The cat family and the hyena family similarly evolved hypercarnivorous top predators. (One might think the bear family, the ursids, should be added to this list, but only the polar bear is hypercarnivorous, and it is a rather atypical member of the family. Most bears are omnivores.) It’s only a slight oversimplification to say that felids almost invariably approach their prey in stealth and try to pounce on it in surprise attacks. Modern canids, by contrast, have a decidedly different tactic, one suited to their ancestors’ lifestyle on the open plains. In that setting, surprise attack is seldom achieved; it is less important to subdue the prey in the shortest possible time than to outrun and exhaust the quarry. Lacking retractable claws, a powerful weapon for most felids, canids rely more on social hunting when confronting large prey—using sheer numbers and coordinated hunting strategies rather than sophisticated weaponry to overwhelm them.
Hyaenids are more closely related to cats, yet they more strongly resemble canids, both behaviorally and anatomically. They kill their prey by consuming them alive, rather than by delivering a killing bite on the neck as felids do. They too are persistent pursuers rather than stalkers that ambush prey, and they tend to be highly social hunters. The similarities are a good example of convergent evolution, an understandable outcome when one realizes that for much of their evolutionary history, the two groups were not direct competitors but were facing similarly open environments. Canids were at first confined to North America, whereas hyaenids arose in Eurasia.
When did canids become so diverse? From Hesperocyon, a descendant of Prohesperocyon, the family experienced its initial radiation in the early Oligocene, about 34 million years ago, splitting into three major subfamilies: the Hesperocyoninae and the Borophaginae (both extinct lineages known only from fossils), and the Caninae, whose descendants survive today. But it is at first only among the hesperocyonines that we see some really dominant dogs, capable of hunting prey larger than themselves. They were the size of small wolves and equipped with teeth specialized for ripping into raw meat, comparable to those of modern African hunting dogs. The early borophagines, on the other hand, were all smaller and tended toward less predatory lifestyles. And biding its time was the Caninae subfamily, comprising only a few inconspicuous species (we’ll avoid calling them “canines,” a term that is usually used in a narrower sense).
Altogether, by about 30 million to 28 million years ago, twenty-five species of canids roamed western North America, a peak of diversity within a continent unequaled before or since by any single family of carnivoran. The dog family was making its mark. Meanwhile, the hyaenodonts and other archaic predators had begun to decline, and they were eventually overtaken by the successful carnivorans.
North American herbivores, the potential prey for canids, steadily diversified during the first half of the following epoch, the Miocene, which lasted from 23 million to 5 million years ago. That was thanks not only to evolution but also to immigration of Eurasian native species via land bridges. The herbivores reached an all-time peak of diversity around 15 million years ago, and perhaps not coincidentally, canids experienced a second peak of diversity (some twenty species) at the same time. But now mostly the borophagines were the ones to flourish. The hesperocyonines were on the verge of extinction, while the Caninae continued to keep a low profile.
Among the factors driving canid evolution was the increasing speed of the grazing herbivores, which in turn was a response to being preyed upon in open habitats. The well-known illustration of this process is how members of the horse family essentially came to run on the tips of their toes, evolving longer toe bones and eventually losing their lateral digits. Even though canids were getting faster, they also had to adjust to competition from new carnivoran immigrants, including members of the cat family; false saber-tooth cats (which were catlike but not true felids); large mustelids; and giant bear dogs (family Amphicyonidae). Bone-cracking became a specialty of the new borophagine species that arose at the time, suggesting that they regularly scavenged carcasses—a kind of resource that is easier to locate in a more open environment. The ability to consume bones may have arisen as a byproduct of group feeding among social predators, in which individuals, trying to consume as much food as quickly as possible, ate bone (or swallowed meat plus bones indiscriminately).