How come dogs bark, even when it's quiet? asks Heather Waslin, a student in Brookville, NY.Dogs fit their barks to the situation. A fire engine screams by in the distance? An alarmed bark. A skunk ambles by the fence? An interested, excited bark. And then there's that random, monotonous, I'm-lonely-out-here bark.

Dogs may growl at a threatening stranger, whine excitedly when a beloved person approaches the front door, or let out an excited short yip during a play tussle with another dog. But barking is an all-purpose vocalization.

Scientists who study dogs note that their closest wild relatives, the wolves, rarely bark as adults. Wolf puppies, on the other hand, bark a lot. Which is a clue, researchers say, as to how domesticated dogs became such enthusiastic barkers.

Human beings shouldn't complain so much about barking since, in a real sense, we're responsible for the racket. By 30,000 to 50,000 years ago, evidence shows that people often brought wild wolf puppies into their nomadic camps. Raised around people from a very young age, the grown-up wolves tended to be tamer than normal. And the friendlier and less fearful the wolf-dog, the more successful he was at coexisting with us, gaining access to tossed-out scraps of food.

As these tamer animals bred with each other, even more docile offspring were born. Dogs helped humans become better hunters, and were used to drag loads of belongings from camp to camp, and to guard settlements. Later, they were used to herd other animals, like sheep. And, of course, dogs also became beloved companions.

But why all the barking? Most baby animals (think bear cubs) are more docile and people-friendly than their (scary) grown-up parents. Scientists who bred extra-docile foxes discovered that friendlier foxes sounded less like foxes, more like dogs. So as wolf-dogs evolved into the equivalent of overgrown puppies, they were more friendly. More playful. And way more barky.

Barks convey both emotional states and other information, and vary in pitch, loudness, and timing. In one study, scientists recorded dogs barking, then played the barks for other dogs. Dogs reacted strongly to "oh-oh, a stranger!" barks, but paid much less attention to "I'm all by myself!" barks.

And after tens of thousands of years, we humans understand our doggy friends pretty well, too. One study found that even 6-month-old infants could match a friendly or angry bark to pictures of a happy or threatening dog.

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