Are Reptiles Cold-Blooded? How Reptiles Control Body Heat

What does cold-blooded really mean?

Animals can be grouped by where their body heat comes from:

• Endotherms, such as mammals and birds, burn food to produce most of their body heat internally and keep a fairly constant temperature.
• Ectotherms, such as reptiles, amphibians, most fish and invertebrates, depend mainly on outside heat from the sun, warm ground, air or water.

“Cold-blooded” suggests that a reptile’s blood is cold, which is often wrong. A lizard basking on a sunny rock can have a body temperature in the mid-30s °C (mid-90s °F), close to or even above the human body temperature of 37 °C (98.6 °F). Some desert lizards are active at about 40 °C (104 °F). What reptiles cannot do is keep that heat when their surroundings turn cold. The differences between reptiles and other ectotherms such as frogs are covered in reptiles vs amphibians.

How do reptiles control their body temperature?

Reptiles control their temperature mainly by behavior. Common methods include:

• Basking: lying in the sun, often on rocks, logs or roads that hold heat.
• Changing position: turning the body broadside to the sun to warm up, or facing it head-on to warm more slowly.
• Shuttling: moving back and forth between sun and shade through the day.
• Retreating: hiding in burrows, crevices or water when it is too hot or too cold.
• Changing color: some lizards darken to absorb heat faster and lighten when warm.
• Gaping: crocodiles and many lizards open their mouths to lose heat through evaporation.

The central bearded dragon of inland Australia is a good example. It basks on branches and fence posts in the morning, retreats to shade or a burrow at midday, and comes out again in the late afternoon. Field studies have shown that it darkens its back when cool to absorb sunlight faster and lightens as it warms.

Each species has a preferred body temperature range in which its muscles, digestion and senses work best. By moving around, reptiles can keep close to that range for much of the day.

What happens when reptiles get too cold?

When a reptile cannot warm up, its body slows down. Muscles weaken, digestion stalls and reactions become sluggish. In temperate regions, reptiles survive winter by becoming dormant in burrows, rock crevices, or the mud and water of ponds, a state often called brumation. Some turtles spend months under ice, as explained in Do turtles hibernate?.

Sudden cold can catch reptiles out. In southern Florida, introduced green iguanas become sluggish when temperatures fall toward about 10 °C (50 °F), and on colder nights they can lose their grip and drop from trees. Most are stunned rather than dead and recover as the day warms; see Why do iguanas fall from trees?.

Why being cold-blooded saves energy

Ectothermy has a big advantage: it is cheap. Because they do not burn food to stay warm, reptiles need far less energy than mammals or birds of the same size. At rest, a reptile typically uses only a small fraction, often estimated at roughly a fifth to a tenth, of the energy of a similar-sized mammal.

This is why many reptiles can go weeks between meals and why large snakes and crocodiles can survive long periods without food. It also lets reptiles thrive in deserts and other places where food is scarce. The cost is that their activity depends on the weather: on cold days they cannot hunt, flee or digest as well as a warm-blooded animal.

The leatherback: a reptile that stays warm

The leatherback sea turtle shows how flexible the rules can be. It is the largest living turtle, with adults commonly weighing several hundred kilograms, and it feeds on jellyfish in cold waters as far north as Canada and northern Europe, sometimes as cold as about 5 °C (41 °F).

Leatherbacks have been measured with core body temperatures up to about 18 °C (32 °F) warmer than the surrounding water. They manage this through a combination of traits:

• Large size: a big body loses heat slowly, an effect called gigantothermy.
• Insulation: a thick layer of oily fat under the skin.
• Heat exchange: blood vessels in the flippers are arranged so that warm blood flowing out passes heat to cold blood flowing back in.
• Muscle activity: constant swimming produces heat.

Leatherbacks are still ectotherms in the strict sense, because they do not generate heat the way mammals do, but they show that “cold-blooded” is far too simple a label.

Are any reptiles warm-blooded?

No living reptile is fully endothermic like a mammal or bird, but a few come close in special situations:

Birds, which descend from dinosaurs and are classified with reptiles by many biologists, evolved full endothermy. Among the animals traditionally called reptiles, though, outside heat remains the main source of warmth.