A Question of

This is a galloping _Crocodylus johnsoni_

Cold-Bloodedness


I'm sure we've all heard the term "cold-blooded" at one time or another. Animals that fit under this category are characterized as having absolutely no control over their own body temperature. They are slaves to the temperature of their environments.

What a load of bunk.

There are few animals alive today that truly fits the description of cold-blooded (e.g. coral). Reptiles, insects, fish and all other so called "cold-blooded" animals (basically anything that is not a bird or mammal) are actually quite capable of controling their body temperatures. They do it through a process known as thermoregulation.

What is thermoregulation?

Thermoregulation is the act of regulating one's body temperature. This term is usually reserved for animals, such as reptiles, that maintain their body temperatures through external means. Reptiles and other "cold-bloods" accomplish this by moving to different areas of their environment to warm up or cool down. They also use certain other behavioural traits, to keep their body temperatures more or less, constant. 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. Crocodylians hold their jaws agape to cool down on hot days. 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 generate an artificial breeze and help cool themselves off.

When thermoregulation was first observed, the researchers studying them viewed the lizards in an enclosed glass cage (to keep them from escaping). Unfortunately glass has this nasty habit of augmenting heat and distributing it evenly around itself. So the lizards in the experiment would be active for the small amount of time that the temperature in the enclosure was tolerable and then inactive the rest of the time. There was no shade or breeze to allow them to regulate their temperatures and panting can only do so much. From this, the false view that reptiles cannot control their body temperatures came about.

Other thermoregulatory mechanisms include burrowing and group living. Yes, contrary to the popular belief, a variety of reptiles are fairly social animals. There are plenty of lizards that bask in large groups (e.g. marine iguanas), much like flocks of birds do. Many reptiles are actually very tolerant of one another. This external means of keeping warm was deemed ectothermy, or "external heating" 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?). Course this doesn't mean that mammals and birds don't occasionally cheat. Lions, deer and other mammals spend a great deal of time in the shade when temperatures get too hot and there are birds species that spend their mornings basking on tree branches to soak up the heat, so endotherms thermoregulate as well, just not to the same extent.

Are things really this clear cut?

While on the surface it seems rather easy to place animals into the category of ectothermic and endothermic, there is more here than meets the eye and the fact of the matter is, thermophysiology on this planet is more of a spectrum than it is categorical. For one thing, the terms endothermic and ectothermic are just one of three measurments used to determine thermophysiology. These terms are used to define active thermal generation and in truth, ectothermy is really endothermy at a lesser degree.

There are two more variables that must be thrown into the mix. That of resting metabolism and of resting temperature. The two terms used for separating resting metabolism are: bradymetabolic and tachymetabolic. A bradymetabolic animal has a low resting metabolism, such as a gila monster (Heloderma suspectum), while a tachymetabolic animal has a high resting metabolism, such as a hamster (Cricetinins). An animal that maintains a near constant internal temperature is referred to as being homeothermic, while an animal with a fluctuating internal body temperature is considered to be poikilothermic. With this in mind let us see how many animals can fit into the all or none perspective.

Our first example is a bee. Bees, like most insects, are considered to be cold-blooded, but their large wing muscles allow them to generate excessive amounts of heat, thus making them endothermic. Yet at rest, there body temperatures drop to temperature much like their environment and when asleep or hibernating they have a low degree of metabolic activity. So bees are considered to be endothermic, bradymetabolic, poikilotherms. Or are they? In the morning, foraging worker bees will raise their internal body temperatures endogenously by shivering their great wing muscles. This raising of body temperature is also used to control the thermal gradient of the hive itself and thus we have to enter a transitional variable known as functional endothermy. That is to say, rather than wait for a sunbeam to warm them up, the bees cheat and use the heat generated by their massive wing muscles, to bring up their core temperatures and metabolisms. So in truth bees are, functionally endothermic, bradymetabolic poikilotherms.

