Y: Today, on A Moment of Science, we ponder what makes Don different from your average snake.

D: Oh, that's a no-brainer. Snakes are cold-blooded, and I'm warm-blooded.

Y: Actually, Don, scientists prefer the terms endothermic and ectothermic. Snakes are ectothermic--they're dependent on their environment for heat. You, on the other hand, are endothermic--your body chemistry regulates your temperature and keeps it constant.

D: And that's good thing, too. 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, and we'd end up having to compete with snakes and other reptiles for the same resources.

Y: Right. But Don, do you know how our bodies manage to stay warm all the time?

D: Hmm! let's see! every chemical reaction gives off some heat as a byproduct. Maybe all the chemical reactions constantly going on inside of us give off enough heat to keep us warm!

Y: Good guess. But our being endothermic isn't a side effect of our metabolism. In fact, about sixty to eighty percent of the chemical activity that takes place inside our cells has no other purpose than to release heat.

D: So the heat isn't a byproduct, it's an end product.

Y: Right. And 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.