At the zoo where I worked, the Gila monster ate a few pinky mice every couple of days. That was it. The crocodiles got a whole chicken now and then, and that was enough.
A mammal that size would starve on those rations. That is the single best lesson I know about cold-blooded animals. They don’t burn food to stay warm, so they need far less of it.
I grew up in Villahermosa, in Tabasco, Mexico, right in front of a river full of crocodiles. Now I’m on a lap around Australia, and the ectotherms are everywhere. Lace monitors walked through our camp at Illaroo. Water dragons sat on the seawall at Evans Head. Two carpet pythons curled up in a toilet-block roof at Pebbly Beach.
These cold-blooded creatures are everywhere once you start looking. So what does “cold-blooded” really mean? Are humans cold-blooded? Are birds? (No, and no.) Here is how it works, and where the rules break.
You can explore the vertebrate branch, from fish to reptiles and birds, in my phylum explorer.
Cold-Blooded vs Warm-Blooded Animals: The Key Differences
Warm-blooded animals, or endotherms, generate body heat from their own metabolic processes. Mammals and birds do this, and so do you. Most warm-blooded animals keep a nearly constant internal body temperature, whatever the external conditions. That lets them sustain high energy levels, but it costs a lot of food.
Cold-blooded animals, or ectotherms, get most of their heat from external sources like the sun, warm rocks or warm water. Their own body temperature rises and falls with the surrounding temperature, unless they move to change it. These ectothermic animals can’t produce heat the way we do, so they borrow it
| Warm-blooded (endotherms) | Cold-blooded (ectotherms) | |
|---|---|---|
| Main heat source | Internal heat from metabolism | External heat sources |
| Body temperature | Constant body temperature | Varies with surroundings |
| Food needed | A lot | Far less |
| Active in the cold? | Yes | Slows down |
| Examples | Mammals, birds | Reptiles, amphibians, most fish, insects |
Birds are warm-blooded animals, just like humans. In fact, most birds run hotter than we do, at around 40–42°C. So do the birds you see every day, from emus to penguins.
What Does Cold-Blooded Actually Mean?
The name is misleading. A lizard basking on a hot rock can have warmer blood than you.
Cold-blooded only means the animal relies on the surrounding environment to set its core temperature. When the temperature drops, its metabolism slows and its metabolic functions crawl. It becomes sluggish. In direct sunlight, it warms up and switches on.
Biologists prefer two other words, and they compare cold-blooded and warm-blooded animals more carefully. Ectothermic means heat comes from outside. Poikilothermic means body temperature varies. Most poikilothermic animals are also ectotherms, but not always. Take a fish in the stable, cold water of the deep sea. It’s an ectotherm, but its temperature barely changes at all.
How Cold-Blooded Animals Regulate Their Body Temperature
Here’s the part most people miss. Ectotherms don’t just accept whatever temperature they get. They regulate body temperature through behavioral adaptations, and most reptiles are very good at it. This temperature control is a remarkable ability.
- Basking. Lizards and snakes angle their bodies to catch the sun rays. Some flatten out to expose more skin.
- Shuttling. Desert reptiles move between sun and shade all day to stay in their preferred range.
- Blood flow. Some lizards widen the blood vessels in their skin to absorb heat fast, then narrow them to keep it in.
- Colour change. Bearded dragons darken when cool to absorb more heat, and lighten when warm.
- Burrows and water. When it’s too hot, ectotherms retreat underground or into cooler water. Under those environmental conditions, a burrow is air conditioning.
Last year at Springbrook, in the Gold Coast hinterland, I saw five red-bellied black snakes on one hike. Two were mating in the open, right beside the track. Snakes are easiest to find exactly where the heat is.
Examples of Cold-Blooded Animals
Cold-blooded animals are incredibly diverse. Many species fall into this group, in fact most animal species on Earth.
Reptiles
Most reptiles are classic ectotherms. Crocodiles bask on riverbanks with their mouths open, which helps them shed excess heat. Monitor lizards need hours of sun before they hunt. Snakes coil to keep heat in, or stretch out to soak it up. You can meet the whole group in my guide to types of reptiles.
2. Amphibians:
Frogs, toads, salamanders and newts are ectotherms with thin, permeable skin. They lose water easily, so they balance heat against drying out. That’s why many amphibians are most active on warm, wet nights.
Cold climates don’t stop them, though. The wood frog lives north of the Arctic Circle.
3. Fish:
Most fish are cold-blooded and match the surrounding water. Blood pumped through the gills loses heat to the water almost instantly, which makes staying warm very hard.
A few have cracked it, as you’ll see below.
4. Invertebrates:
Insects, spiders, crustaceans and most other invertebrates are ectotherms. Even so, many insects can warm themselves. Honey bees and bumblebees shiver their flight muscles before take-off. Some moths do the same.
The Exceptions: Cold-Blooded Animals That Make Their Own Heat
The line between warm-blooded and cold-blooded is blurrier than textbooks suggest.
- Tuna and some sharks. Bluefin tuna, mako sharks and great white sharks keep the internal temperatures of their swimming muscles warmer than the water. A heat-exchanging web of blood vessels traps the heat inside.
- The opah. In 2015, scientists found this deep-water fish warms its whole body, using its constantly flapping fins as a heater. It was the first known fully warm-bodied fish.
- Tegus. Argentine tegus are lizards, but in the breeding season they warm themselves up to 10°C above their burrow temperature. I worked with tegus at the zoo, and this still amazes me.
