Dear Science,

Why does my dog's tail wag? When I was a kid, I was told dogs wag their tails when they're happy, but she really wags it a lot and all the time. No one can be that happy, not even a dog. Can they?

Barking

The answer lies in why there is a dog in your house at all. The genetic evidence tells us that dogs were the first domesticated creatures, around the time modern humans evolved. How did that happen? Trash. According to the in-vogue theory, dogs started as wolves with a hankering for the piles of refuse surrounding human settlements. Only after wolves tamed themselves could primitive humans selectively breed or train dogs. It's a beautiful idea, but difficult to prove. (A colleague sardonically quipped, upon hearing this theory, "Those primitive hominids sure took good notes, didn't they?")

Luckily for us, Siberian winters are long. Russian geneticist Dr. Dmitry Belyaev, presumably tired of being bit and bored, started a long-running breeding experiment with wild foxes. In the experimental group, only the foxes least afraid of or most likely to approach humans were allowed to breed. In the control, breeding was random. No big surprise, the selectively bred population became increasingly tame toward humans. More unexpectedly, the foxes also gained mottled hair, floppy ears, and curly tails—similar to dogs. The biggest treat: The tame foxes started to understand human social cues.

Dogs are the most humanlike creature, at least when it comes to social skills—closer than our genetic cousins chimpanzees and bonobos. Try to direct your dog to some hidden toy in your apartment just by looking or pointing. She'll have no trouble. Fourteen-month-old human children, well before language develops, also find the game easy. Chimps—smarter than dogs in most ways, but more volatile—fail miserably. Dogs come preprogrammed to recognize where a human is pointing, gazing, or nodding; a chimp must first learn each of these tricks. Such abilities require the temperament of a dog, something a chimp or a wolf lacks. Take a litter of wolf or dog pups and raise them the same way. The dogs will inherently respond to humans with much more grace than the wolves, even with the same training. One of the biggest differences: Wolves rarely wag their tails, doing it only when they're actively submitting; dogs cheerfully wag their tails all the time.

So why does your dog wag her tail? To show a deeply selected trait for benign temperament—a rare, socially lubricating spirit shared by humans and dogs. It makes Science wonder how our dispositions—relatively agreeable compared to those of chimps, and required for human-to-human interaction—were pulled by our canine companions.

Evolutionarily Yours, Science

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