Cold-blooded & warm-blooded

The animal kingdom can be split based on how body temperature is controlled (thermoregulation): ‘cold-blooded’ ectotherms use external sources of heat, such as solar energy, whereas ‘warmblooded’ endotherms generate internal heat via metabolism. But while this difference helps define features and behaviour, some species show that the distinction between the two groups isn’t so clear-cut.

What’s wrong with saying ‘cold-blooded’ and ‘warm-blooded’?

Some people don’t like those terms, with good reason! Most invertebrates are ecotherms and can’t be ‘blooded’ as they don’t have actual blood, so are instead classified as ectotherms. The labels ‘cold’ and ‘warm’ only really apply to vertebrates.

Another problem is that the terms are relative, not absolute: in the desert, a cold-blooded lizard might have a higher body temperature than a warm-blooded rodent. But despite what textbooks may tell you, many scientists still use cold/warm as synonyms for ecto/endo because the meanings are almost interchangeable.

Why regulate body temperature?

Because the biochemical reactions that sustain life work best within a certain range. Many animals are homeotherms, which means they aim to maintain bodies near an optimum temperature using physiological and/or behavioural strategies. Endotherms can avoid overheating by surface evaporation (dogs pant, for instance) and gain warmth through physical activity, while ectotherms (such as a crocodile basking in the sun) can cool down by moving to shade.

How do endotherms generate heat?

All vertebrates can warm the body by shivering (repeated muscular contraction), but birds and mammals are classed as endotherms because they’re also capable of generating heat from metabolism by ‘nonshivering thermogenesis’. The processes that produce heat in each group are distinct, so many biologists think that they evolved independently: true mammals burn a type of fat called brown adipose tissue and birds flap their wings to heat pectoral muscles.

Are ectotherms at a disadvantage?

No! It’s tempting to assume that the more sophisticated heating mechanisms used by mammals and birds have allowed them to be more successful than ‘primitive’ fish and reptiles. In fact, being cold-blooded has its benefits. Notably, because their heat doesn’t come from metabolism, ectotherms need far less energy to function.

Lizards, such as this Peter’s rock agama, rely on external heat

If a warm-blooded animal is like a motorbike then a cold-blooded creature is a pushbike: lower performance, but also lower fuel consumption. Ectotherms lead an economical lifestyle – an advantage when food is limited or unpredictable, when an endotherm would have to either starve or migrate. This enables cold-blooded species to occupy ecological niches that aren’t available to warm-blooded ones.

Are there exceptions?

Plenty. Some species from cold-blooded groups could be considered warm-blooded as they raise their body temperature above ambient. Large fish such as mackerel sharks (including the great white), billfishes (such as swordfish and marlin) and tunas heat body regions as a by-product of contracting red muscles while swimming, and a bloodflow system that limits heat loss. Opahs can even warm their whole bodies.

Among reptiles, leatherback turtles are insulated by a layer of subcutaneous fat – similar to blubber in seals and whales – that helps keep body temperature at about 25ºC during deep dives or in sub-polar waters under 5ºC. Big animals have less surface area (where heat is lost) relative to size, so some scientists believe that large reptiles maintain body temperatures through a phenomenon called gigantothermy.

Were dinosaurs warm-blooded?

Large ones are often depicted in films and documentaries as slow and lumbering beasts under the assumption that their physiology was similar to living reptiles. But based on growth rates (estimated from fossil bones), dinosaurs weren’t cold-blooded: their metabolic rates – and in turn their ability to generate heat – were halfway between those of ectotherms and endotherms. So instead, dinosaurs may have been mesotherms – what you might call ‘lukewarm-blooded’ vertebrates.