The Order Carnivora

The domestic dog, gray and red wolf, coyote, dingo, jackals, dhole, and others are interfertile and frequently cross in their native habitats all over the world. They have been assigned to various genera over the years depending on the criteria used and will probably continue to be a problem for systematists for many years. It is not possible to assign the domestic dog with certainty to any one progenitor, but it is clear that the dog and the wolf are closely related and evolved together.

The order Carnivora is worldwide in distribution and is an assemblage of intelligent, mostly flesh-eating mammals with prominent canine teeth; molars adapted for crushing, cutting, and grinding; and a relatively short alimentary canal. Members of the order have digits provided with claws and sometimes with webs. Behavioral characteristics identify most of them as predators with strong family ties, devoted to the care of their young. Many of the species adapt readily to domestication (Clutton-Brock & Jewell, 1993; Ewer, 1973).

Since the time of Linneaus the levels of classification have become more accurate and useful and we can now also consider genomic evidence for relationships of animals.

Wayne and Ostrander (2007) have investigated the dog genome sequence and associated genomic resources and said that these studies “will revolutionize the study of dog evolution, population structure and genetics.” They presented a chart of canid relationships that shows the major groupings of species. A more accurate phylogeny of the Dog family, Canidae, is in the making and molecular markers that identify gene pools now allow the assignment of mixed “breeds” to specific gene pools. The WISDOM Panel MX of Mars Veterinary, Inc., has a deoxyribonucleic acid (DNA)–based test that can identify 134 American Kennel Club (AKC)–registered breeds that may be present in a mixed breed dog (Giger et al., 2007).

The most complete treatment of fossil and living families and genera of mammals can be found in Classification of Mammals above the Species Level, by Malcolm McKenna and Susan Bell (1997) with contributions from George G. Simpson. This classification project was begun by Simpson (1945) in 1927 at the American Museum of Natural History in New York City. Many people cooperated in assembling this tome, which includes all mammals of the world. The number of extinct taxa is much greater than living forms and many fossil animals are known only from a few bones or teeth.

In a more recent compilation of species, Wozencraft (2005) revised the Order Carnivora in Wilson and Reeder (2005) Mammalian Species of the World. He raised the Giant Panda, the Madagascan Carnivores, the African Palm Civet, the Skunks, and the Walrus each to family rank.

This pictorial family tree (Fig. 1-1) is an approximation of how animals in the Order Carnivora are related to each other according to the most recent classification. The major divisions are the Feliformia (cats) and the Caniformia (dogs), which may have arisen from Miacid stock between the Eocene and the Paleocene approximately 40 to 60 million years ago. This time frame witnessed the evolution of modern mammals and birds. Every year new fossils are found (as well as extant forms) and we are able to improve our understanding of animal phylogeny. For photographs of various mammals see Nowak (1991).

The Family Canidae

The family Canidae is a distinct group of dog- and foxlike animals distributed throughout the world (Gittleman, 1989; Mivart, 1890; Sheldon, 1992). Although there is general agreement about which genera are included in the family, there has been disagreement about the generic status of several species and their groupings into subfamilies. Clutton-Brock et al. (1976) reviewed the family Canidae using numeric methods and suggested a classification at the generic level based on 90 characters of the skeleton, pelage, internal anatomy, and behavior. Their results indicated that the three largest genera—Canis (dog, wolf, coyote, jackal), Vulpes (foxes), and Dusicyon (South American foxes and foxlike animals)—all are closely related but merit the separate designations they now have.

Wayne (1993) and Wayne and colleagues (1989, 1999, 2007) have investigated the evolutionary relationships of carnivores using karyologic and molecular procedures. Their techniques, such as DNA hybridization, protein electrophoresis, albumin immunologic distance, and high-resolution G-banding of karyotypes, have helped define relationships. Our current understanding of the taxonomy of carnivores and dogs, in particular, is discussed by Wayne and Ostrander (2007) and Boyko et al (2010).

Behavior is a feature that is sometimes characteristic of a breed. Takeuchi and Mori (2006) compared the behavior of purebred dogs in Japan with those in the United States and the United Kingdom. Stockard (1941) also took note of behavior in the purebred crosses he was producing.

Hybrids

Because all dogs are interfertile, many intermediate physiognomies are not assignable to any particular breed (Stockard, 1941; Van Gelder, 1977). Even first-generation hybrids of purebred parents may exhibit bizarre combinations of body characteristics (Figs. 1-2 and 1-3).

Differences in the conformation of the body or its parts are common and are considered as malformation only when extreme. Susceptibility of the developing embryo or fetus to malformation varies with developmental stage or age and may affect only one part or organ of the body. Malformations may be due to genetic, endocrine, metabolic, infection or mutagenic influences.

Charles R. Stockard of the Cornell Medical College undertook an investigation, starting about 1926, of the genetic and endocrinic basis for differences in form and behavior in dogs. He made crosses between many purebred dogs of various breeds and their F₁ and F₂ generations to study the physiognomy, skeleton, and endocrine organs of the resulting litters. Unfortunately, like Malcolm E. Miller, who started “Anatomy of the Dog,” Stockard died before his work was completed, but from his extensive notes, photographs, and specimens his colleagues were able to publish many of his findings as a monograph of the Wistar Institute (Stockard, 1941). What remains of the records and the extensive skeletal collection is now housed and cataloged in the Museum of Natural History on the campus of the University of Georgia in Athens and can be examined by appointment. Stockard’s monumental study has left a legacy of numerical comparisons of body and skeletal parts, documented photographs of parents and hybrids, and many disarticulated skeletons. The purebred crosses he produced raise many questions as to the interactions of a large number of breed factors on external features and deformations of the skeleton.

Cartilage growth distortion (achondroplasia) in the skeleton may be restricted to the appendicular or the axial skeleton, or it may affect both. For example, the lower jaw is developmentally independent of the upper jaw. Thus brachygnathic, prognathic, and normal-muzzled pups may appear in the same litter. Such a disharmony between the upper and the lower jaws usually results in malocclusion, difficulty in eating, uneven wear, and loss of teeth.

In many instances Stockard was able to produce and examine F₂ generations from back-crosses to parent stocks. These back-crosses indicated that reversions were common and that the head and tail ends of the axial skeleton often act independently. For example, the very extreme type of Bulldog head may be present in a dog with a long straight tail, or a dog with a normal long muzzle may have a short screw-tail. All F₁ hybrids of the screw-tailed Bulldog X straight-tailed Basset Hound develop long, straight tails. However, in the F₂ generation the tail deformity reappears in several different patterns.

Dachshund-Pekingese hybrids are good examples of genetic influences on variations in skull form (see Figs. 1-2 and 1-3). The Pekingese dog has a greatly reduced muzzle and a bulging forehead. This is probably a result of achondroplasia of the basicranium and the cartilaginous forerunner of the upper jaws, a condition similar to that seen in the Bulldog. The Asiatic origin of the Pekingese suggests descent from Chow-Chow ancestors, whereas the Bulldog was developed at a later date from European stock. Thus very different breeds of dog can have very similar mutations in various parts of the world, and, if selected, these mutations can be enhanced.

Because the dominant factor for achondroplasia in the appendicular skeleton results in a more severe shortening of the bones in Bulldog types than that in Hounds, Stockard suggested that the “breed quality” of bone may depend on a large number of factors in the genetic constitution of the animal. Thus “Bulldog bone” and “Hound bone” may be more complex entities than we currently imagine.

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