Why do we shiver when we get cold?

Shivering is the cold-weather equivalent of sweating. We sweat to cool off; we shiver to warm up. When our body runs out of fuel and energy out in the cold, it tries to produce more internal heat through the involuntary muscle movement we know as shivering. Moving our muscles burns calories and generates heat, which is why we get so warm when we exercise. Shivering, however, is pretty inefficient in us humans. It does a little bit of good, but mostly it is a sign that we should get in from the cold. You may notice some birds shivering quite a lot. In some species, shivering is their main defense against the cold—and it works for them.