This is often related to locomotion. Think about what body shape is best adapted to swimming. Be sure you understand the adaptative characteristic of the birds feet illustrated on page 82. Endotherms usually maintain a constant body temperature. As an animal increases in body size, the relative amount of surface area, in relation to its volume, decreases, even though the absolute value of the surface area increases.
Heat is lost in proportion to the surface area of the organism.
Heat is produced in proportion to the volume of the organism. Remember, the rule about body size can be used only to compare closely-related animals. For instance, a rabbit from a cold climate will probably be larger than a rabbit from a warm climate. Large ears, for instance, can radiate a lot of heat to the environment. This is an advantage in warm climates and a disadvantage in cold climates.