You probably already know that humans are warm-blooded, while creatures like snakes are cold-blooded. Scientists prefer the terms endothermic and ectothermic. Snakes are ectothermic–they’re dependent on their environment for heat. Humans, on the other hand, are endothermic–our body chemistry regulates our temperature and keeps it constant.
Chemical reactions like the ones that enable us to move our muscles run slowly when it’s cold and more quickly when it’s warm. If we were forced to live at the temperature of the world around us, we would lose the advantage of being able to function at night and in lots of different habitats. Instead, every time it got cold, we’d become sluggish and less able to fend for ourselves; we’d end up having to compete with snakes and other reptiles for the same resources.
You might be wondering how our bodies manage to stay warm. About sixty to eighty percent of the chemical activity that takes place inside our cells has no other purpose than to release heat. The cost of all that activity is high–that’s why endothermic animals need more food to keep functioning than ectothermic animals of the same weight.