Warm- and Cold-Blooded Animals

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No matter what the outside temperature may be, your body, like a living furnace, works to maintain a constant internal temperature. It generates heat by burning the food you eat. All mammals and birds are capable of generating this internal heat and are classed as homoiotherms (ho-MOY-ah-therms), or warm-blooded animals. Normal temperatures for mammals range from 97° F to 104° F. Most birds have a normal temperature between 106° F and 109° F.

A portion of the brain known as the hypothalamus (hi-po-THAL-ah-mus) is the thermostat that controls your body’s furnace. This thermostat is set at 98.6° F, but a degree or so higher or lower is within the normal range for a human. In fact, your body temperature varies with the time of day. It is at its lowest just before you get up in the morning, rises to a peak in the afternoon, and then falls again while you sleep at night. Strenuous activity raises the body temperature. Illness also may cause a greater rise or drop in the normal temperature.

Nerves in the skin and deep within the body send temperature messages to the hypothalamus. It compares the temperatures of these areas with that of the brain and, if they are too low or too high, it sends messages to nerves and glands to help increase or decrease the heat. When you are cold, a message from the brain causes your muscles to shiver. This generates a little heat and starts warming the body. When you are too hot, a message triggers your sweat glands. Evaporation of the resulting perspiration cools the skin. Another message may dilate (enlarge) the blood vessels under the skin so more blood can come to the surface and more heat can escape through the skin to the air.

Panting is another cooling method used by mammals with few sweat glands. Moisture evaporates from the mouth and tongue to cool the overheated body. Birds cannot sweat, but they get rid of excess body heat by breathing it out. Special air sacs, which extend from the lungs, increase the amount of air the birds can breathe in and out.

Warm-blooded animals can be as active in winter as summer, but their bodies must have plenty of food to burn for additional heat. Birds, with their higher body temperatures, often find it difficult to locate enough food when winter’s lower temperatures arrive, so most of them migrate to warmer climates where their bodies do not have to work as hard to maintain heat.

Heat escapes from the body through the skin. Layers of clothing help you retain your body heat in the winter. Other mammals must rely on layers of fat or a fur covering to insulate them from the cold and retain their body heat. In extremely cold climates, you won’t find mammals with large ears or long tails. A lot of extra food would be required to replace the heat lost from these large surfaces—food that would be extremely hard to find.

Smaller animals must produce more heat to keep warm than larger ones. To understand this, pretend that a 3-inch-square box is a small animal and a 6-inch-square box is a larger animal. On its six exposed sides, the small animal has 54 square inches of skin. The larger animal has 216 square inches of skin, or four times as much. The inside heat-producing area of the small animal is 27 cubic inches, but the inside of the larger animal contains 216 cubic inches, which is eight times bigger. If it takes one unit of energy for each cubic inch to warm 1 square inch of skin, the smaller animal must burn twice as much energy to keep its skin at the temperature of the large animal’s skin. This means it must produce twice as much heat.

Because small bodies must produce so much heat to stay warm, the size of warm-blooded animals is limited. If the animal were too small, it could not digest food fast enough to produce heat as quickly as warmth could be lost through the skin. During the day a tiny hummingbird refuels its furnace with food every ten to fifteen minutes. If it were not able to slow its body down at night to about one-twentieth of its daytime energy by going into a hibernation-like torpor, the cool night air of even a warm climate would endanger the hummingbird’s life.

Torpor is a type of sleep from which an animal cannot be awakened quickly. Its body temperature drops to that of its surroundings, and the heartbeat and breathing are slowed down greatly. If the temperature drops too low, the animal will freeze and never awaken from torpor. True hibernators pass in and out of torpor throughout the winter.
Animals that cannot generate internal heat are known as poikilotherms (poy-KIL-ah-therms), or cold-blooded animals. Insects, worms, fish, amphibians, and reptiles fall into this category—all creatures except mammals and birds. The term cold-blooded is a little misleading because poikilotherms can have very warm body temperatures in the tropics. Cold-blooded actually means the animal’s body temperature is basically the same as its surroundings. A fish swimming in 40° F water will have a body temperature very near 40° F. The same fish in 60° F water will have a body temperature near 60° F.

Since cold-blooded animals cannot generate their own heat, they must regulate their body temperature by moving to different environments. You probably have seen a lizard, turtle, or alligator lying around basking in the sun. It does this to raise its body temperature. When it gets too warm, it moves into the shade, takes a dip in the water, or burrows under a rock or into the ground to cool off. When temperatures drop, cold-blooded animals become less active, even sluggish. If an insect becomes too cold, its wing muscles cannot move fast enough for it to fly. Some moths vibrate their wing muscles, an action similar to your shivering, and the contracting muscles produce enough heat for takeoff. After a cold night, a grasshopper often is too stiff and cold to hop. However, once the sun’s rays have warmed it up, it can leap around as usual.

Extreme changes in environmental temperatures can be fatal to the cold-blooded animal. As water temperatures increase, oxygen content is reduced. Raising the temperature from 41° F to 95° F will cut the oxygen level in half. A fish experiencing this drastic rise in temperature must pump twice as much water across its gills to get the same amount of oxygen it received when the temperature was lower. The increased activity also increases the fish’s need for oxygen, which compounds the problem. As a result, the fish may die from a lack of oxygen, not heat.
Many insects die when temperatures drop, but next year’s supply winters in eggs, cocoons, or some other protective covering. They emerge or hatch when spring or summer temperatures return. Reptiles burrow into the ground or find a den in which to live until surface temperatures are more favorable. In fact, sunny winter days bring many of them out to warm themselves and look for food. Extremes of heat and cold are hard on all animals. But both warm-blooded and cold-blooded animals have adapted to normal weather changes.