Inside of a Dog_ What Dogs See, Smell, and Know - Alexandra Horowitz [20]
The original dogs were mongrels, in the sense that they didn't come from a controlled lineage. But many of the dogs we keep, mutts or not, emerged from hundreds of years of strictly controlled breeding. The consequence of this breeding is the creation of what are nearly subspecies, varying in shape, size, lifetime, temperament,* and skills. The outgoing Norwich terrier, ten inches high and ten pounds strong, is but the weight of the calm, sweet, enormous Newfoundland's head. Ask some dogs to retrieve a ball and you get a puzzled look; but a border collie doesn't need to be asked twice.
The familiar differences between modern breeds aren't always the result of intentional selection. Some behaviors and physical features are selected for—retrieving prey, smallness, a tightly curled tail—and some just come along for the ride. The biological reality of breeding is that the genes for traits and behaviors come in clusters. Mate a few generations of dogs with particularly long ears and you might find that they all share other characteristics: a strong neck, downcast eyes, fine jowls. Coursing dogs, bred to gallop swiftly or long, are leggy—their leg length matches (in the husky) or surpasses (in the greyhound) the depth of their chests. By contrast, dogs who track on the ground (as the dachshund) wound up with legs much shorter than their chest is deep. Similarly, selecting for one particular behavior inadvertently selects for accompanying behaviors. Breed dogs who are very sensitive to motion—who probably have an overabundance of rod photoreceptors in their retinae—and you may also get a dog whose acute sensitivity to motion leads to their being temperamentally high-strung. Their appearance might change, too: they may have large, globular eyes for seeing at night. Sometimes what comes to be desirable in a breed is a trait that first appeared inadvertently.
There is evidence of distinct dog breeds as early as five thousand years ago. In drawings from ancient Egypt, at least two kinds of dogs are depicted: mastiff-looking dogs, big of head and body, and slim dogs with curled tails.* The mastiffs may have been guard dogs; the slender dogs appear to have been hunting companions. And so the designing of dogs for particular purposes began—and continued along these lines for a long while. By the sixteenth century, there were added other hounds, bird dogs, terriers, and shepherds. By the nineteenth century, clubs and competitions sprouted, and the naming and monitoring of breeds exploded.
The various modern breeds have probably all emerged with this proliferation of breeding in the last four hundred years. The American Kennel Club now lists nearly one hundred fifty varieties, grouped according to the purported* occupation of the breed. Hunting companions are distributed into "sporting," "hound," "working," and "terrier" categories; there are, in addition, the working "herding" breeds, the plainly "nonsporting" breeds, and the rather self-explanatory "toys." Even among dogs bred to join the hunt, there are subdivisions, by the very kind of assistance they provide (pointers point out the prey; retrievers retrieve it; Afghan hounds exhaust it); by the specific prey they're after (terriers are ratters, and harriers go after hares); and by the medium preferred (beagles chase on land; spaniels will swim in water). Worldwide, there are hundreds more breeds still. Breeds vary not just by our uses of them but physically: by body size, head size, head shape, body shape, type of tail, coat kind, coat color. Go searching for a purebred dog and you'll confront a new-car-worthy list of specs,