Cold-blooded animals are those that cannot generate significant body heat on their own. Instead, their body temperature rises and falls with the temperature of their surroundings. Reptiles, amphibians, fish, and invertebrates are all cold-blooded. The term is a bit misleading, though, because these animals don’t actually have cold blood. A lizard basking on a sun-baked rock can have blood just as warm as yours.
Why Scientists Prefer “Ectotherm”
The scientific term for a cold-blooded animal is ectotherm, meaning “outside heat.” The distinction matters because a cold-blooded animal’s blood isn’t inherently cold. It’s simply heated from external sources rather than internal ones. Some desert lizards in the South American genus Liolaemus can raise their body temperature 30°C (54°F) above the surrounding air temperature just by angling their bodies toward the sun. That’s not cold at all.
There’s actually a common misconception that cold-blooded animals struggle mainly to stay warm. In reality, research published in the Proceedings of the National Academy of Sciences found that for most ectotherms on Earth, especially the enormous diversity of species living in tropical and desert regions, the bigger challenge is staying cool. The label “cold-blooded” reflects a bias toward thinking about animals in cold climates, when most ectotherm species actually live where overheating is the greater risk.
How Cold-Blooded Bodies Work
Your body burns calories constantly to maintain a temperature around 37°C (98.6°F), whether you’re sleeping or running. A cold-blooded animal doesn’t do this. Its metabolism runs at a fraction of the rate, roughly one-tenth or less compared to a similarly sized warm-blooded animal. This means a snake or a frog needs far fewer calories to survive. A lizard can go weeks between meals, while a mouse-sized mammal would starve in days.
That energy efficiency comes with trade-offs. Because ectotherms channel so little energy into generating heat, a much larger share of their caloric intake goes toward growth and reproduction. Fish, for example, spend about 35% of their total metabolizable energy on producing offspring. Birds and mammals spend only 2% to 6% on reproduction, with the rest consumed by the metabolic furnace that keeps their bodies warm.
How They Control Their Temperature
Cold-blooded animals aren’t passive victims of the weather. They use sophisticated behavioral strategies to manage their body temperature throughout the day.
- Basking (heliothermy): Absorbing heat directly from sunlight. Lizards, turtles, and snakes position themselves in sun-exposed spots, sometimes flattening their bodies to maximize the surface area catching solar radiation.
- Contact warming (thigmothermy): Pressing against warm surfaces like sun-heated rocks, roads, or soil. Snakes stretched across pavement at dusk are using heat stored in the asphalt.
- Shuttling: Moving back and forth between sunny and shaded areas to fine-tune body temperature, much like adjusting a thermostat.
- Protected basking: Some nocturnal species tuck into crevices or vegetation gaps during the day where sunlight still reaches them, absorbing heat while remaining hidden from predators. They then emerge at night with enough stored warmth to hunt.
These behaviors can be remarkably precise. Many lizard species maintain body temperatures within a narrow preferred range for hours, despite wide swings in air temperature around them.
What Happens When It Gets Too Cold
Temperature governs the speed of chemical reactions in every living cell. For a cold-blooded animal, dropping temperatures don’t just make movement sluggish; they slow digestion, immune function, and reproduction. Atlantic salmon, for instance, stop eating when water temperatures fall below about 7°C (45°F), well before the cold would actually kill them. Some fish species, like the cunner, become fully dormant in winter, shutting down digestive enzymes and barely moving.
Reptiles and amphibians in temperate climates enter a state called brumation, their version of hibernation. During brumation, heart rate, metabolism, and energy demands all drop dramatically. The animal tucks into a burrow, rock crevice, or pond bottom and essentially waits out the winter. This isn’t just the body passively slowing with the cold. Research shows that ectotherms actively suppress their metabolic processes beyond what temperature alone would cause, deliberately conserving energy stores to survive months without food.
Surviving Freezing Temperatures
Some cold-blooded animals take cold survival to an extreme. The wood frog can survive being literally frozen solid. Ice forms in its body cavity, between its cells, and its heart stops beating. It shows no brain activity. By every outward measure, it appears dead.
The trick is chemical. As freezing begins, the wood frog floods its cells with glucose and urea, which act as natural antifreeze. These molecules stabilize cell membranes and proteins, preventing the kind of ice crystal damage that would destroy tissue. At the same time, the frog shuts down key metabolic enzymes to minimize energy consumption and protect cells from the damage that comes with losing blood flow. When temperatures rise in spring, the frog thaws, its heart restarts, and it hops away. Researchers have found that this process involves upregulation of mitochondrial genes and enhanced antioxidant defenses to handle the stress of blood suddenly flowing again through previously frozen tissue.
Which Animals Are Cold-Blooded
Among vertebrates, the cold-blooded group includes fish, amphibians (frogs, salamanders, newts), and reptiles (snakes, lizards, turtles, crocodilians). All invertebrates, from insects to spiders to crabs, are also ectothermic. This makes cold-blooded animals the overwhelming majority of animal species on Earth. Warm-blooded animals, birds and mammals, are the exception rather than the rule.
The line isn’t perfectly clean, though. Some fish, like the great white shark and bluefin tuna, can warm specific body regions above water temperature using heat generated by their muscles. These species blur the boundary between cold-blooded and warm-blooded, which is one more reason scientists moved away from those everyday terms. Biology tends to operate on a spectrum, and body temperature regulation is no different.