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A Question of
This is a galloping _Crocodylus johnsoni_

I'm sure we've all heard about reptiles and other "cold-blooded" animals. These creatures are characterized by having absolutely no control over their own body temperature. They are slaves to the temperature of their environments.

What a load of bunk.

There is no animal alive today that fits that description. No, reptiles, insects, fish and all other so called "cold-blooded" animals are actually quite capable of controling their body temperatures. The do it by a process called thermoregulation.


Thermoregulation is the process that reptiles use to keep their body temperatures at their optimum. They do this by moving to different areas of their environment to warm up or cool down. For instance if a lizard is starting to feel the burn of the tropical sun, it might head into the shade or take a dip in a pool of water. Likewise that same lizard would also bask on a leaf or in a warm current of air to warm up. One very interesting behaviour that has been noticed by researchers is that some animals like Frilled Dragons (Chlamydosaurus kingii) and Collared lizards (Crotaphytus collaris) will run on their hind legs in the heat of the day. This seems to occur on very hot days with no breeze. By running around like this these lizards are able to create an artificial breeze and help cool themselves off. Neat huh?

When thermoregulation was first observed the researchers were viewing labratory lizards in an enclosed glass cage (to keep them from escaping). Unfortunately glass has this nasty habit of augmenting heat and distributing it all around itself. So the lizards in the experiment would be active for the small amount of time that the temperature in the enclosure was good and then inactive the rest of the time. This was how the false view that reptiles can't control their body temperatures came from.

Other ways to thermoregulate include burrowing and staying in a group. Yes contrary to the popular belief many reptiles are social animals. There are plenty of lizards that bask in large groups, much like bird flocks do. Reptiles are actually very tolerant of one another. This external way of keeping warm was deemed ectothermy and is a much more correct term for these animals.

So What are Mammals and Birds?

Mammals and birds keep their temperatures even by internal means such as shivering muscles to warm up and sweating to cool down. This internal way of keeping warm is called endothermy (Did you see that one coming?)

Which is Better?

It has always been thought that mammals being "warm-blooded" were one step above the reptiles and usurped their role as the dominant land animals by the end of the Cretaceous. Then some scientist had to go and show us that mammals had been living under reptile rule for over 185 million years. So maybe endothermy isn't the advantage. If your an ectotherm then you can, instead of using most of your energy towards keeping warm, devote almost 100% of it to hunting, foraging, mating, and all those other things. When things cool down, you slow down to compensate for it. If your endothermic on the other hand you can stay active in any weather. Of course in order to stay active you'll have to keep eating and the cooler it gets the more you'll have to eat. So when even the most resilient ectotherm is in torpor, you can stay awake, just so you can search for more food to keep you going.

So if reptiles aren't active in cold temperatures then how do leatherback sea turtles survive in the arctic? Well reptiles found a way to deal with the cold weather without having to change their metabolisms. It's called Gigantic endothermy.

Not Another Big Word

Gigantic endothermy or Gigantothermy is another temperature conserving technique that is used by the larger reptiles. The basic formula for this is that the larger you are the easier it is to keep your body in a normal temperature range. Gigantic endothermy is practiced by reptiles like the crocodylians (the large forms) and the leatherback sea turtle (Dermochelys coriacea) along with other large ectothermic animals such as the white pointer (Carcharodon carchararius). By being large these animals have more mass for heat to deal with and thus it takes heat a lot longer to leave the body.

Gigantothermy was proposed as one way that deinosaurs kept warm without resorting to endothermic techniques, but paleontologist Robert Bakker protested against it using animals like elephants as examples along with the idea that the longer it takes for an animal to cool down the longer it takes for it to warm up. So if a deino is caught in a monsoon season then it's core temperature could fall dangerously low and the animal might never recover. The problem with this idea is that there are gigantothermic reptiles today that spend long periods of time in cold areas. These animals handle it by simply moving their body. When you move your body uses energy and creates heat so by simply moving a gigantothermic animal can keep it's body temperature a constant.

In fact, the larger you get, the easier it seems to be to keep a stable temperature.

Anything Else?

The term cold-blooded extends beyond just metabolism talk. Since it's inception society has viewed cold-blooded animals as being slow moving, sluggish, stupid and sometimes even evil animals that were inferior to mammals and birds, with their warm-bloodedness. Indeed a popular term in society today for a cruel heartless act is cold-blooded (He killed him in cold blood!) Indeed research into reptiles is actually very recent. For the most part reptiles weren't considered that interesting to study. In some cases the only reason we know what we know about some reptiles is because they are going extinct and we need to know about their natural history in order to save them.

With this recent surge in reptile studies, we have found that most of what we knew about reptiles was untrue. They are not cold-blooded, sluggish, inferior creatures, but instead are active, adaptive, intelligent animals with one of the longest success stories in the history of the planet.

Cold-blooded? I think not.