Besides the heavy eyelids and lashes and the slit nostrils that protect it against desert winds, a camel’s body exhibits many special adaptations to life in hot, dry regions. Thus, camels have unusually low metabolic rates. They can exist on dry food for two weeks or more, depending on the temperature, because they tolerate a much greater depletion in body water than most other mammals.
A camel may lose about 30% of its weight in body water without ill effects, as compared to about 12% in man. During periods of desiccation the blood of most mammals becomes increasingly viscous; it therefore circulates more and more slowly, until it cannot carry away metabolic body heat to the skin quickly enough, leading to “explosive heat death”. This is avoided in camels by a physiological mechanism that ensures that water is lost from the body tissues alone, while the blood’s water content remains fairly constant.
The camel does not lose its appetite during periods of desiccation, and can graze over a wide area away from water. When presented with water after a moderate dry period, it takes in, at one time, as much water as was lost-a camel can drink 25 gallons (100 liters) or more in a very short time. The body fluids rapidly become diluted to an extent that could not be tolerated by other mammals, which would die from water intoxication even if they took in a much smaller amount.
Because of its ability to lose water from body tissues alone, and because of its relatively small surface area, the camel can afford to sweat. The coarse hair on the back is well ventilated, allowing the evaporation of sweat to occur on the skin and provide maximum cooling. (The hair also acts as a barrier to the sun’s radiation and slows the conduction of heat from the environment.)
Undue water loss from sweating is avoided because the camel’s temperature can vary over a range greater than that of other mammals. In the North African summer a camel may have a morning temperature of 93°F, and an afternoon maximum of 105°F. Sweating does not commence until the higher temperature is reached; therefore the camel is able to store heat during the day, which can be lost at night without expense of water.
The rate of urine flow is low in camels, and little water is lost with the feces. Moreover, investigations of kidney function in the camel have revealed an extremely low secretion of urea when its food is low in protein. The camel, like most ruminants, can utilize urea for microbial synthesis of protein-a valuable asset to animals that have to exist on low-grade diets in deserts.
Thus, the adaptations of the camel to its hot environment do not involve independence from drinking water but the ability to economize the water available and to tolerate wider variations in body temperature and water content. In winter, when the temperature is comparatively low and water is not needed for heat regulation, camels become independent of drinking water for periods of several months.