Our other example: bats. Bats are considered to be warm-blooded animals. Their have active lifestyles and generate internal heat and so could thus be called endothermic, tachymetabolic homeotherms, but when a bat rests, they bring their metabolisms down to a much lower state and rely on the cave and heat generated by their neighbors to keep their temperatures constant. Thusly, bats are endothermic, homeothermic and bradymetabolic.

Other animals that defy the rules include, varanids, naked mole rats, lamniforme sharks, tuna, practically every insect on the planet, crocodylians, the leatherback, pythons, etc. Creatures such as these are living breathing examples of the dangers in using an arbitrary system of measurement to categorize life.

Which is Better?

It had originally 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 paleontology went and showed 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 that make life interesting. 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.

With this in mind, one should expect to find a lack of reptiles the further north one goes, yet this is not always the case. While the majority of reptiles live around equatorial regions of the planet and while their numbers do decrease towards the poles (both of these things are also common of birds & mammals as well), we still do find reptiles that survive in rather cold areas of the world. From lizards in Tibet to the leatherback sea turtles surviving in arctic waters. How do these animals accomplish these feats while still remaining "cold-blooded?"

For Tibetan animals such as Thermophis baleyi, the trick lies in where they are located. This species maintains it's survival on the cold Tibetan plateus by living near hot springs where it lives off of fish and invertebrates. Lizards and snakes of the region (i.e. Laudakia sacra) tend to have longer basking times and give birth to live young. This does not explain all the animals here though, for instance the little gecko Cyrtodactylus tibetanus, which attains a length of only 10.16cm, not only survives on these montane places, but does so while maintaining the nocturnal existence so common to Gekkonidae. Just how these lizards survive the 4.4º C (40º F) and cooler nights, while maintaining their 10º C (50º F) body temperatures, is still a bit of a mystery. One theory on how they handle the cold is by absorbing heat on the rocks in the day and using it as a bit of a thermal battery. Another thermally enigmatic gecko is Palmatogecko rangei from the Namib desert. This little reptile is also active on 4º nights with an added wind chill from the Atlantic as well.

One of the most thermally adaptive of all the reptiles though, is the Leatherback sea turtle (Dermochelys coriacea). One of the means that allows these turtles to maintain internal temperatures 18º higher than their surrounding environment (64º F) is their sheer size. The term used for this is known as gigantic endothermy or gigantothermy.

What is gigantothermy?

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 at a constant temperature range. Gigantic endothermy is practiced by other reptiles as well, such as the crocodylians (the large forms) 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. [Note: This is a simplification & a generalization. In both Dermochelys and lamniformes sharks, the use of muscles to keep the body warm is also used. D.coriacea has special circulatory adaptations along with a blubbery body as well].

Gigantothermy was proposed as one way that dinosaurs kept warm without resorting to endothermic techniques. Paleontologist Robert Bakker (Dinosaur Heresies, 1986) protested against this using animals such as 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 dino 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 its body temperature a constant. Besides that fact, it has also been shown that reptiles can gain heat faster than they lose it (Brattstrom 1973, et al). This is one of those little known facts about reptiles. The problem for them is not so much gaining heat, as it is losing heat (reptiles lack sweat glands). So the general rule is that the larger you get, the easier it is to keep a stable body temperature.

More than just metabolism

The term cold-blooded extends beyond just metabolism talk. Since its 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!). 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 why 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 help save them. With this recent surge in reptile studies, it has been shown that most of what we thought we knew about reptiles is untrue. They are not cold-blooded, sluggish, inferior creatures, but are instead, active, adaptive, intelligent animals with one of the longest success stories in the history of the planet.

Cold-blooded; I think not.


Bakker, R.T. 1986 The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and their Extinction

Brattstrom, B.H. 1973. Rate of heat loss by large Australian monitor lizards. Bull. So. Calif. Acad. Sci. 72(1): 52-54.