HOW COME? Cold weather makes us shiver

How come your teeth chatter and your body shivers in the cold? Also, how come fingers wrinkly when wet? - asks reader Matthew Levine.

If you've ever suffered through a bout of uncontrollable teeth chattering, you're in good company: Shivering and shaking are simply side effects of being a warm-blooded human (or a dog, cat or rabbit) in winter.

The body begins shivering when its core temperature of around 99 degrees begins dropping in the cold. A shivering center in the brain receives signals of COLD COLD COLD! from the skin and spinal cord. And we begin to vibrate.

Shivering happens when body muscles begin contracting and releasing. Since the muscles are working, they generate heat. Usually, the heat created by working muscles is wasted. On a warm day, the body tries to rid itself of the unwanted heat by sweating.

But with shivering, the heat is the whole point, helping the body maintain its core temperature in chilly air. Teeth chatter, too, as jaw muscles join in the tiny shaking motions.

We also shiver when we have a fever. With an infection to fight off, the brain's hypothalamus may reset our temperature to, say, 101 degrees F. The hypothalamus sends a message to the 98.6 degree body that it's actually too cold. Shivering, teeth-chattering, and chills follow, at least until your temperature rises to the new setpoint.

Many (cold-blooded) insects also shiver to warm up. When a bumblebee rests, its body temperature drops or rises to that of its surroundings. But even on a chilly May morning, a bumblebee's wing muscles must register a toasty 86 degrees F. before she can lift into the air.

Basking in the sun may not do the trick, so a bee shivers her way to a higher temperature. By rhythmically contracting and releasing her abdominal muscles - faster and faster as her temperature rises - a bee generates enough heat to take flight.

We can cure our shivering by taking a hot bath. But we may also emerge with weird, prune-y fingers. Blame the wrinkling on skin structure. The skin's outer layer, the epidermis, is attached to the thicker dermis underneath. However, there's some "give" between the two.

Submerge your hands in warm water for awhile, and the oil that protects the skin - and makes it a bit waterproof - washes away. Since the undersides of fingers and toes are hairless, they don't have much oil to begin with. As the oil washes away, the extra-thick skin on hands and feet becomes more permeable. So while you relax in the tub, the keratin protein in the epidermis is busily soaking up 6 to 10 times its own weight in water.

What happens then, according to the latest theories? The extra water alters the skin's balance of electrolytes (such as sodium and potassium). This affects the functioning of nerve fibers, which in turn cause blood vessels leading to fingers and toes to narrow. As the vessels shrink, the negative pressure tugs down the plumped-up epidermis. Presto: ridges and furrows.