Cold-blooded animals

Cold-blooded animals include all invertebrate animals. Insects, spiders, other “bugs,” worms, and all animal sea life except fish are examples of invertebrates. Of the five classes of vertebrate animals, fish, amphibians, and reptiles are cold-blooded. Only birds and mammals are warm-blooded. The proper scientific term for cold-blooded animals is poikilotherms (or ectotherms), while the scientific term for warm-blooded animals is homeotherms (or endotherms).

The best way to understand and appreciate cold-blooded animals and how they adapt is to compare them with warm-blooded animals. While a few cold-blooded animals have some rudimentary abilities to achieve temperature control internally, only birds and mammals are truly homeothermic.

Both birds and mammals maintain quite high core (internal) body temperatures with a very narrow range of variability. Mammals maintain a temperature of about 37 degrees Celsius with only about one degree of variation. Birds’ temperatures are about 2 degrees higher. Homeotherms control their high and constant temperatures with an elaborate system of neural and chemical feedback systems. Most homeotherms have highly developed systems of insulation (feathers, fur, or fat).

Mechanisms of Ectothermic Temperature Control

Poikilotherms have little or none of the internal mechanisms of the homeotherms, and they usually have poor insulation or surfaces that enhance heat transfer. They maintain their temperature control primarily through external exposure to the environment, called thermoconformers. They have a much lower basal metabolism, and most species maintain a much lower temperature range than homeotherms.

Some cold-blooded animals live in very constant environments and maintain their temperature range with little or no adjustment. For sea life and other organisms that live in the ocean or very large bodies of freshwater, this is especially true. The entire surface of the organism is in contact with water, which conducts heat at least five times as fast as air. Fish can easily adjust their thermal environment in lakes and estuaries by changing their depth since water temperatures can vary greatly below the surface. Some fish have internal mechanisms to become somewhat dormant with extreme seasonal changes in lakes whose surfaces freeze during the winter.

Amphibians and reptiles have more complicated mechanisms and use more adaptations than fish. Amphibians and reptiles can operate within a much wider core body temperature range than mammals or birds. This is especially true of some amphibians. Some salamanders’ body temperatures range from -2 degrees Celsius to 36 degrees Celsius (mean twenty-seven). The range of external temperatures that some amphibians can tolerate, at least for varying lengths of time, varies from 28 degrees Celsius (below freezing) to 38 degrees Celsius (about 100 degrees Fahrenheit).

Both amphibians and reptiles maintain their body temperatures primarily by changing their locations to gain or lose heat from the sun, shade, or substrate. Some species achieve control by moving in and out of the water. This is an especially effective method since water conducts heat so rapidly. Many of these animals change body positions to increase or decrease exposure to the sun or to decrease surface area to gain or lose heat more rapidly. Reptiles have scaly and often thin skins, while amphibians have moist secretory skins, both of which are very effective for heat transfer. An adaptation present in some amphibians and reptiles is the ability to change the color of their skins. A darker skin color will absorb heat, and a lighter skin color will reflect the heat. Chameleons are one of the best-known color-changing animals and often change their skin color to absorb and reflect heat.

Cold-blooded animals have some natural advantages over warm-blooded animals. Because their natural temperature is much lower than that of mammals and birds, they are less susceptible to parasites and other illnesses. Additionally, because they do not use their energy to produce heat, many cold-blooded species can go for weeks or months between meals if needed. However, as the average temperature rises with climate change, scientists have raised concerns about the preservation of cold-blooded animals.

Reptiles

Reptiles are the most interesting of the poikilotherms for several reasons. Lizards and snakes are very adept at changing their locations frequently to maintain temperature control. They may be in holes or in the shade, hidden in the grass, or among rocks and vegetation, quietly conserving energy and waiting for prey. They will bask in the sun or lay on warm surfaces in the evening as needed. Some lizards will actively generate heat by doing multiple push-ups.

Reptiles have a wider body temperature range than homeotherms, but usually not nearly as wide as amphibians. The crocodile is one of the more advanced reptiles. One species was found to have a mean body temperature of 25 degrees Celsius with a maximum range of 6 degrees above or below the mean. Being so large, the surface area to body ratio is not very great compared to smaller reptiles, and the surface exchange of heat takes much longer. Crocodiles sometimes spend long periods in the water and will be at their lower temperature range. Then, they will lay for hours in the sun on warm sand or dirt. They have learned to use the water to cool down, and they may submerge or be partially in and out of the water. On hot days, they will open their huge jaws, not to pant like fur-bearing animals, but to expose the very large surface area of the mucous membranes in their mouths, which creates an effective evaporative cooling effect. When crocodiles or alligators have their jaws tied shut for transport by their captors, many die from overheating. They do have some ability to slow or speed up their metabolic rate. In severe drought conditions, they will burrow into deep, wet mud and sand and change their metabolic rate enough to survive.

Principal Terms

Core Temperature: internal body temperature around the heart, brain, and spinal cord

Ectotherm: an animal that depends on environmental heat sources, usually solar radiation, to maintain body temperature

Homeotherm: an animal that maintains a constant, steady body temperature

Metabolic Rate: the rate (expressed as calories per minute) at which an animal produces and consumes energy

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

Poikilotherm: an animal that has a changing body temperature

Bibliography

Barrett, Kim E., et al. Ganong Review of Medical Physiology. 26th ed., McGraw-Hill Education, 2019.

Bates, Mary. “How Do Chameleons Change Colors?” Wired, 11 Apr. 2014, www.wired.com/2014/04/how-do-chameleons-change-colors. Accessed 1 July 2023.

Bennett, Howard. “Ever Wondered about Warm-Blooded and Cold-Blooded Animals?” The Washington Post, 29 Nov. 2015, www.washingtonpost.com/lifestyle/kidspost/ever-wondered-about-warm-blooded-and-cold-blooded-animals/2015/11/27/575d30ca-6c57-11e5-aa5b-f78a98956699‗story.html. Accessed 1 July 2023.

"Cold-Blooded Animals." International Fund for Animal Welfare, 26 July 2024, www.ifaw.org/journal/cold-blooded-animals. Accessed 20 Sept. 2024.

Dudley, Karen. Alligators and Crocodiles. New York: Raintree/Steck Vaughn, 1998.

Hine, Robert. The Facts on File Dictionary of Biology. 4th ed., Facts on File, 2004.

Rahn, Joan E. Keeping Warm, Keeping Cool. New York: Atheneum, 1983.

Stebbins, Robert C., and Nathan W. Cohen. A Natural History of Amphibians. Princeton UP, 1995.