MONTAGUE, Mass. — Think of all the dogs out there: labradors and poodles and labradoodles; huskies and westies and dogues de Bordeaux; pit bulls and spaniels and lovable mutts that go to doggy day care.

Add them up, all the pet dogs on the planet, and you get about 250 million.

But there are about a billion dogs on Earth, according to some estimates. The other 750 million don’t have flea collars. And they certainly don’t have humans who take them for walks and pick up their feces. They are called village dogs, street dogs and free-breeding dogs, among other things, and they haunt the garbage dumps and neighborhoods of most of the world.

In their new book, “What Is a Dog?,” Raymond and Lorna Coppinger argue that if you really want to understand the nature of dogs, you need to know these other animals. The vast majority are not strays or lost pets, the Coppingers say, but rather superbly adapted scavengers — the closest living things to the dogs that first emerged thousands of years ago.

Other scientists disagree about the genetics of the dogs, but acknowledge that three-quarters of a billion dogs are well worth studying.