ADAPTATION TO WATER LACK
1. Why are Indian cattle breeds more tolerant of heat than European breeds?
2. How has the camel adapted to limited water availability?
3. Contrast the water-lack adaptive mechanisms of sheep and donkeys with each other and with the camel.
Throughout history, certain animals have had to adapt to conditions of water lack because of their habitat (little adaptation has been necessary, however, for cattle, swine, dogs, and cats). The problem is compounded by exposure to high temperatures. Indian cattle breeds (Zebu and Brahman) are more tolerant of heat than European breeds because of greater sweating (and hence cooling) and not because of any special water conservation mechanism. Adequate water must be provided. Camels, donkeys, and sheep, however, have adapted for coping with periods when water is not available.
Camels
The means whereby the dromedary (one-humped camel) has adapted to water lack has received the most interest. Many legends have been associated with this camel and its ability to survive for long periods in the desert without water. It was thought that the metabolism of hump fat, and the greater metabolic water yield from it, provided the extra water needed, but this notion has generally been discredited. The amount of fat in the hump is not great and, even though more metabolic water is derived from fat metabolism, more energy (ATP) is also produced. Consequently, only half as much fat is metabolized as would be the case for protein and carbohydrate, resulting in about the same water production.
The most important finding is the camel’s ability to endure a degree of dehydration equal to about 30% of its body weight, compared with 10 to 12% for most other animals. This permits it to survive longer when water is not available. Another adaptive mechanism is the camel’s ability to store body heat during the day (resulting in a body temperature increase) rather than dissipating it.
In one day, the camel’s body temperature might range from 34.2 to 40.7 °C,,a much greater range than the 38 to 39.3 °C for the dairy cow. Water is thus conserved because heat dissipation requires the evaporation of water. The camel awaits the.cool desert night to dissipate the stored heat (see Chapter 13). The camel also has summer fur, which is most prominent on its back; this is effective in reducing solar heat gain. Finally, the camel rapidly ingests water, after a period of dehydration, up to 25% of its body weight, which permits rehydration at the infrequently found watering spots. The lowering of plasma osmotic pressure that occurs when such a large volume of water is absorbed after rapid ingestion does not cause the hemolysis that might otherwise occur because of the following reason. During dehydration, plasma osmolality increases and would be associated with a decrease in erythrocyte volume. With rehydration, plasma osmolality returns toward normal, allowing for erythrocyte volume to return to predehydration volume (i.e., erythrocytes do not exceed normal volume, which could predispose to rupture). Although the camel can concentrate its urine and dehydrate its feces, these are not significant factors in regard to the camel’s ability to withstand water deprivation.Sheep and Donkeys
Sheep and donkeys are also notable in their ability to withstand water lack. They are similar to the camel in that they can endure dehydration up to about 30% of their body weight. Also, the sheep and donkey are similar to the camel in being able to drink almost 25% of their body weight in water at one time without harmful effects. The sheep is protected from solar heat gain by its wool covering and excretes dry feces and relatively concentrated urine. The donkey dissipates heat by sweating more than the camel and sheep; its survival time is correspondingly less. Because sheep do not sweat as much as camels and donkeys, evaporative heat loss by way of the respiratory passages (panting) is a more important factor in sheep.
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