There is much diversity in the animal kingdom in terms of outward, physical characteristics from scales and feathers to fur and skin, but when it comes to internal workings, animals usually fall into one of two general groups: warm-blooded and cold-blooded. If you’ve recently brought home a reptilian pet, you may already know that they are cold-blooded. But what does this mean?
Yes, most reptiles are cold-blooded animals, meaning they cannot regulate their internal body temperature, so it varies according to their environment. Cold-blooded reptiles include lizards, snakes, and turtles, although Leatherback turtles are actually considered warm-blooded as their size allows them to maintain a core temperature.
Although the vast majority of reptiles are cold-blooded, each species will find comfort in different surroundings i.e. certain snakes prefer cooler temperatures and wetter environments than others. Let’s find out more about what it means to be cold-blooded, for reptiles and other animals.
What is ‘Cold-blooded’?
Being cold-blooded – or ‘ectothermic’ to give it its scientific term – essentially means that you cannot generate your own body heat, so your body temperature will always be determined by how warm or cold your surroundings are.
When we say ‘cold’ blooded, confusingly, this does not refer literally to the temperature of an animal’s blood, but rather, these animals did not evolve to maintain their internal temperature to a constant level like mammals can.
Why Are Reptiles Cold-blooded?
Reptiles are usually much smaller in size compared to mammals and need to produce more heat to keep warm than warm-blooded animals do because they lack the surface area and insulating layers of fat or fur.
As reptiles lose warmth through their skin much faster than warm-blooded animals, they would need to digest their food twice as fast to generate the same amount of constant internal heat. For this reason, most reptiles evolved to survive with less food and adapt to the temperature of their surroundings, relying more on their climate (instead of merely food) to give them energy.
So the Desert Iguana found in northwestern Mexico, for example, will have an internal body temperature of around 80 degrees Fahrenheit if it is basking in 80 -degree sun.
How Do Reptiles Maintain Their Body Temperature?
Reptiles maintain their body temperature using a process known as ‘thermoregulating’, whereby they will move to a warmer location to heat up and a cooler location to cool down.
In a similar way we, as humans, might add or remove a sweater to help us reach our ideal temperature at the moment, reptiles will choose to lie on a hot sun-baked rock or move to a shady spot to cool off. So depending on the species, some reptiles like small burrowing lizards and snakes may crawl underground to stay cool, whereas alligators may plunge into a nearby body of water.
Moving between the warmth and the shade throughout the day like this helps reptiles manage their internal bodily temperatures by keeping their core as warm as it needs to be to move around.
Difference Between Cold and Warm-blooded Animals
Warm-blooded animals (humans, birds, and most land and marine mammals) have a constant and reliable internal temperature of around 99 to 104°F. A bit like a continual boiler system running inside them, warm-blooded or ‘endothermic’ animals maintain this body temperature range no matter what the surrounding climate, as long as they consume food that is burned off for energy and warmth.
Cold-blooded animals, on the other hand, get their energy from their surrounding environment rather than their food and because they cannot maintain a constant internal temperature, they cannot survive extreme weather conditions.
There are many upsides to being cold-blooded though:
- Firstly, reptiles and other cold-blooded animals do not need to feed as often as we do which is good news when food is scarce in the desert and other harsh landscapes, since they don’t need to rely on this for their energy.
- Secondly, ectothermic animals are less vulnerable to illness and disease since their lack of a constant bodily temperature doesn’t attract many parasites or viruses that love a reliably warm host!
- And thirdly, cold-blooded animals can actually travel more quickly and more often in hot temperatures than warm-blooded animals. Have you ever noticed how slow and ‘low on battery’ you feel when carrying some heavy shopping during a heatwave?
Lizards and snakes, meanwhile, can be seen scuttling across the desert floor in boiling temperatures for hours because their muscles and energy levels become activated by the warmer temperatures. Concurrently, extreme dips in temperature can see cold-blooded animals become sluggish, even perfectly still as they rest and conserve energy, which is why so many hibernate throughout winter.
Other Cold-blooded Animals
In addition to reptiles, there are many more animal groups that are classed as cold-blooded which include:
Insects
- Dragonflies
- Bees
- Spiders
- Mites
- Ants
- Beetles
- Grasshoppers
Amphibians
- Frogs
- Salamanders
- Toads
- Newts
- Caecilians
Arthropods
- Millipedes
- Centipedes
- Lobsters
- Crab
- Shrimp
- Barnacles
- Isopods
- Krill
Fish
Practically all fish species are cold-blooded (apart from the colorful ‘Opah’ or ‘Sunfish’ often found in Hawaii, which is thought to be the only warm-blooded fish in the world).
Just as a terrestrial cold-blooded animal’s body temperature will match its environment, a fish swimming in 30°F waters will have an internal temperature of roughly 30°F too!
According to the National Oceanic and Atmospheric Administration, there are some modern-day swordfish, shark, and tuna species that are somewhere in between warm and cold-blooded since they are somewhat able to maintain their body temperature in changing waters. However, their internal organs soon begin slowing and cooling if they remain in deep cool waters for too long, compelling them to swim back up to the warmer, shallow waters.
Dinosaurs
Yep, despite technically classing as reptiles, many dinosaurs were also cold-blooded as they shared the complex metabolism that can now be found in modern-day bird species.
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