How Come: It's no sweat for a hot dog to cool off

glands. As sweat glands pump out water, it turns into a vapor and wafts off into the air. Sweating cools us because the work of evaporating water - changing it from liquid to gas - requires heat energy. And that heat comes from our bodies.

But dogs have a tougher time keeping cool in hot weather. Dogs have very few sweat glands designed to dissipate body heat - just a sprinkling on the nose and footpads. A dog can radiate some heat through dilated blood vessels under the skin of his face and ears. But to vent most excess heat, the ancestors of domesticated dogs evolved panting. How does a lolling tongue and fast breathing help a dog chill out? Like sweat disappearing from skin, saliva evaporates into the air from a dog's tongue and mouth. Heat energy is lost, and cooled blood flows from the mouth's blood vessels to the rest of the body.

But it's not just a dog's mouth cavity that provides the cooling. The entire upper respiratory tract and lining of the lungs also lose moisture as a dog pants. Dogs avoid hyperventilating by breathing shallowly during panting, and by alternating bouts of fast panting with slow, normal breaths.

And it's not just our doggy companions that pant. Our dogs' closest wild relatives, wolves, also pant to cool down. Likewise, a sleepy wolf is a circling-in-place wolf. Scientists call the before-bed turning a "modal action pattern" - a behavior shared by a whole species. Dogs and wolves are part of the same species (Canis lupus), and share most behaviors, such as marking territory with urine, or burying tasty bones.

So wolves make a comfy bed in tall grass by turning and trampling it down, scaring off stray mice and toads. (They may also be sniffing the wind in every direction for threats.) And domestic dogs turn circles on the carpet (which doesn't really need flattening), or paw a blanket up into a cozy nest. Circling may also help both animals curl up into a compact ball.

Outdoors, both wolves and dogs carve out bowls in the dirt on hot days, unearthing cooling mud to nap on. Indoors, your dog may simply scratch at the rug before snoozing.