The ancestor of today’s canids arose in North America 8 to 12 million years ago and looked like a fox. Within a million years, that animal crossed the Bering land bridge into Europe and diverged into a number of new species, including the gray wolf, which later migrated back into North America. Some 3 million years ago the first ancestral dogs and cats reached South America by way of the Isthmus of Panama. Finding a paradise full of prey and free of competing predators, the canid invaders soon radiated into several new species, each adapted to South America’s unique habitats. Alone, the voracious, diminutive bushdog of today’s Amazon rainforest brings down pacas larger than itself. The maned wolf is often called the “fox-on-stilts” because of the disproportionately long legs that allow it to peer over the tall grass of the pampas. The hoary fox and the crab-eating fox also emerged at this time.
The 35 species of living canid include the gray wolf (the largest), the coyote, the jackal, the African wild dog, the Ethiopian wolf, the fox, the dhole and the raccoon dog. Although so closely related to the gray wolf as to be the same species, we continue to call the domestic dog, Canis familiaris, a separate species because it is nearly always reproductively isolated, is subject to artificial selection and only exists in the wild as a feral animal, like the dingoes of Australia.
Dog breeds, however, are not considered separate subspecies because without human interference they freely interbreed, within size constraints. Most breeds were created during the past 200 years, not nearly enough time to cause a significant genetic divergence. Artificial selection by humans only involves concentrating traits already present in the wolf/dog genome—morphological characteristics like size and coat color, and behavioral features like herding, hunting, retrieving or guarding. In addition, certain types of genetic exoticism—dwarfism, giantism, and neoteny, or the retention of juvenile features into adulthood—have been fixed through selective breeding.
What Makes Diversity Possible
The genetic, dietary, physiological, behavioral and social flexibility of canids, combined with a relatively unspecialized dental structure that allows them to be generalist eaters, has made them adaptable to different habitats and to human societies.
Canids cover much of the dietary field, from the highly carnivorous wolf, dhole and African wild dog to the insectivorous bat-eared fox and the largely frugivorous raccoon dog of Japan. There are also generalist munchers, like foxes, coyotes and, of course, dogs. Yet even the wolf will eat grass and fruit, though perhaps not as much as dogs, and feed on prey ranging from moose to mice.
Canids can do that because, on the whole, their teeth have not evolved to perform specialized tasks, like crushing heavy bones (in the hyena) or grinding grasses and grains (in herbivores). Canids have in addition to the distinctive canines, carnassial teeth for shearing and molars for crushing—a sort of broad-based dentition that allows them to consume a wide variety of foods. In domestic dogs, tooth size is reduced, as are jaw strength and overall relative size, probably as a result of relaxed selection for these attributes relative to wild canids. Still the dog’s varied and sometimes eccentric culinary choices reflect the ability of canids to adapt to the available food.
Nearly all canids form pair bonds. Larger species, like wolves, African wild dogs and dholes, hunt cooperatively, while many of the smaller fox-size canids do not. However, packs may form even in relatively antisocial species when there is a resource like a garbage dump or when prey is too large for a single individual; thus, coyotes tend to form packs and hunt cooperatively when deer are abundant. In other cases, coyotes may be nearly solitary and not form pair bonds, for example in tight quarters of cities, where prey is small and extended families are not practical. Pack size also varies among wolves, depending on the food base. When deer-size prey or larger is scarce, wolves tend not to form packs. This kind of social flexibility is key to surviving in diverse and changing environments.