BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.5. Endothermic animals

19.5.5. Adaptations to extreme climatic conditions

Bergmann's rule

In endothermic animals, total heat production depends on body volume, while The rate of heat loss depends on its surface area. As an Organism grows larger, its body volume increases at a faster rate than its surface area. For this reason, species living in cold regions (such as polar bears and whales) tend to be large in size, whereas inhabitants of hot countries (such as many insectivorous mammals) are usually smaller. This phenomenon, known as Bergmann's rule, is also observed within a single species: for example, the Amur tiger from the Russian Far East is larger than the Sumatran tiger from Indonesia. Of course, small animals can still be found in polar regions, and true giants in the tropics, but in such cases they possess additional adaptations for conserving or dissipating heat. For instance, small mammals in temperate or polar zones have a voracious appetite, enabling them to maintain a High Metabolic Rate. They have short protruding body parts (ears, limbs) to reduce heat loss, and in winter they are forced to hibernate. Large mammals in hot countries, such as elephants and hippos, face the opposite challenges. Elephants are aided by very large, highly vascularized ears. By constantly flapping its ears, an elephant enhances heat loss through radiation and convection. Hippos lack Sweat Glands and employ a behavioral strategy similar to that of crocodiles—alternating between land and Water to mitigate the effects of Temperature fluctuations.

Allen's rule

Species inhabiting cold regions have smaller protruding body parts than their close relatives in warm countries. This pattern is known as Allen's rule and is illustrated in Fig. 19.17 using foxes as an example.

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Fig. 19.17. Differences in ear length among three fox species inhabiting different climatic regions. This example provides a clear illustration of Allen's rule. A. Arctic fox, inhabiting the Arctic. B. Red fox from Western Europe. C. Bat-eared fox from Botswana.