Chapter 1 “Temperature Regulation in Animals” There are, broadly speaking, two kinds of animals with regard to body temperature: exothermic (cold-blooded) and endothermic (warm-blooded) animals. The exothermic animals, such as reptiles, do not supply body heat by metabolic conversion of food to heat. Reptiles allow their surroundings to determine their body temperature. They lie out in the sun to warm their body. If they are too hot, they seek the shade or even burrow into the ground. At night they hide from the cold in burrows or squeeze into cracks between rocks or hide in leaf cover. Reptiles avoid the extremes of temperature. When reptiles become cool, their movements slow down, and chemical processes in their bodies, such as digestion, are inhibited. Predators, such as hawks and eagles, find it easier to prey on lizards and snakes in cooler weather. The distribution of reptiles is somewhat limited by their exothermic character. They do not thrive in cold climates1. What are the advantages and disadvantages in being exothermic? When the lizard is in a cool environment and cannot find a warmer spot, its body simply cools to the temperature of the surroundings. It is not necessary for the exothermic lizard to generate heat to increase its body temperature. This means that the lizard uses less energy and does not have to eat as much. As the lizard cools its digestion, breathing rate and heart rate slow, saving energy. A disadvantage occurs when the cool lizard is attacked by a predator. If warm, he could run fast and have a much better chance of 1 St. Patrick did not chase the snakes out of Ireland. Ireland was already completely free of snakes. St. Patrick was instrumental in converting pagans to Christianity. Since the snake was a symbol used in pagan rituals, St. Patrick was influential in ridding Ireland of the ritual use of symbolic snakes. 10 Verne A. Simon evading capture. A warm lizard being chased by a predator can move quite fast for a short distance, but like other exotherms, lacks endurance and soon tires. When the exotherm is running fast, its effort is anaerobic, that is, is not using oxygen, and lactic acid is building up in its body. It soon tires and is unable to exert itself. It must recover by taking in oxygen to rid the body of lactic acid. Another disadvantage of exothermic life is that cold climates are not available as habitat. If there is a sudden climate change, an exothermic animal wouldn’t be able to mount the sustained effort needed to migrate to a better environment. The exothermic creature might simply perish. About 180 million years ago, mammals appeared. Mammals are endothermic (warm-blooded) and are able to maintain a nearly constant body temperature regardless of the temperature of their surroundings within wide limits. Their bodies will not tolerate too high or too low a temperature. If the surroundings are too hot or cold, causing the body temperature to exceed allowed limits, the animal will die. Mammals have furry coats to help them tolerate low temperatures. Sea-dwelling mammals—whales, seals, and walrus—have thick layers of blubber for insulation. Birds are endothermic and have feathers to protect them from the cold. Many types of birds and mammals survive in cold climates. Emperor penguins even live in the Antarctic, in the coldest climate on earth. Under normal circumstances, mammals and birds manage to keep this very nearly constant body temperature regardless of the temperature of their surroundings. Mammals are characterized by having body hair and suckling their young. This latter behavior gives the class its name; mammals must have mammary glands. A second advantage is that endothermic animals are not limited to activity only in daylight hours. In many locations, it is too cold at night for exotherms to be active. Even very cold temperatures do not exclude endothermic animals such as mammals and birds from nocturnal activity. Exothermic animals are not normally found in cold climates, though there are a few exceptions. Mammals with their hair can grow warm fur coats as in the polar bear or beaver. Mammals with little or no hair often have a thick subcutaneous layer of fat for thermal insulation as in walrus and whales. Birds have feathers for insulation, like the snowy owl, whose white feathers match the snow. During long periods of cold weather (winter), some, but not all, mammals hibernate. Their body temperature drops to a few degrees above freezing, and their breathing and heart rates almost cease. Other mammals, such as bears, undergo estivation. Their body temperature may drop by 40oF (22oC). While in estivation, they live off the fat they have accumulated in preparation for winter. They may become active for short periods on relatively warm days. Some animals tough it out in winter, like emperor penguins and moose. Adaptations in the Animal Kingdom 11 Mammals in cold latitudes develop furry coats to retain heat during cold winters. Mammals in warm climates may have very sparse hair, for example, elephants and naked mole rats. Some mammals (sheep, horses, cattle) give birth to single young with well-developed senses of sight and hearing and with musculature ready for walking within minutes of birth. Such well-developed young are said to be precocial. In contrast to precocial young are altricial young, born in litters, with closed eyes, closed ear canals, having no capability of locomotion, and often sparsely covered with hair or feathers. Some examples of endothermic altricial young are dogs, cats, rats, mice, and polar bears. Human babies are neither clearly altricial nor precocial. At birth a human baby’s eyes are open and the baby can hear. On the other hand, a human infant is naked and unable to move about. Humans usually are born singly. Altricial animals are usually born in litters and are small. Because of their small size they have a large surface to volume ratio, which makes it more difficult to maintain a constant body temperature as required by endothermic animals. How do altricial young cope with temperature fluctuations? Some animals such as puppies and kittens have numerous litter mates with which they can huddle, thereby decreasing their exposed surface area. Mothers share bodily heat with their offspring and provide warm milk as a buffer against the cold. There is a special organ for heat production in young altricial animals, that is, brown adipose tissue (BAT). Heat production by BAT is stimulated by cold temperatures and by norepinephrine2 (noradrenaline). BAT is present in large amounts in the young, constituting about 5% of body weight. In adults, BAT constitutes only 1% of body weight and has traditionally been regarded as unimportant. Adults are able to raise their body temperature by the muscular activity, including shivering, but infants seem unable to shiver. Human babies have BAT deposits in the neck and between the shoulder blades. Heat production by BAT in human babies can be demonstrated by infrared photography. An unclothed baby lying prone at room temperature shows hot spots between his shoulder blades and in the area of his neck in an infrared photo. The distribution of BAT has important advantages. It is very helpful in keeping the heart and brain from cooling. Both of these organs show decreased activity if the temperature drops. The placement of BAT on neck and shoulders aids in keeping the brain and heart warm. BAT is brown due to pigmented organelles called mitochondria within this special tissue. The BAT is very rich in mitochondria. Mitochondria are 2 Epinephrine (adrenaline) is a hormone produced by glands at the top of the kidneys. Epi means “above” and nephrine refers to the kidney. Norepinephrine is an active metabolite of epinephrine. Both epinephrine and norepinephrine prepare animals for “fight or flight.” 12 Verne A. Simon present in most body cells and have the function of producing energy which can be chemically stored in the form of adenosine tr
