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Man and Wolf
The Process of Domestication

TODAY, almost 40 percent of households in the United States own at least one dog, comprising a total of more than 77 million dogs.1 In the year 2020, pet owners spent more than 40 billion dollars on food for their animals, and about 30 billion dollars on veterinary care.2 It is undeniable that the dog is a valued and important member of our society. Unlike any other nonhuman species, the dog has become fully integrated into our lives, and it appears that he is here to stay. So, what exactly was it that brought man and dog together so many years ago? And more important, what characteristics of these two very different species enabled them to forge the strong and ongoing partnership that is still so important to us today?

The Dog's Phylogeny (Evolutionary History)

The dog, like the cat, is a member of the order Carnivora, which includes a diverse group of animals that are all predatory in nature. Carnivores are so named because of their enlarged carnassial teeth. These include the enlarged upper fourth premolar and the lower first molar on each side of the mouth. These adaptations make the teeth efficient at shearing and tearing prey. All carnivores also have small, sharp incisors for holding prey, and they often have elongated canine teeth for stabbing and tearing.

During the time when dinosaurs dominated the earth, a group of animals called the miacids were evolving. The Miacidae family included a very diverse group of predatory mammals, many of whom were small, tree-dwelling animals. This group existed about 62 million years ago and formed the ancestral family for all members of the order Carnivora. The miacids all walked on the palms/soles of their feet (plantigrade), were long bodied and slim, and were the first animals with carnassial teeth-an indication of their predatory nature.

Over time, a group called the viveravines branched off from the miacids. The viveravines are now known to be the oldest ancestor of the domestic cat. A second branch that evolved from the miacids was the miacines. Animals in this group were the ancestors of all extant canid species, as well as the bear, raccoon, and weasel. The miacines existed about 60 million years ago and eventually gave rise to Hesperocyon (meaning western dog), who is designated as the oldest member of the Canidae family. Remains of Hesperocyon have been found in South Dakota, Nebraska, Colorado, and Wyoming and are estimated to have existed about 36-38 million years ago. Interestingly, current evidence indicates that the Canidae family evolved completely in North America and did not migrate into Eurasia until much later in its development. Hesperocyon was a digitigrade mammal (walking on its toes) and was long bodied and long legged, obviously adapted for speed. Its dentition (including the presence of carnassial teeth) and body structure showed it to be an agile predator.

By the end of the Oligocene period, about 23 million years ago, Hesperocyon had evolved into Leptocyon. Leptocyon is thought to be the most recent common ancestor of all of today's canids, although there is some controversy over this mammal's eventual fate.3 Some accounts claim that Leptocyon gave rise to Tomarctus, who became the wolf's and our dog's primary ancestor. Other records depict Tomarctus and Leptocyon as two separate branches of Hesperocyon. Regardless, it appears that Leptocyon, and probably Tomarctus, gave rise to the dominant group of canids in North America, who were destined to become all of our modern-day canid species.

The Dog's Taxonomy (Naming the Dog)

Today, the domestic dog is classified as a member of the Canidae family (Table 1.1). This family also includes the wolf, coyote, dingo, fox, jackal, and Cape hunting dog. The dog's genus is Canis, and its species is familiaris. Other members of Canis are the coyote (Canis latrans), two species of wolf (the gray or timber wolf, Canis lupus, and the red wolf, Canis rufus), and four species of jackal. The extreme regional variations that are observed in wolves all represent varieties (subspecies) of Canis lupus, rather than separate species. Twenty to thirty subspecies have been identified, several of which have become extinct in the last century. The genetic plasticity of the wolf as a species is illustrated by the great variation in physical and behavioral attributes in various subspecies. For example, Alaskan timber wolves (Canis lupus pambasileus) typically weigh more than 100 pounds at maturity and exist as well-organized packs consisting of an average of five to eight adults. In contrast, the small Asian wolf (Canis lupus pallipes) weighs only about 45-50 pounds and travels alone or in very small packs. For many years, there was scientific dispute over whether the red wolf (Canis rufus) should be classified as a separate species of wolf or as a subspecies. This question was finally put to rest in 2019 when a comprehensive study of red wolf genetics, anatomy, and behavior came to the conclusion that the red wolf is indeed a separate species (Canis rufus) and is currently protected under the Federal Endangered Species Act.4

Table 1.1 Taxonomy of the Dog.

There is similar dispute regarding the domestic dog. The immediate common wild ancestor of Canis familiaris continues to be the subject of some debate. At one time, it was believed that the dog was descended from the interbreeding of ancestral wolves, coyotes, jackals, and possibly other wild canids.5 During the 1940s, the Nobel Prize-winning ethologist Konrad Lorenz wrote that some breeds of dogs were descended from the golden jackal, whereas others, those that he called "lupus" breeds, were directly descended from the wolf.6 This theory has been largely discarded, however. During the 1970s, wolf and dog expert Michael Fox developed a "missing link" theory. He believed that the dog is descended from a now-extinct, European, dingo-like dog. However, little fossil evidence of this ancestor has been found. Yet another theory suggests that our present-day domestic dog arose from a type of semiwild dog similar to the Australian dingo (classified as either Canis lupus dingo or Canis familiaris dingo) and the New Guinea singing dog (classified as Canis familiaris hallstromi).

Current behavioral, morphological, and molecular biological (genetics) evidence supports the theory that today's gray wolf, Canis lupus, is the domestic dog's closest relative. Although it is often stated that the wild wolf is our domestic dog's immediate wild ancestor, in evolutionary terms this is impossible. More correctly, the present-day wolf and the present-day dog share their most recent ancestor, which was probably very wolflike in appearance and behavior. This distinction is important because the wolf that is extant today has been evolving for the same period that today's domestic dog has been evolving. Therefore, the present-day wolf is actually the present-day domestic dog's closest relative.

Overall, the most compelling evidence in recent years about how to accurately classify the domestic dog comes from analyzing genetic information. Mitochondrial DNA (mDNA) is genetic material that is passed from mothers to their offspring (in the ovum), with no genetic recombination. Analysis of mDNA allows the reconstruction of matrilineal histories and can provide an estimate of a species' evolutionary history. These studies have shown that although there are morphological and behavioral differences between wolves and dogs, from a genetic standpoint, the domestic dog is virtually identical to the other members of the Canis genus. In fact, there are greater mitochondrial DNA differences between some breeds of dogs than are found between dogs and wolves! This knowledge, coupled with the fact that dogs, wolves, coyotes, and jackals are still reproductively interfertile, provides strong evidence that there is very little phylogenic distance between these groups of canids.

Both dogs and wolves have 39 pairs of chromosomes (78 total), as is true for the four species of jackal and the coyote. Because of this very close genetic relatedness, some argue that the domestic dog should not be classified as a new species but, rather, as a subspecies of wolf (i.e., Canis lupus familiaris).6 Conversely, another criterion for species classification is adaptation to different ecological niches. Some biologists and ecologists, although accepting the close genetic relationship between the dog and the wolf, maintain that because dogs, wolves, coyotes, and jackals all adapted to occupy and thrive in very different ecological niches, they should each represent a separate species.7

Additional evidence for the dog's close relationship to the wolf lies in the existence of physical, genetic, and behavioral similarities between the two species. One of the most basic is the social nature of dogs and wolves. Both species readily establish and maintain social groups....