No observer can help but remark upon the incredible variety of sizes, shapes, temperaments and behaviors of the dog—from the one-pound Chihuahua to the 200-pound Mastiff; the stubby-legged, placid Basset Hound to the long-legged, fleet Greyhound. Nowhere else in the animal kingdom does so much morphological diversity exist within a single species.
In large measure, the 400 or so breeds of dog extant today are products of human breeders, who, as Charles Darwin pointed out nearly 150 years ago, have selected consciously and unconsciously for specific physical and behavioral traits. But dogs are also the products of 40 million years of canid evolution through natural selection. The forces of evolution created the unique physiological and behavioral characteristics— the senses, physical abilities, social and individual behaviors and brains—that made the wolf the ideal progenitor of the dog. Those attributes resonate in human myths from around the world that ascribe to canids wild and domestic central roles in the creation of humans, guarding the dead or guiding them to the afterlife and serving as intermediaries between humans and nature.
Various legends of the dog as a fell beast and spreader of violence and disease are also widespread, reflecting a less exalted place in human affairs. But it is the diversity and malleability of canid characteristics that have made dogs indispensable allies of humans for more than 100,000 years—longer than any other domestic animal.
From an evolutionary perspective, the diversity found in the domestic dog echoes trends in wild canid evolution, albeit on a much different time scale. Rising from a common ancestor, new species of wild canids spread over millions of years into nearly every type of habitat on Earth. Today, 35 species of canids are found on every continent but Antarctica. However, centuries of persecution to protect domestic livestock and harvest furs and trophies, along with habitat destruction, have brought several of those species, including the African wild dog and the little Ethiopian wolf, close to extinction. Other species, like the gray wolf, have been extirpated from much of their historic range. Yet the dog, the coyote and some foxes continue to flourish. The signal difference is that dogs have evolved almost exclusively through artificial selection by humans while wild canids have evolved through natural selection.
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The full details of canid evolution, including development of the dog, remain unclear because of the incomplete nature of the fossil record, but what we do know reveals how a remarkable group of predators evolved and what they have meant to the natural and built worlds. In an effort to capture this rich story, the Natural History Museum of Los Angeles County has created an interactive multimedia exhibition exploring canid evolution and the role of dogs in human societies. As scientific advisors of “DOGS: Wolf, Myth, Hero & Friend,” we will be working on a book to accompany the exhibition.
The exhibition and national tour is made possible by Pedigree® Food for Dogs and is supported by a generous grant from the National Science Foundation. The exhibition premieres at the Natural History Museum of Los Angeles County on October 13, 2002, and over the next five years is scheduled to travel to San Diego and San Bernardino, California; Seattle; Mesa, Arizona; Omaha; Washington, D.C; Milwaukee; Philadelphia; Cleveland; and Chicago, with more cities to be announced.
The evolutionary history of dogs begins some 40 million years ago in North America, when Hesperocyonines, looking like a cross between a fox and a weasel, emerged from the soup of carnivores. Hyena-like canids, the Borophagines, or “bone-eaters,” with bone-crushing jaws, followed and persisted until around 2.5 million years ago, when the last one vanished.
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.
The social nature of wolves provides the evolutionary template for dogs. Wolves are highly social and live in a complex society maintained by systems of communication, cooperation, and aggressive and submissive behaviors. They exist in small family units consisting of a single reproductive (dominant or alpha) mated pair and their adult offspring from consecutive years. Some packs may also contain non-relatives or “strangers.”
Generally only the alpha pair reproduce, while adult offspring assist in all aspects of pack life including hunting, provisioning and guarding of offspring, defending pack territories, and attending to the den. Such packs maintain territories and aggressively repel interlopers—so aggressively, in fact, that interpack aggression may be the largest cause of non-human-induced mortality among wolves.
These wolfish behaviors are a kind of pre-adaptation for human/dog relations. Dogs can be readily submissive to their owners, but show various degrees of aggression toward strangers or territorial interlopers. They form long-term bonds with humans, as they would often do in a pack or with mates, and show cooperative and altruistic behavior, sacrificing for humans just as they would for kin.
How Old Is Our Oldest Friend?
Exactly when the dog split off from the wolf and hitched its future to that of humans is subject to debate, as are the reasons humans and wolves joined forces. The earliest archaeological evidence of dogs dates from 12,000 to 14,000 years ago. By 8,000 to 10,000 years ago dogs were found throughout the world, their presence increasingly recorded in early rock art from Africa, Eurasia, North America and Australia. This art shows them used for hunting and ceremonial purposes. Compared with wolves, these dogs possessed a foreshortened face, crowded teeth, a smaller brain, reduced bulla—the bony case surrounding the ear —and a prominent “stop,” or break between forehead and face.
But there is new evidence that the archaeological record does not tell the full story of the dog’s origin. Research by one of us (Robert Wayne and colleagues) on mitochondrial DNA, inherited through the mother and involved in the cell’s energy system, indicates that genetically the dog split from the wolf perhaps over 100,000 years ago and today differs in genetic (DNA) composition from the wolf by no more than 1 percent. This finding suggests that behavioral features may have been key in the domestication process and indicates that for much of the history of anatomically modern humans, there have been dogs.
The Domestication Question
Evidence suggests that ancient human hunters and gatherers had great respect for the abilities of top animal predators, like wolves and big cats, not least because they sought the same prey and probably scavenged each other’s kills. Proximity combined with the similar social structure of human clans and wolf packs doubtless created opportunities for humans to tame wolf puppies and for wolves to grow more accustomed to human encampments and activities. Wolves scavenging around the camps could also have warned of other predators and even driven them away.
