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Why are Reptiles Cold-Blooded? (Plus Fun Facts About Reptiles)

Did you know that being cold-blooded doesn’t mean that reptiles actually have cold blood? I didn’t know this either! So what does it really mean and why are reptiles cold blooded in the first place? This is what I found…

Reptiles are cold-blooded animals, (also called ectotherm), which gives them an advantage as part of their survival strategy. They don’t use their energy to warm themselves like we do. Instead, most of their energy goes to growth and activity. Being cold-blooded affects the reptile’s metabolism in a way that they can go without food for long periods of time.

But because reptiles cannot warm themselves they are very dependent on their surroundings. They require specific surroundings in temperature, humidity, and food, (also called husbandry), to survive. And there are also different types of cold-blooded when it comes to animals…

What does Cold-Blooded Mean?

There are three types of thermophysiological characteristics when speaking of a cold-blooded animal:

  1. Ectothermy is when the animal controls its body temperature through external processes like laying on a warm rock in the sun. 
  2. Poikilothermy is when an organism can function in a wide temperature range internally.
  3. Bradymetabolism is when the animal can respond to need by altering its metabolism, like hibernation, for example.

Reptiles are ectotherms, also called cold-blooded. This means the energy acquired from food is not carried into body heat. Instead, the reptile needs to gain body temperature from its surroundings, like from the sun. Their body temperature before being sun-warmed is usually close to the temperature of the surrounding water. Once sun-warmed, the reptile’s temperature is close to the human body temperature. 

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So being cold blooded doesn’t actually mean reptiles have cold blood. It means that they use outside surroundings to warm their blood rather than internally warming their own blood.

Reptiles can have a body temperature ranging between 41-50° to 95-104°F. But reptiles living in the arctic seas have temperatures ranging between below 32° F to 50-59° F. 

These cold-blooded animals are dependent on the sun to heat their body, but they can also receive too much heat during heat stress which can be dangerous. That is why ectotherms release heat-shock proteins since excessive heat can damage proteins in the reptile’s body. The release of heat-shock proteins stabilizes other proteins and prevents their molecular structure from breaking, called denaturation.

Colder environments can be dangerous to the animal. Some ectotherms can alter their body chemistry when exposed to freezing temperatures. They can use cryoprotectants (ice-inhibiting compounds of protein, sugars, and sugar alcohols) or already dissolved substances like salts to flood their bloodstream and tissues. The altering of body chemistry will lower the freezing point of their water and, in that way, prevent ice crystals from growing or forming in their cells and tissues. 

The Pros and Cons of being a Cold-Blooded Animal

Reptiles have a number of advantages and disadvantages due to them being cold-blooded. Here are some advantages:

  1. The most significant advantage of being cold blooded is that reptiles can go for long periods of time without food because of their cold-bloodedness. The cooler the temperature of the blood, the lower the metabolism will be. Low metabolism means that the reptile needs fewer calories. Not needing food regularly is advantageous in harsh environments where there is a lack of food or water. Reptiles can simply wait around for better times to come around. Many reptiles have also developed a sit-and-wait strategy, which saves energy and is basically an ambush strategy. 
  2. It’s energetically efficient to be ectotherm because the energy absorbed through food can be distributed to other areas rather than warming up the body. Reptiles get their warmth for free through the heat of the sun and can turn their energy to growth and activity instead, (growth meaning in growing physically, and activity meaning mating, running from danger, hunting, etc). 
  3. Every reptile has a Proffered Optimum Temperature Range (POTR) where they are physically at their best. A reptile’s temperature affects its immune system as a result. By moving to different locations to control its temperature, the reptile is also optimizing its immune system. A reptile’s immune system is at its best when within its POTR. 
Reptiles are limited to the kind of environment they can survive in due to their dependence on external surroundings to regulate body/blood temperature

One of the disadvantages of being cold blooded is the dependence of external temperature for survival. The advantage of controlling body temperature by moving to different locations also shows how dependent the reptile is on the external temperature.

It is evident when a reptile is cold and lacks enough heat because it seems tired and slow. Reptiles that are active in the day need to heat up in the sun before carrying on with their activities. Because of this heat dependence, you can find most reptiles in environments where the weather and temperature are practically constant.

Take Australia, for example; it is the country with the highest number of reptile species. So, ectotherms are not very flexible with changing environments; they are adjusted to a specific environment. 

The adaptation to a specific environment is why reptiles need very specific husbandry, referring to food, care, and shelter. You might have come across these particular requirements when owning a reptile pet.

Not only is heat from a UVB/UVA light systems necessary to care for a reptile, but also humidity and water regulation. Humidity regulation can be achieved with a humidity box with moist moss. But you also don’t want to minimize ventilation when trying to humidify the environment. Ventilation is required to avoid the growth of fungus and bacteria which cause infections. 

Are All Reptiles Cold-Blooded?

There are about 6,500 species of reptiles, and they have all been known to be cold-blooded. Reptiles are animals like snakes, lizards, alligators, crocodiles, turtles, and a very long time ago, dinosaurs

However, New Scientist reported in 2016 that the Argentinian black and white tegu, a lizard, heats its body temperature by itself during mating season. The ability to control body heat from within means that this lizard is the first know warm-blooded reptile. The tegu has the ability to raise its body temperature by 50°F. Researchers speculate that hormones during the mating season cause the body temperature to rise. An increase in body temperature could help the lizard be more active and do more eating, running, or mating. 

What other Animals are Cold-Blooded?

Other than reptiles, cold-blooded animals include fishes, amphibians, and insects.

  • Most fishes are ectothermic, meaning that they manage their body temperature using their environment. But other fishes are endothermic, meaning that they can manage their body temperature themselves using their metabolism.
  • Amphibians, like toads and salamanders, are also ectothermic, using their surroundings to control body temperature. If there is no opportunity to bask in the sun and the animal needs to save energy, amphibians can brumate, a hibernation-like state. 
  • Insects have traditionally been seen as poikilotherms which makes them more adaptable to their changing environment. But some insects can also be warm-blooded, so-called endotherms. Interestingly, the body temperature and adaption vary depending on if the insect is flying or preparing to fly. 

What Animals are Warm-Blooded?

Warm-blooded animals, so-called endotherms, include mammals and birds. These animals have a higher body temperature than its surroundings and are regulated from within by metabolic processes

  • Mammals, including humans, regulate heat loss and heat production internally according to need. Sweating, panting, and drinking water are ways to cool down, while goosebumps reduce the amount of heat leaving the body, and shivering generates heat. However, keeping the body temperature steady regardless of the environmental situation takes a lot of energy produced metabolically
  • Birds are endotherms and have the ability to adapt well to different surrounding temperatures with other strategies. They can control the temperature of their legs and feet separately from their body. By standing on one leg, the bird reduces the unfeathered surfaces to keep body heat. And building nests away from or in the sun alters heat loss or gain. 


Reptiles are cold-blooded because they regulate their body heat ectothermically to have an advantage in survival. They have found ways to be energy efficient by gaining heat from external sources.

They are slower when lacking body heat but use their energy savings to go for long periods without food when needed. Because of their ability to wait, reptiles can survive in harsh environments when food is absent. But the reptile is very dependent on its environment to regulate body heat, thus making it hard to change environments

Warm-blooded animals have other qualities and cannot go for more extended periods without food because their body temperature is regulated through metabolic processes. But warm-blooded animals are more flexible in different environments because the body temperature is controlled internally

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