Dogs, also called canids, have large canine teeth, long muzzles, and blunt, nonretractable claws; all but the African hunting dog have five toes on the forefeet and four toes on the hind feet. They vary in size from the tiny desert-dwelling fennec, which weighs 1.5 kg (3.2 lb), to the gray or timber wolf, which weighs 20 to 80 kg (44 to 175 lb). Canids are found throughout all the continents except Antarctica and in nearly every climatic zone; the arctic fox lives on ice floes and above timberlines, while the rare bush dog inhabits savannas of equatorial South America.

Canids show remarkable genetic plasticity; that is, strains or varieties of species rapidly adapt their size and other physical characteristics to different climates and habitats. The Australian dingo has probably adapted twice, first to become domesticated in prehistoric Asia and then to become a wild species in Australia when it was introduced there about 10,000 years ago.

General Behavior

Canids have highly developed senses of smell and hearing, enabling them to hunt by night as well as in the daytime. They work territories, usually at a tireless trot or canter, breaking into a gallop to pursue prey. The long red fox leaps into the air to spot mice, then pounces on them. Canids are essentially carnivorous, but many species will also eat fruit and other vegetable matter to tide them through prey-scarce periods. They feed mainly on mice, voles, and larger rodents, especially rabbits, and will also eat large insects and carrion. Large canids also prey on hoofed animals, such as antelope, caribou, and deer.

Some species of canids work primarily in relays or packs. Relay hunters, such as the African hunting dog, take turns running down hoofed animals. Wolves and Indian dholes hunt in packs. Dhole packs, which consist of several families and number up to 30 members, have been known to take on tigers and Himalayan bears. Few if any reports exist of unprovoked wolf or dhole attacks on humans.

Canids have territories, or home ranges, which they mark off by urine or scent posts. The range of the gray wolf varies from 18 to 13,000 sq km (7 to 5000 sq mi), whereas that of a fox may be only 5 to 50 sq km (2 to 20 sq mi). In more solitary canids such as foxes, the range may depend on gender, age, and food availability.

Barks, growls, yelps, whimpers, and howls are associated with different behaviors such as greeting, submission, play, or courtship. The raising or lowering of the ears or tail and the ruffling the neck hairs are also forms of communication among canids.

Reproduction

The gestation period of most canids ranges from 50 to 70 days and occurs once a year. Canids produce from 2 to 13 offspring, which usually are reared in a burrow. The pups are born blind, and those of some foxes are suckled for as long as ten weeks. Canids become sexually mature in one or two years.

Evolution

Members of the dog family, like other carnivores, evolved from a genet-like, tree-climbing carnivorous mammal of the Eocene Epoch (55 million to 38 million years ago). Although dogs were once grouped with carnivores such as the bears and raccoons, they are now considered to be more closely linked in their evolution with cats. During the early Oligocene Epoch (34 million to 24 million years ago), as many as 50 dog-like animals appeared, with good running legs and well-developed, blunt-clawed toes. They were probably the most successful line of the carnivores, and remained so until about 1.6 million years ago. The first wolves and foxes appeared at about the upper Pliocene Epoch (5 million to 1.6 million years ago). The African hunting dog, the dhole, and the South American bush dog probably developed along separate lines that branched off from the Eocene Epoch dog at about the same time as hyenas.

Ecological Value

Until recent decades, most wild canids were hunted for bounties and considered scourges to livestock and other wildlife. The elimination of coyotes and foxes from some areas, however, has resulted in huge increases in rodent populations that eat vegetation intended for desirable livestock and wildlife. Because wolves, coyotes, and foxes usually eliminate the weaker, less adaptable members of rodent and ungulate (hoofed-mammal) species, some wildlife biologists consider these predators necessary to maintain the genetic strength of such prey species.

Dogs and Wolves

The eight species of dogs and wolves include the well-known gray wolf. This animal, which formerly ranged throughout Europe, Asia, and North America, has been driven in reduced numbers to wilderness areas of these continents. The red wolf, once found from Pennsylvania to Texas and Florida, is now an endangered species. The coyote, on the other hand, has expanded its population into the eastern United States and is now found in every state except Hawaii. Three species of jackal, also a true canid, primarily inhabit Africa. Until recently the jackal was thought to be an ancestor of the domestic dog, but because the jackal is less social than the domestic dog, has a narrower, fox-like head, and howls unlike any domestic dog, this possibility is now doubted. The gray wolf appears to be the only ancestor of the modern domestic dog.

Foxes

The red fox is the best known of the 21 fox species. Red foxes can live in close proximity to humans, and their wariness and keen senses of sight, smell, and hearing have earned them the reputation of cleverness. They have good endurance and run gracefully, reaching a speed of 48 km/h (30 mph). Foxes emit a strong "foxy" odor from the caudal glands near the tail. When seen in strong light, the pupils of their eyes are elliptical, or oval shaped; those of wolves are round.

Other Canids

The raccoon dog, named for its masked facial markings, lives in eastern Siberia, Japan, and parts of China. A long-haired, bushy-tailed animal, it is one of the least dog-like canids. In northern climates it hibernates through much of the winter.

The maned wolf of Brazil and northern Argentina looks like a stilt-legged red fox with an erect mane on the back of the neck and top of the shoulders. Although it is close to the height of the gray wolf, it has a much slighter build and is not a true wolf. Maned wolves are nocturnal and usually solitary; they are not swift-running, and stalk then pounce on prey.

By contrast the South American bush dog is a stocky, short-legged, short-eared, and short-tailed canid that lives in forests and savannas. Bush dogs hunt by night in packs of ten or less and have been observed pursuing pacas (large South American rodents) into water. One bush dog in captivity acted much like a domestic dog except that it could dive and even swim under water.

The Falkland Island dog, now extinct, was reportedly an unwary and curious animal, which followed early explorers through these islands off the southern coast of Argentina. When the islands were settled, the dogs were killed for fur or to protect sheep herds, and by 1876 they had been exterminated.