- Brooding pythons. Some female pythons shiver around their eggs to keep them warm.
- Leatherback sea turtles. Their sheer bulk and thick fat hold heat, letting them dive into cold water where other sea turtles can’t follow.
Warm-blooded animals bend the rules too. Hummingbirds drop into torpor on cold nights and let their temperature fall to save energy.
Advantages of Being Cold-Blooded
- Far less food. Ectotherms conserve energy. They can need as little as a tenth of the energy of a mammal the same size. My Gila monster’s diet proves it.
- Better growth. More of each meal becomes body mass instead of body heat.
- Long fasts. Many reptiles survive weeks or months without enough food. Large pythons can go months between meals, sometimes more than a year.
- Small sizes are possible. Tiny endotherms lose heat too fast. Tiny ectotherms don’t need to worry about it.
Disadvantages of Being Cold-Blooded
- Activity depends on the weather. In colder temperatures, hunting, digestion and escape all slow down.
- Limited range. There are few reptiles in polar regions, and most species live in warmer climates.
- Vulnerable when cold. A chilled lizard can’t outrun a predator.
- Overheating risk. In extreme temperatures, an ectotherm without shade or water can die quickly. They have no sweat glands to cool down.
How Cold-Blooded Animals Survive Winter
When temperatures drop for extended periods, ectotherms have three main tricks.
Brumation. Reptiles slow right down in burrows or crevices for the colder seasons. Their metabolism slows dramatically, but unlike true hibernators they may wake on warm days to drink or bask.
Breathing underwater. Some freshwater turtles overwinter under ice, barely breathing at all. Australia’s Fitzroy River turtle goes further. It takes up much of its oxygen through its cloaca, earning it the nickname “bum-breathing turtle”.
Freezing solid. The North American wood frog lets up to about two-thirds of its body water freeze. Its heart stops. It survives by flooding its cells with glucose, which works as a natural cryoprotectant. Come spring, it thaws and hops away.
Fish in Antarctic waters take the opposite approach. Instead of tolerating ice, they make antifreeze proteins that stop ice crystals from growing in their blood.
Climate Change and Cold-Blooded Animals
Because their body temperature depends on external factors, ectotherms feel climate change directly. Few environmental factors matter more to them than heat.
Tropical lizards and frogs already live close to their upper heat limits, and warmer regions get hotter every decade. Temperature fluctuations and extreme environments push them further. A 2019 study found marine ectotherms are especially at risk. They have fewer cool places to hide than land animals.
Sea turtles show a stranger effect. Their nest temperature decides their sex, with warmer sand producing more females. Take the northern Great Barrier Reef population, which nests mostly on Raine Island. A 2018 study found that 99% of its young green turtles were female. The effects of climate change on wildlife are rarely this easy to measure
FAQs About Cold-Blooded Animals
No. Humans are warm-blooded, with a nearly constant body temperature of about 37°C. We generate heat internally and cool ourselves with sweat glands.
No. Birds are warm-blooded, and most keep a body temperature of around 40–42°C, warmer than ours. Some, like hummingbirds, let it drop at night to save energy.
Most are. A few, including great white and mako sharks, are partly warm-blooded. They keep their swimming muscles, eyes and brain warmer than the surrounding water.
No. Their blood is the same temperature as the rest of their body, which depends on their surroundings. A lizard in direct sunlight can be warmer than you.
Some can. The wood frog survives with about two-thirds of its body water frozen, using glucose as antifreeze. Antarctic fish use antifreeze proteins to stop ice forming at all.
Watch the Rocks on a Sunny Morning
Next time you walk a bush track on a warm morning, look at the rocks, logs and track edges. That’s where the ectotherms are, charging up for the day.
Give them space, and watch how carefully they choose their spot. A cold-blooded animal is never just lying there. It’s working.
Wildlife Nomads Challenge
The Warm-Up Challenge 🦎
Ten questions on basking, freezing frogs, warm-bodied fish and bum-breathing turtles. Instant feedback after every answer.
References:
Jensen, M. P., Allen, C. D., Eguchi, T., Bell, I. P., LaCasella, E. L., Hilton, W. A., Hof, C. A. M., & Dutton, P. H. (2018). Environmental warming and feminization of one of the largest sea turtle populations in the world. Current Biology, 28(1), 154–159.e4. https://doi.org/10.1016/j.cub.2017.11.057
Pinsky, M. L., Eikeset, A. M., McCauley, D. J., Payne, J. L., & Sunday, J. M. (2019). Greater vulnerability to warming of marine versus terrestrial ectotherms. Nature, 569(7754), 108–111. https://doi.org/10.1038/s41586-019-1132-4
Storey, K. B., & Storey, J. M. (2017). Molecular physiology of freeze tolerance in vertebrates. Physiological Reviews, 97(2), 623–665. https://doi.org/10.1152/physrev.00016.2016
Tattersall, G. J., Leite, C. A. C., Sanders, C. E., Cadena, V., Andrade, D. V., Abe, A. S., & Milsom, W. K. (2016). Seasonal reproductive endothermy in tegu lizards. Science Advances, 2(1), e1500951. https://doi.org/10.1126/sciadv.1500951
Wegner, N. C., Snodgrass, O. E., Dewar, H., & Hyde, J. R. (2015). Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science, 348(6236), 786–789. https://doi.org/10.1126/science.aaa8902
