Chris has a master's degree in history and teaches at the University of Northern Colorado.
What Animals are Cold-Blooded?
If you've ever had a pet lizard or a pet dog, you may have noticed that both of them spend a lot of time laying in the sun. While lizards and dogs share this behavior, the reasons for it are very different. Dogs basically just enjoy being in the sunshine, while for lizards it's a matter of survival.
Lizards are an example of animals that cannot control their own body temperature. Biologists describe these animals as being ectothermic, which is a fancy way of describing animals that get their body heat from external sources. We often call these animals cold-blooded. Basically, if the environment is warm, a cold-blooded animal will be warm. If the environment is cold, the animal is cold. While these animals can't naturally regulate their own body temperatures, they do have some other interesting adaptations, so maybe calling somebody cold-blooded shouldn't be such an insult.
Which animals actually live without control over their body temperatures? Basically, cold-blooded animals include the insects, arachnids, reptiles, fish, and amphibians. These are the oldest groupings of animals on Earth, evolving much earlier than birds and mammals, which tells us that being cold-blooded is an ancient biologic system. Warm-bloodedness, or being able to naturally regulate body temperature, is a more recent adaptation.
Cold-blooded animals must rely on external sources like sun or shade to control their body temperatures. This means that there are many behavioral traits found commonly amongst cold-blooded critters. For one, they behave very differently in different environments. Many cold-blooded animals, from trout to turtles to salamanders, will be active in the sun, but become very sluggish in colder environments.
Another behavioral trait we often see is absorbing heat. To absorb heat, cold-blooded animals will often spend hours basking in the sun. Lizards are especially known for this. A lizard may spend hours basking, re-positioning itself so that the maximum amount of its body surface is directly facing the sun. This maximizes the amount of heat it can gather. Then, when the lizard becomes too hot, it jumps into the shade or a cool puddle. Changing its body temperature requires a change in the environment.
As they move between hot and cold environments, their body temperatures can change drastically. We know that people have a nearly constant body temperature of 98.6 degrees Fahrenheit. If your temperature goes up or down by a degree, that's a big deal. For cold-blooded animals like snakes, body temperature can go up or down much more dramatically throughout the year.
So, what do they do when there's no sun? Many cold-blooded creatures have evolved some tricks to survive without constant heat. Some fish, for example, have a protein in their blood which acts like an antifreeze and lets them survive in cold waters. Many cold-blooded animals, however, have simply evolved the ability to drop into torpor, which is a state of sleep that takes lots of time to emerge from, because most of the body functions are basically in low-energy mode. It's basically hibernation, or sort of like putting your computer to sleep but not turning it off. In this state, the animal requires much less energy to survive. This is how animals like reptiles or amphibians survive winter months. As long as its blood doesn't literally freeze solid, it can survive.
Being cold-blooded means that animals have to spend a lot of time moving between various environments, and those extreme changes in temperature can quickly become deadly. So, why haven't these animals evolved to become warm-blooded? Being cold-blooded actually has its advantages.
One of the biggest advantages is that cold-blooded creatures need much less food to survive. Warm-blooded animals have to eat a lot so that they can draw the energy from food and turn it into heat. Cold-blooded animals require much less food, and therefore can survive in harsh and extreme environments like the desert. Since food isn't being used for heat, the energy just goes towards powering their bodily systems and towards controlling their body size. Have you ever seen a mammal the size of a mosquito or minnow? No. Warm-blooded animals can't be too small or they wouldn't be able to consume enough food to create heat energy and energy for body functions. On the other hand, you can have very large fish and very small fish, large reptiles and small reptiles. Cold-blooded animals have more options in terms of body size. Overall, the benefits of being cold-blooded make this adaptation a hot commodity.
Let's review. Animals that cannot naturally regulate their body temperatures are considered to be ectothermic or cold-blooded. Insects, arachnids, fish, reptiles, and amphibians are generally cold-blooded. These animals have a higher range of body temperatures than warm-blooded animals, but most move between environments to make themselves warmer or cooler. Many cold-blooded animals have evolved to go into torpor, or hibernation, to conserve energy in colder seasons. While they're more susceptible to extreme temperature changes, being cold-blooded means that they require less food and can use more energy from their food to control body size and other bodily processes. So while they can't regulate their body temperature without external factors, there are good reasons why this is the regular body system for the majority of animals in the world today.
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