rates (Schmidt-Nielsen, 1964).
Metabolic rates are lower during torpor and aestivation. However, mammals adapted to desert conditions have lower metabolic rates in general than similar mammals that live in less extreme conditions (Schmidt-Nielsen, 1972). This reduces the internal heat load, and therefore, the water used for evaporation.
Water and Food Consumption
Humans obtain about 60% of the water they need from ingested liquid, 30% from ingested food, and 10% from metabolism (Campbell et al., 1999). A rodent adapted to arid conditions obtains approximately 90% from metabolism and 10% from ingested food (Campbell et al., 1999). It is estimated that the Euro can go 2-7 days without water and possibly much longer (Schmidt-Nielsen, 1964). The predaceous marsupial mulgara can go its whole life without ingesting water (Costa, 1995).
These mammals still need water, but they have adapted to obtaining water from the food they eat and from metabolism. The fawn hopping mouse eats seed, small insects, and green leaves for moisture, and Kowaris eat insects and small mammals to obtain water (Vandenbeld, 1988). Both of these animals, and most other desert animals, are generalist (Costa, 1995). Generalist feed on varied food sources, which is important when food resources are scarce, as they often are in arid regions (Costa, 1995).
The ability to excrete concentrate urine and dry faeces is an important adaptation to arid conditions. Mammals that are adapted to the desert have very long loops of Henle compared to animals that live in aquatic environments and less arid regions (Campbell et al., 1999). A longer loop of Henle allows urine to become very concentrated due to the osmotic gradient in the kidneys (Farid, 1989). Desert rodents can have urine five times as concentrated as that of humans (Schmidt-Nielsen, 1964).
Antidiuretic hormone (ADH) is important in regulating the volume of urine excreted and its concentration. ADH is produced in the hypothalamus and is released into the bloodstream in response to increased blood osmolarity (Campbell et al., 1999). A larger release of ADH leads to a fast renal response that causes increased reabsorption of water (Schmidt-Nielsen, 1964). This leads to a smaller volume of more concentrated urine being excreted.
Camels produce dryer faeces than other ruminants (Farid, 1989). For example, sheep produce faeces with 45% water after 5 days of water deprivation, while camels produce faeces with 38% water after 10 days of water deprivation (Farid, 1989). Even when fresh, the droppings of camels and desert rodents are almost dry to the touch (Schmidt-Nielsen, 1964). This water reabsorption takes place in the alimentary canal and the colon, and functions to help maintain an animal’s water balance (Farid, 1989).
Body Features and Circulatory Adaptations
Some Australian mammals have long legs to hold their bodies as far away from the solar heated ground as possible. Many have light colored coats to help reflect solar radiation. Others, such as bilbies and rabbits, have large ears that help cool the animal. The ears are covered with tiny blood vessels that help radiate heat from the body (Costa, 1995). When the animal is hot, vasodilation occurs. This increases the diameter of superficial blood vessels resulting in escalated blood flow (Campbell et al., 1999). More heat will then be transferred to the environment through convection, radiation, and conduction.
Camels, the fat-tailed marsupial mouse, and other herbivorous mammals store fat as a morphophysiological adaptation to arid conditions (Costa, 1995). This fat can be turned into metabolic water during times of water or food scarcity.
Energy is required to overcome friction and gravity for all types of locomotion. However, different types of locomotion require varying amount of energy. Many mammals in Australia hop, which is an energy-efficient type of locomotion. When animals go from walking to running, there is an increasing energy cost; however, once kangaroos start moving there is no additional energy cost (Campbell et al., 1999). This is because when a kangaroo lands, energy is stored in the tendons of its hind legs. This stored energy is used to power the next hop (Campbell et al., 1999). For example, when a kangaroo is hopping at 30 kph, its energy costs are about half that of a similar sized animal running at the same speed (Campbell et al., 1999).
A recent study was done on how the Western chestnut mouse repletes its store of glycogen after physical activity. Rats and humans must eat following activity to totally restore their muscle glycogen (Bräu et al., 1999). However, this is a problem for the Australian mammal because it mainly feeds on carbohydrate-poor grasses (Bräu et al., 1999). In lower vertebrates, such as fish, frogs, and lizards, muscle glycogen can be resynthesized from endogenous carbon sources (Bräu et al., 1999). It would be advantageous if the Western chestnut mouse and other mammals that have diets poor in carbohydrates could restore their glycogen completely without food (Bräu et al., 1999).
Bräu et al. (1999) designed and performed an experiment to test if the mouse could completely restore its glycogen without food. The mice were fasted for forty hours and then chased for three minutes on a track. A control group was fasted but not chased (Bräu et al., 1999). Following this, the mice were anaesthetized after being allowed to recover zero or fifty minutes with no food. Muscle samples were taken and examined for glycogen, glucose, and lactate (Bräu et al., 1999).
It was found that these Australian rodents could completely recover, to pre-exercise levels, their levels of muscle glycogen without food. This is an important adaptation considering the lack of carbohydrate rich food in the rodents diet, and the need to respond quickly in ‘fight or flight’ responses (Bräu et al., 1999). The researchers state that they do not know the molecular mechanism of how this works or how widespread this adaptation is yet (Bräu et al., 1999).
The mammals of Australia are not only able to survive in arid environments, but they are able to thrive due to a wide array of adaptations. These adaptations allow the mammals to maintain a balance between thermoregulation and water balance. Mammals use evaporative cooling techniques to maintain a constant body temperature, while at the same time they use behavioral adaptations to reduce heat load and water loss. Many mammals do not need to ingest water to survive. Instead, they get it from the food they eat. Nasal counterflow, concentrated urine, and dry faeces also reduce the amount of water an animal loses. All these adaptations and more, play an important role in the animal’s ability to conquer the desert.
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<http://www.bom.gov.au/climate/averages/> Accessed 2003.
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