Warm-blooded animals

Warm-blooded animals like birds and mammals were so named because they are warm to the touch. However, a cold-blooded animal lying in the sun would also feel warm, so additional terms have been created to differentiate between different types of animals. Homeotherm (same temperature) and poikilotherm (changing temperature) have been used for warm- and cold-blooded animals, but hibernating warm-blooded animals reduce their body temperature to a few degrees above their surroundings, and some cold-blooded deep-sea fish live in a constant temperature environment, thus maintaining a constant body temperature.

More accurate terms have been developed focusing on the heat source maintaining body temperature. Warm-blooded animals use the heat generated from their body’s metabolism to maintain body temperature, whereas cold-blooded animals depend more on outside heat sources, such as solar radiation, to maintain body temperature. Thus, warm-blooded animals are called endotherms (heat from within), and cold-blooded animals are called ectotherms (heat from outside). Warm-blooded animals also use solar radiation to help warm themselves on a cold day, and cold-blooded animals have some internal metabolic heat to maintain their body temperature, but endotherm and ectotherm best describe the primary difference between warm- and cold-blooded animals.

The Metabolic Furnace and Body Temperature

Warm-blooded animals (endotherms) are characterized by using metabolism to keep their body temperature relatively constant and independent of the environmental temperature. The overall metabolic process is about 25 percent efficient, which means 25 percent of the food energy accomplishes work, and the other 75 percent is lost as heat. The internal organs, such as the heart, kidneys, brain, and lungs, generate the majority of the heat, which is then distributed throughout the body by the circulatory system.

Because warm-blooded animals produce heat, they must also lose heat to maintain a steady body temperature. The rate of heat loss from the body is dependent on the difference between the skin temperature and the surroundings. The colder the surroundings, the faster heat is lost. To maintain a steady body temperature, warm-blooded animals can adjust their behavior, insulation, and metabolic rate.

Small rodents and insectivores seek shelter in burrows to reduce heat loss, while larger animals, such as elk and deer, graze on warm, sunny slopes on cold winter days to gain solar radiant heat. Penguins huddle together to reduce heat loss in cold Antarctic winds; otherwise, the metabolic heat required to keep warm would deplete their body fat stores before the winter’s end.

The arctic fox reduces its heat loss by growing a thick, insulating fur coat. In contrast, diving mammals like the Weddell seal use a thick layer of fat (blubber) under the skin to insulate and reduce heat loss to cold ocean water.

If behavioral adjustments and increased insulation cannot prevent excess heat loss, warm-blooded animals increase their metabolic rate to generate additional heat to maintain body temperature. Shivering produces an increase in muscle metabolism, and the heat produced adds additional warmth. Some animals have unique ways of maintaining their body temperature. For example, swordfish can warm their brain and eyes to 15° Celsius (59° Fahrenheit) warmer than the water around them using a countercurrent exchanger (a unique blood vessel structure). This allows them to see and think better than their prey and predators. However, these metabolic adjustments are costly and indicate that heat loss is greater than metabolic heat production. In this situation, an animal will have difficulty maintaining a steady body temperature in its surroundings.

While all mammals and birds are warm-blooded and most species in a group are typically classified as either warm- or cold-blooded, some notable exceptions exist that can raise their body temperature using specialized processes. Most fish species are cold-blooded, but a few, like the Atlantic bluefin tuna and the swordfish, are partially warm-blooded because they use a process called regional endothermy to warm specific body parts or organs. However, the only known truly warm-blooded fish is the opah, also called the moonfish. This fish circulates warm blood to all its organs and remains about 5 degrees Celsius (9 degrees Fahrenheit) warmer than its habitat. Similarly, most sharks are cold-blooded, but a few species of mackerel sharks have warm-blooded abilities, including the great white, shortfin mako, longfin mako, porbeagle, and salmon sharks. Unlike most reptiles, the Argentine black and white tegu lizard can raise its body temperature using hormones.

Cooling the Body

During periods of cold weather and reduced food supply, small warm-blooded animals will decrease their body temperature to a few degrees above their surrounding temperature to reduce heat loss and conserve bodily energy stores. This can be a hibernation lasting throughout the winter months or a period of torpor lasting for the night. This lower body temperature, however, is not a decrease in body temperature as observed in cold-blooded animals but a new, highly regulated body temperature attained by lowering the animal’s metabolic rate. It is a very important strategy for energy conservation and survival.

To maintain a steady body temperature, warm-blooded animals depend on a balance between heat produced by metabolism and heat lost to their surroundings. When the ambient temperature exceeds body temperature, an animal will gain heat instead of losing heat to the surroundings. This situation can then elevate body temperature. Body temperature, however, is held constant by heat loss through the evaporation of water from the animal. Water, in changing phases from liquid to vapor, requires energy (heat), and this represents the only mechanism available to an animal if the surrounding temperature is greater than the body temperature.

Two mechanisms for cooling the body by increasing water loss are panting and sweating. Zebras, gazelles, and bison are examples of closed-mouth panters. These animals rapidly move air through the nostrils, which cools the tissues lining their upper airways (nasal cavities). Dogs are the best example of open-mouth panters; they move air rapidly over the moist tongue to remove heat. In contrast to panting, sweat glands secrete a hypotonic salt water over the skin's surface. The sweat evaporates and cools the body's surface. Both panting and sweating can reduce an animal’s bodily water reserves, resulting in dehydration. Thus, increasing water intake during periods of hot weather is especially important.

Camels are known for their tolerance of hot environments. Although they are closed-mouth panters, they also allow their body temperature to rise during the day by 3 to 4 Celsius (37.5 to 39 Fahrenheit). This helps the camel tolerate hot environments in two ways. By increasing its body temperature, the camel will gain less heat from the environment because there is less of a difference between the surroundings and the camel’s temperature. Second, the camel can reduce the amount of panting and water loss needed to keep a steady body temperature. A camel stores body heat during the hot day and loses this heat during the cool desert night. The camel’s humps are not stores of water, but fat. This makes the camel’s skin thinner, which allows for better heat loss at night.

Principal Terms

Core Temperature: the internal body temperature around the heart, brain, and spinal cord; warm-blooded animals maintain a consistent core temperature; however, the skin or peripheral temperature varies with their surroundings

Ectothermic: heat from without; cold-blooded animals that depend on environmental heat sources, usually solar radiation, to maintain body temperature

Endothermic: heat from within; the preferred term for warm-blooded animals because it describes how they maintain their body temperature

Hibernation: a sustained period of torpor (lack of activity) where an animal reduces its metabolic rate

Homeotherm: an animal with a constant, steady body temperature

Metabolic Rate: the rate (expressed in calories per minute) at which an animal produces and consumes energy; can be indirectly measured by the amount of oxygen consumed per minute

Metabolism: the conversion of carbohydrates, proteins, or fats into chemical energy that can be used to accomplish work and generate heat

Poikilotherm: an animal with a changing body temperature

Bibliography

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