Guide Cold-blooded

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Larger animals, such as these prairie dogs, do not use as much energy to produce the heat required to keep their larger bodies warm.

This thermostat is set at 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. A tiny hummingbird must refuel its body furnace every ten to fifteen minutes during the day to maintain its body heat.

Panting is another cooling method used by mammals with few sweat glands.

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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.

Cold-blooded animals cannot generate their own body heat, but they do regulate it by changing their environment. Alligators and other reptiles often lie in the sun to warm themselves. On the other hand, they cool off by taking a dip in the water, moving into the sade of a rock or crawling into a burrow in the ground.

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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. 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 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 cubic inches, which is eight times bigger.


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. They then used statistical analyses and energetic models to determine the relationship between growth rate and energy use. Annual growth rings in fossils were used to determine growth rates, while metabolic rates were estimated by using changes in body size as an animal grows from birth to adult known as ontogenetic growth. Warm-blooded endothermic mammals grow 10 times faster than cold-blooded ectothermic reptiles, and metabolise 10 times faster; in general doubling one's metabolic rate leads to a doubling in growth rate," Grady explains.

However, when they examined the growth rates of dinosaurs, although there was some variation in the rate they grew, they had neither the high metabolic rate of mammals and birds, nor the low metabolic rate of reptiles. Today, mesothermic animals are uncommon, but living species come from across the evolutionary spectrum, and include leatherback turtles, tuna, great white sharks and the echidna. These animals at times rely on internally-generated metabolic heat to maintain body temperatures, while being subject to external temperatures in others.

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Dinosaurs evolved around million years ago, and competed for resources with ectothermic animals like lizards. Their higher metabolic rate meant they could move faster making them a more dangerous predator, or more elusive prey, says Grady. As well as helping us understand how warm-blooded animals evolved, understanding dinosaurs' energy use challenges our understanding of how life operates, Grady explains.

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Against today's polarised landscape, dinosaurs stand out as a successful middle way. Tags: animals , biology , dinosaurs. Email the editor.

Cold-Blooded or Warm-Blooded? Dinosaurs May Have Been in Between

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