Mutual tolerance and respect, combined with the proto-dogs’ willingness to submit to human direction, could well have set up the dynamic relationship that has changed and flourished through many human cultures and adaptations. Certainly the similarities between the wolf’s pack structure and the extended families of early humans made it easy for proto-dogs to fit into human societies.
Proto-dogs may have remained unchanged in appearance for tens of thousands of years, perhaps until what we know as the domestic dog began to appear in the fossil record. We know that many European travelers described the dogs of the Inuit and many North American Indian tribes as indistinguishable from wolves. That might reflect not only a lack of selection for traits we identify with dogs but also the intermingling of dogs and wolves. In this view, the look of proto-dogs began to change in places where people started to establish more permanent settlements and change their ways of living at the end of the last Ice Age.
Dogs Extend Our Abilities
The key to the dog/human relationship doubtless lies in the way dogs extend human abilities while providing companionship. At an apparently early date, humans learned to breed dogs for certain characteristics—trainability, sociability, size and coat color among them. They also began to create dogs who concentrated certain inherent wolfish talents and were thus even more valuable as hunters, guardians, warriors or herders. By 7000 B.C., Egyptian tombs show hunting, herding, war and guard dogs, as well as esteemed pets.
Dogs extend virtually all human senses. They detect odors at concentrations that are 1,000 to 100 million times lower than what humans can perceive, and they perform better than any machine. Today, dogs are used to detect explosives, guns, money, drugs, underground oil and water leaks, contraband agricultural products, termites, and almost as many objects as one can imagine. Dogs continue to find game and humans, including people trapped in rubble, and even to track endangered species.
Dogs see better at night, dusk and dawn than humans, and they can recognize moving objects at up to 540 meters (900 yards). Humans created gaze, or sight, hounds thousands of years ago to take advantage of the dog’s ability to see and run down large, fast prey, like gazelles and deer. Dogs hear over a much broader range of frequencies than humans, which enhances their value as watchdogs and as helpers for the hearing-impaired.
All canids exhibit both skeletal and physiological adaptations for running, and some species/breeds are better built for running than others. In the wild, canids run to catch prey and to avoid predators. For millennia, humans have taken advantage of the dog’s ability to run for hunting, hauling and sport racing.
Canids are also highly vocal, communicating through barks, bays, yodels, yelps, whines, growls and howls, with domestic dogs being the champion barkers. When it comes to howling, though, many of them come up short, and some dogs lack even a full range of barks. Humans do not always understand or appreciate these vocalizations in their house dogs, but these sounds are important in hunting and guard dogs, and dogs understand their meaning.
Looking at the work dogs perform and the companionship they provide, it is easy to romanticize their place in human society. Numerous myths and legends from many cultures attest to the importance of dogs and various wild canids—they are presented as creators of the world or of men, bringers of fire, healers, guardians or guides to the underworld, the inseparable companions of gods as well as men.
The Deadly Relationship
But there is a dark side to human interactions with canids. Despite enjoying a popular fascination among many groups, wild canids have long been subject to persecution on the grounds that they kill livestock and threaten people. The slaughter has decimated wolf populations around the world, even while it has failed to suppress coyotes, who are expanding their range through much of North America. Other wild canids continue to suffer from hunting and habitat loss. Negative images, like that of the big bad wolf in the fairy tale “Little Red Riding Hood,” are nearly as common as positive portrayals.
Dogs, too, have suffered persecution as spreaders of rabies and killers of livestock—in some areas dogs have long killed more livestock than wild predators, who are more frequently blamed. Millions of dogs are abandoned and abused each year or sacrificed in research. In some parts of the world, health officials continue to slaughter tens of thousands of stray dogs during rabies epidemics, despite the presence of a vaccine. Even without the fear of rabies, dog bites remain a major public health problem in the United States and other countries, serving as a constant reminder that for all of his virtues, the dog, like every canid, often acts with his teeth.
Dogs can also spread diseases like parvovirus and distemper to their wild cousins, a particular problem for endangered canids such as the Ethiopian wolf. Hybridization with domestic dogs is a problem for that imperiled wolf as well.
At the behest of their human companions dogs have served heroically in war; in fact, prior to the invention of firearms, they were a lethal part of any arsenal. But war dogs have also been turned to torture and brutality—against Native Americans during the Spanish conquest, for example, or runaway slaves or innocent civilians and protestors in many nations. Once popular sports, bull and bear baiting and dog fighting are now generally considered cruel spectacles, although they persist as illegal blood sports.
The dog’s attributes, including its sagacity, so highly praised in the 19th century, have helped it remain the chief, enduring animal companion and helper for humans. Whether pulling sleds; tracking endangered Florida panthers; assisting disabled people as their eyes, ears, stabilizers and guardians; protecting flocks and property; locating disaster victims or explosives; playing Frisbee; appearing in a show; or bringing joy to someone’s life, dogs continue to figure prominently in human society. They also continue to serve as a vital physical and spiritual link between the tame and the wild, human society and nature.
As scientific advisors of the exhibit by the Natural History Museum of Los Angeles County, we have attempted to present a full portrait of the rich history of human’s best friend and the rich, natural family of canids from which it came. We recognize that it is a story without end as scientists, veterinarians, historians, archaeologists and anthropologists continue to fill in the often significant missing details and as humans continue to shape dogs to new purposes through breeding and training. We have a responsibility to dogs and their wild cousins that we cannot ignore, for in ways nearly too numerous to count, they have helped us get where we are. They bring balance to our lives and to the natural world, and they provide an insight into the mechanisms of evolution itself.
Visit “Dogs: Wolf, Myth, Hero & Friend” online.