Warm and Cold Blooded Animals


With a few exceptions, all mammals and birds are warm-blooded, and all reptiles, insects, arachnids, amphibians and fish are cold-blooded. What does it mean to be warm-blooded or cold-blooded? The temperature of an animal's blood is related to its body temperature.

Warm-Blooded vs. Cold-Blooded

Warm-blooded creatures, like mammals and birds, try to keep the inside of their bodies at a constant temperature. They do this by generating their own heat when they are in a cooler environment, and by cooling themselves when they are in a hotter environment. To generate heat, warm-blooded animals convert the food that they eat into energy. They have to eat a lot of food, compared with cold-blooded animals, to maintain a constant body temperature. Only a small amount of the food that a warm-blooded animal eats is converted into body mass. The rest is used to fuel a constant body temperature.


These thermal infrared images of warm-blooded animals, show how birds and mammals maintain body temperatures well above the surrounding, cooler air temperature.

Cold-blooded creatures take on the temperature of their surroundings. They are hot when their environment is hot and cold when their environment is cold. In hot environments, cold-blooded animals can have blood that is much warmer than warm-blooded animals. Cold-blooded animals are much more active in warm environments and are very sluggish in cold environments. This is because their muscle activity depends on chemical reactions which run quickly when it is hot and slowly when it is cold. A cold-blooded animal can convert much more of its food into body mass compared with a warm-blooded animal.


The infrared images above, show how cold-blooded animals take on the temperature of their surroundings. Both the gecko and the scorpion are at the same temperature as the air surrounding them. Notice the difference between these cold-blooded creatures and the warm-blooded humans holding them.

Staying Cool and Keeping Warm

To stay cool, warm-blooded animals sweat or pant to loose heat by water evaporation. They can also cool off by moving into a shaded area or by getting wet. Only mammals can sweat. Primates, such as humans, apes and monkey, have sweat glands all over their bodies. Dogs and cats have sweat glands only on their feet. Whales are mammals who have no sweat glands, but then since they live in the water, they don't really need them. Large mammals can have difficulty cooling down if they get overheated. This is why elephants, for example, have large, thin ears which loose heat quickly. Mammals have hair, fur or blubber, and birds have feathers to help keep them warm. Many mammals have thick coats of fur which keep them warm in winter. They shed much of this fur in the summer to help them cool off and maintain their body temperature. Warm-blooded animals can also shiver to generate more heat when they get too cold. Some warm-blooded animals, especially birds, migrate from colder to warmer regions in the winter.


The above infrared images show some ways in which warm-blooded animals try to maintain a constant temperature. The dog in the left image pants when he is too warm. Notice the extra heat radiating from the dog's mouth as he pants. The flamingos in the right image are covered with feathers which help keep these birds warm when it is cool outside.

Cold-blooded animals often like to bask in the sun to warm up and increase their metabolism. While basking, reptiles will lie perpendicular to the direction of the sun to maximize the amount of sunlight falling on their skin. They will also expand their rib cage to increase their surface area and will darken their skin to absorb more heat. When a reptile is too hot, it will lie parallel to the sun's rays, go into a shady area, open its mouth wide, lighten its skin color or burrow into cool soil. Some cold-blooded animals, such as bees or dragonflies, shiver to stay warm when in a cold environment. Fish who live in areas where the winters are cold move to deeper waters during the colder months or migrate to warmer waters. Some fish have a special protein in their blood which acts like anti-freeze to help them survive very cold water temperatures. Snakes, lizards, toads, frogs, salamanders and most turtles will hibernate during cool winters. Some insects die when it gets too cold, however others survive by migrating to warmer areas or moving underground. Honeybees stay warm by crowding together and moving their wings to generate heat.


These thermal infrared images of a collard lizard show a cold-blooded animal's body temperature in a cooler and warmer environment. In the image to the left, the lizard is just above room temperature, being warmed by the human hand holding it. To warm up lizards will seek a sunny area and bask in warm sunlight, as in the image to the right.

Advantages and Disadvantages

There are many advantages to being warm-blooded. Warm-blooded animals can remain active in cold environments in which cold-blooded animals can hardly move. Warm-blooded animals can live in almost any surface environment on Earth, like in arctic regions or on high mountains where most cold-blooded animals would have difficulty surviving. Warm-blooded animals can remain active, seek food, and defend themselves in a wide range of outdoor temperatures. Cold-blooded animals can only do this when they are warm enough. A cold-blooded animal's level of activity depends upon the temperature of its surroundings. A reptile, for example, will increase its body temperature before hunting and is better able to escape predators when it is warm. Cold-blooded animals also need to be warm and active to find a mate and reproduce.

Being cold-blooded, however, also has its advantages. Cold-blooded animals require much less energy to survive than warm-blooded animals do. Mammals and birds require much more food and energy than do cold-blooded animals of the same weight. This is because in warm-blooded animals, the heat loss from their bodies is proportional to the surface area of their bodies, while the heat created by their bodies is proportional to their mass. The ratio of a body's surface area to its mass is less the larger the animal is. This means that larger warm-blooded animals can generate more heat than they loose and more easily keep their body temperatures stable. Smaller warm-blooded animals loose heat more quickly. So, it is easier to stay warm by being larger. Warm-blooded animals cannot be too small or else they will loose heat faster than they can produce it.

Being large requires a greater food supply, but for mammals, being small also requires a lot of food to generate more heat. Small mammals need to eat very often to survive because they need more energy to keep a constant body temperature. They also need to eat high energy foods such as fruit, seed, and insects and even other small mammals. Larger mammals can get by with eating lower energy foods or eating less often. In an environment where food is scarce, such as in deserts, reptiles have an advantage. Since cold-blooded animals do not have to burn a lot of food to maintain a constant body temperature, they are more energy efficient and can survive longer periods without food. Many cold-blooded animals will try to keep their body temperatures as low as possible when food is scarce.

Another disadvantage to being warm-blooded is that warm-blooded bodies provide an nice warm environment for viruses, bacteria and parasites to live in. Mammals and birds tend to have more problems with these infections than do reptiles, whose constantly changing body temperatures make life more difficult for these invaders. However, an advantage of this is that mammals and birds have developed a stronger immune system than cold-blooded animals. A reptile's immune system is more efficient when the animals is warmer, however, since bacteria probably grow more slowly in lower temperatures, reptiles sometimes lower their body temperatures when they have an infection.

Exceptions

Some animals do not fall neatly into the categories of being warm or cold-blooded. Bats, for example, are mammals who cannot maintain a constant body temperature and cool off when they are not active. Echidnas maintain a range of body temperatures which usually lies between 77 and 98.6 degrees Fahrenheit and have difficulty cooling down. Mole Rats are another group of mammals who are unable to regulate their body temperature, however, since they live underground, the temperature of their environment does not change much. Some warm-blooded animals, such as bears, groundhogs, gophers and bats hibernate during the cold winter. During hibernation these animals live off of stored body fat and can drop their body temperatures by as much as 50 degrees Fahrenheit. The Hawk Moth is an insect which can raise its body temperature well above the temperature of its surroundings when it is flying because of its huge wing muscles which generate heat when in use. Bees are another example of insects that can raise their body temperatures above that of their environment by moving their wings rapidly to generate heat.


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