I'm a zookeeper, and I take care of reptiles. We're a different sort of a zoo - a mobile zoo - the zoo that comes to you! We pack reptiles into bags and containers and drive out to your; house, school or event. To ensure the wellbeing of our animal wards we need to negotiate specific conditions with our clients, one of these is shade. I get asked all the time, 'is the shade for you or the animals?'
The answer to that question is well both! I definitely need to be in the shade, but perhaps more importantly still we need to be especially mindful of the temperature that our animals are exposed to because they cannot modify their body temperature. That's what it actually means to be cold-blooded. The idea that reptiles 'like' the warm weather because they are cold blooded is an oversimplification at best because reptiles don't actually have cold blood, or rather they don't have cold blood all of the time, or even most of the time (depending on climate).
A new way to describe, 'Cold-Bloodedness'
The term cold-blooded is misleading, and perhaps not the best way to describe what's actually going on here, a better word is ectothermic or ectothermy. I promise this part is fairly easy to understand, so please stay with me. Our new word is of Greek origin and can be broken into two components; ecto (ektos) and thermy (thermos). You might already be familiar with the later part of the word, thermos which is ofcourse a container used to keep liquids warm. It means heat, you'll also likely recognise this derivative in the word thermometer - a tool used to measure heat. The former part of the word ecto (ektos) means outside. Together the word translates to 'outside heat', and that's the hokey pokey! It means you get heat from the environment, not from within your body like warm-blooded animals or rather endothermic (inside heat) animals.
Practically, what does this all mean? Let's briefly discuss what life is like for endothermic animals - like you! It means you use A LOT of energy keeping your body warm; it's incredibly inefficient. But it also means that there is plenty of energy available at a moment's notice. Human animals have a habit of thinking that whatever is the norm for them must be the typical condition, but that's not true here, warm-bloodedness or endothermy is not the rule, but rather the exception. Far more animals exchange heat within their environment. Warm-blooded animals including mammals and birds have so much available energy, that we live our lives in fast forward. And the human propensity for thinking our experience is the status quo means that animals like reptiles are often viewed as sluggish, which is probably where the idea of a 'lazy lizard' comes from. To me that feels a bit unfair; he's not lazy, you're just like...super duper energised!
How metabolisms differ
Let's flesh this out the details a bit more. Basically the metabolisms of endotherms and ectotherms differ. The energy guzzling endotherms including people employ a variety of methods to maintain a stable thermal temperature including; sweating, goosebumps and shivering, other mammals have different strategies such as panting, growing fur or coating their bodies in blubber. Generally speaking these animals also require huge amounts of food to fuel this sort of metabolism, while ectotherms require significantly less. In fact reptiles can go for weeks, months or even a year without food. Instead of using food as fuel to maintain body temperature, ectotherms, especially reptiles, make use of the sun's energy; they are solar-powered creatures. Reptiles have this behaviour where they sit in the sunshine - it's called basking. It may feel nice to sit in the sun and warm up your body, but reptiles don't do this simply because it 'feels good', they are compelled to bask.
No right or wrong way, only consequences
Without basking a reptile such as a snake or lizard will stay cool and a cool body for an ectotherm is lethargic and doesn't allow for other metabolic processes to work, so a reptile can't eat or rather it can't digest food when it's cold. But basking in the sunshine might be dangerous, it may expose you to potential predators. Whereas a mouse doesn't have to worry about sitting in the open, it can hide in the dark and cool and forage for food when the coast is clear.
Temperature and climate will limit the places that animals both ectothermic and endothermic can survive. Large endotherms, like mammals, are generally rare in extreme environments like the desert. When present, they often require massive territories and have structural adaptations to survive. By contrast, reptiles seem to have an edge on these extreme climates. They can tolerate huge variability in temperature without a fur coat or wide ears to dissipate heat. Reptiles generally have a preferred body temperature similar to the human body, but unlike humans, they can tolerate significant shifts in internal temperature without experiencing hyper- or hypothermia. In some cases, reptiles can even survive exposure to freezing conditions or incredibly high heat environments. Lake Eyre Dragons, found in South Australia are exposed to temperatures reaching 60℃. Their tiny lizard bodies may exceed 40℃ - a life-threatening condition for humans.
And there are consequences for both survival strategies. There is no right or wrong kind of metabolism, it’s simply a matter of trade-offs. You can be a busy mammal, but have to find huge amounts of food to survive or you could be an energy saving reptile, and be at the mercy of external factors; exchanging heat (energy) with the environment. To be efficient or inefficient with energy that is the question!
The rebels of the reptile world
Now let's discuss a few of the oddballs in the reptile world. One turtle in particular stands out, it’s known as the Leatherback Turtle, and as its name suggests it’s quite unique in that it has a leathery carapace, not a hard shell like other turtles. But the reason I bring it up now, has got to do with the fact that this turtle runs warm - generally. I mean it’s body temperature is higher than surrounding waters - simply put this is NOT an ectothermic reptile. It employs a number of mechanisms to regulate body temperature including; insulating tissues, large body size and circulatory changes. This has been coined, ‘gigantothermy’, which in case you haven’t realised translates to ‘big heat’. And this is truly a massive turtle, weighing as much as 900 kg. It roams the world's oceans and can dive to depths of over 1000 metres, its lifestyle and anatomy reminds me more of a whale than a turtle. This form of thermoregulation may offer a solution for how dinosaurs (once thought to have been ectothermic) maintained such enormous bodies (Paladino, F. 1990).
Speaking of dinosaurs, it would be very remiss of me to not mention the elephant in the room, which is of course not an elephant at all, but rather a bird. For you see, as the direct descendants of dinosaurs, modern day birds are (as far as phylogenetics are concerned) also members of the reptilian family, and all of them are warm-blooded or endotherms, maintaining a consistent body temperature.
Summary
So hopefully by now, you’ll agree with me, that cold-blooded/ness is just a word crime, and it doesn’t adequately describe the complex and nuanced survival strategy employed by MOST animals on the planet that exchange heat with their environment.
All creatures have evolved different life strategies, including thermoregulation. Neither endothermy nor ectothermy is correct or better, it’s just different, and there are consequences for animals in both camps. And for every rule, there are rule-breakers including reptiles such as the warm-bodied leatherback turtle and of course the birds, which are reptiles too, but that explanation is for another blog post.
True Blue Reptiles are a mobile reptile zoo specialising in fun and educational reptile experiences! These blog posts are a nice change of pace and allow us to flex our academic brains! Want to learn more about reptile physiology, anatomy and adaptations, consider having True Blue Reptiles visit your school or institution, head over to: www.truebluereptiles.com.au
1.) Paladino, F., O'Connor, M. & Spotila, J. Metabolism of leatherback turtles, gigantothermy, and thermoregulation of dinosaurs. Nature 344, 858–860 (1990). https://doi.org/10.1038/344858a0