BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.5. Endothermic animals

19.5.7. Adaptations for life in high temperatures

Animals inhabiting regions where ambient temperatures exceed Skin Temperature absorb heat, and their only way to dissipate this heat is through the evaporation of Water from the body surface. However, hot regions can be extremely arid or, conversely, very humid, which creates additional challenges. In hot, dry climates, evaporative cooling requires an adequate replenishment of the body's water loss, which is inherently scarce there. Under hot, humid conditions, water is plentiful, but it evaporates poorly because the air is heavily saturated with water vapor. In such cases, behavioral thermoregulation mechanisms, such as seeking shade and breezy spots in or beneath forest canopies, often play a crucial role.

Bergmann's rule

As noted in section 19.5.5, animals living in hot regions are smaller than their relatives from colder climates. This can be explained by the fact that The amount of heat gained from the environment (as well as lost to it) is roughly proportional to the body surface area. Therefore, most desert-dwelling endotherms are small—such as the kangaroo rat (Dipodomys)—and face fewer difficulties than large animals like the camel. Furthermore, small animals can live in burrows dug into sand or soil, where the microclimate¹ is more favorable. Adaptations of kangaroo rats for water conservation are discussed in section 20.3.1.

The camel as an example of thermoregulation in a hot climate

The camel is exceptionally well adapted to hot, arid climates, utilizing the following mechanisms to cope with these conditions:

1. In a dry and hot climate, provided there is free access to water, a camel can maintain its body temperature within the 36–38 °C range through cutaneous water evaporation.

2. If water is unavailable, such as during multi-day journeys across the desert, the fluctuation between morning and evening body temperatures increases as the animal dehydrates; body temperature may range from 34 °C in the early morning to 41 °C in the afternoon. In other words, the animal stores heat during the day to conserve water, eliminating the need to dissipate it through evaporation. In a series of studies, Schmidt-Nielsen established that a 500 kg camel capable of tolerating a 7 °C rise in body temperature stores approximately 12,000 kJ of thermal energy. If the camel had to get rid of this amount of heat via evaporation, it would have to lose 5 liters of water just to keep its body temperature constant. Instead, the accumulated excess heat is dissipated at night through radiation, conduction, and convection.

3. An additional advantage of this "temporary ectothermy" observed in camels during the daytime is that it reduces the temperature gradient between the hot desert air and the camel's body, thereby minimizing the influx of external heat.

4. The camel's coat serves as an effective thermal insulation material that reduces both heat absorption and water loss. One experiment demonstrated that a shorn camel loses 50% more water than a control animal.

5. Finally, another crucial advantage of the camel is its high tolerance for severe dehydration. While most mammals cannot survive water loss that reduces body mass by more than 10–14%, the camel can lose up to 30% of its body mass through water depletion while maintaining a normal Blood volume. Death from overheating during dehydration typically results from a drop in blood volume, as the Circulatory system becomes unable to rapidly transfer heat from the body's core to its surface, causing Tissues to overheat².

6. Contrary to popular belief, camels cannot pre-emptively store water against future shortages. Some researchers also question the camel's ability to obtain metabolic water from the fat stored in its hump.

7. Following a period of severe dehydration, a camel can drink a massive amount of water in a short time to rehydrate its severely depleted tissues. For instance, a 325 kg camel has been observed drinking 30 liters of water in less than 10 minutes. This is roughly equivalent to a person of average height and weight drinking about 7 liters of water in the same span of time!

¹ Bergmann's rule applies to closely related taxa, making this comparison ill-suited. Additionally, small body forms predominate in extremely hot, arid regions primarily due to a scarcity of plant forage, which forms the base of food chains. — Transl. note.

² A decrease in blood volume under any climatic conditions is dangerous primarily because it impairs blood gas exchange—tissues will die from oxygen starvation much faster than from overheating. — Transl. note.