Body heat
Reference: Thermoregulation
Generate heat internally and pay for it with food, or take it from the surroundings and pay for it with dependence on the weather. Both were arrived at more than once.
The trade, stated as a bill
Endotherms — birds and mammals — hold a constant internal temperature and can therefore be active at night, in winter, and at altitude. The cost is enormous: a mammal needs roughly ten times the food of a reptile of the same mass, and most of it is burned purely to stay warm.
Ectotherms — reptiles, amphibians, fish, invertebrates — take heat from outside and regulate it behaviourally, by moving into sun or shade. The saving is so large that a crocodile can go months between meals. The cost is that a cold reptile is a slow reptile, and the weather decides.
Birds and mammals reached endothermy separately, which puts it alongside wings, the camera eye and the four-chambered heart on the list of answers found more than once.
The boundary is not clean, and the catalogue should not pretend otherwise: tuna and some sharks keep their swimming muscle well above sea temperature, and a brooding python raises its own temperature by shivering.
Every creature on this axis
| Creature | Body heat |
|---|---|
| Horse | Endotherm |
| Bat | Endotherm |
| Whale | Endotherm |
| Eagle | Endotherm |
| Shark | Ectotherm (some partly warm) |
| Octopus | Ectotherm |
| Dragonfly | Ectotherm |
| Frog and salamander | Ectotherm |
| Snake | Ectotherm |
| Oak | Ambient |
| Dragon Traditional | Unspecified; often described as hot |
| Pegasus Traditional | Endotherm |
| Griffin Traditional | Endotherm |
| Centaur Traditional | Endotherm |
| Sphinx Traditional | Endotherm |
| Phoenix Traditional | Endotherm |
| Mermaid Traditional | Endotherm implied |