The dog (Canis lupus familiaris, pronounced /ˈkeɪ.nɪs ˈluːpəs fʌˈmɪliɛərɪs/) is a domesticated form of the Wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The domestic dog has been one of the most widely kept working and companion animals in human history. Amongst canine enthusiasts, the word "dog" may also mean the male of a canine species, as opposed to the word "bitch" (the female of the species).
The dog quickly became ubiquitous across culture in all parts of the world, and was extremely valuable to early human settlements. For instance, it is believed that the successful emigration across the Bering Strait might not have been possible without sled dogs. Dogs perform many roles for people, such as hunting, herding, protection, companionship, and, more recently, assisting handicapped individuals. Currently, there are estimated to be 400 million dogs in the world.
Over the 15,000 year span that the dog had been domesticated, it diverged into only a handful of landraces, groups of similar animals whose morphology and behavior have been shaped by environmental factors and functional roles. As the modern understanding of genetics developed, humans began to intentionally breed dogs for a wide range of specific traits. Through this process, the dog has developed into hundreds of varied breeds, and shows more behavioral and morphological variation than any other land mammal. For example, height measured to the withers ranges from a few inches in the Chihuahua to a few feet in the Irish Wolfhound; color varies from white through grays (usually called "blue'") to black, and browns from light (tan) to dark ("red" or "chocolate") in a wide variation of patterns; coats can be short or long, coarse-haired to wool-like, straight, curly, or smooth. It is common for most breeds to shed this coat, but non-shedding breeds are also popular.
Dog is the common use term that refers to members of the subspecies Canis lupus familiaris. The term is sometimes used to refer to a wider range of species: it can be used to refer to any mammal belonging to the family Canidae, which includes wolves, foxes, jackals, and coyotes; it can be used to refer to the subfamily of Caninae, or the genus Canis, also often called the "true dogs". Some members of the family have "dog" in their common names, such as the Raccoon Dog and the African Wild Dog. A few animals have "dog" in their common names but are not canids, such as the prairie dog and the dog fish.
The English word dog comes from Middle English dogge, from Old English docga, a "powerful dog breed". The term may derive from Proto-Germanic *dukkōn, represented in Old English finger-docce ("finger-muscle"). The word also shows the familiar petname diminutive -ga also seen in frogga "frog", picga "pig", stagga "stag", wicga "beetle, worm", among others. Due to the archaic structure of the word, the term dog may ultimately derive from the earliest layer of Proto-Indo-European vocabulary, reflecting the role of the dog as the earliest domesticated animal.
Dog ousted traditional English hound by the 16th century, before which time it had the meaning of "dog" rather than modern "hunting dog", as in other Germanic languages – it is cognate to German Hund, Dutch hond, common Scandinavian hund, and Icelandic hundur. Hound itself is derived from the Proto-Indo-European *kwon- "dog", found in Welsh ci (plural cwn), Latin canis, Greek kýōn, Lithuanian šuõ, just to name a few.
In breeding circles, a male canine is referred to as a dog, while a female is called a bitch. A group of offspring is a litter. The father of a litter is called the sire, and the mother is called the dam. Offspring are generally called pups or puppies until they are about a year old. The process of birth is whelping.
The domestic dog was originally classified as Canis familiaris and Canis familiarus domesticus by Carolus Linnaeus in 1758, and was reclassified in 1993 as Canis lupus familiaris, a subspecies of the gray wolf Canis lupus, by the Smithsonian Institution and the American Society of Mammalogists. Overwhelming evidence from behavior, vocalizations, morphology, and molecular biology led to the contemporary scientific understanding that a single species, the gray wolf, is the common ancestor for all breeds of domestic dogs; however, the timeframe and mechanisms by which dogs diverged are controversial.
History and evolution
Domestic dogs inherited a complex social hierarchy and behaviors from their wolf ancestors. Dogs are pack animals with a complex set of behaviors related to determining each dog's position in the social hierarchy, and they exhibit various postures and other means of nonverbal communication that reveal their states of mind. These sophisticated forms of social cognition and communication may account for their trainability, playfulness, and ability to fit into human households and social situations, and these attributes have earned dogs a unique relationship with humans despite being potentially dangerous apex predators.
Although experts largely disagree over the details of dog domestication, it is agreed that human interaction played a significant role in shaping the subspecies. Shortly after domestication, dogs became ubiquitous in human populations, and spread throughout the world. Emigrants from Siberia likely crossed the Bering Strait with dogs in their company, and some experts suggest that use of sled dogs may have been critical to the success of the waves that entered North America roughly 12,000 years ago. Dogs were an important part of life for the Athabascan population in North America, and were their only domesticated animal. Dogs also carried much of the load in the migration of the Apache and Navajo tribes 1,400 years ago. Use of dogs as pack animals in these cultures often persisted after the introduction of the horse to North America.
The current consensus among biologists and archaeologists is that no one can be sure when dogs were domesticated. There is conclusive evidence that dogs genetically diverged from their wolf ancestors at least 15,000 years ago but some believe domestication to have occurred earlier. It is not known whether humans domesticated the gray wolf as such to initiate dog's divergence from its ancestors, or whether dog's evolutionary path took a different course already prior to domestication. Lately the latter view has gained proponents such as biologists Raymond and Lorna Coppinger. They theorize that some wolves started gathering around the campsites of the paleolithical man to scavenge his refuse. There, an evolutionary pressure developed that favored those who were less frightened by and keener in approaching humans.
The bulk of the scientific evidence for the evolution of the domestic dog stems from archaeological findings and mitochondrial DNA studies. The divergence date of roughly 15000 years ago is based in part on archaeological evidence that demonstrates that the domestication of dogs occurred more than 15,000 years ago, and some genetic evidence indicates that the domestication of dogs from their wolf ancestors began in the late Upper Paleolithic close to the Pleistocene/Holocene boundary, between 17,000 and 14,000 years ago. But there is a wide range of other, contradictory findings that make this issue controversial.
Archaeological evidence plays a large role in this debate. In 2008, a team of international scientists released findings from an excavation at Goyet Cave in Belgium declaring that a large, toothy canine existed 31,700 years ago and ate a diet of horse, musk ox and reindeer. Prior to this Belgium discovery, the earliest dog fossils were two large skulls from Russia and a mandible from Germany, that dated from roughly 14,000 years ago. Remains of smaller dogs from Natufian cave deposits in the Middle East, including the earliest burial of a human being with a domestic dog, have been dated to around 10,000 to 12,000 years ago. There is a great deal of archaeological evidence for dogs throughout Europe and Asia around this period and through the next two thousand years (roughly 8,000 to 10,000 years ago), with fossils uncovered in Germany, the French Alps, and Iraq, and cave paintings in Turkey.
Thus, the archaeological evidence suggests that the latest dogs could have diverged from wolves was roughly 15000 years ago, although it is possible that they diverged much earlier.
DNA studies have provided a wider range of possible divergence dates, from 15,000 to 40,000 years ago, to as much as 100,000 to 140,000 years ago. This evidence depends on a number of assumptions that may be violated. Genetic studies are based on comparisons of genetic diversity between species, and depend on a calibration date. Many estimates of divergence dates from DNA evidence use an estimated wolf-coyote divergence date (roughly 1 million years ago) as a calibration. If this estimate is incorrect, and the actual wolf-coyote divergence is closer to 750,000 or 2 million years ago, then the DNA evidence that supports specific dog-wolf divergence dates would be interpreted very differently. Furthermore, it is believed that the genetic diversity of wolves has been in decline for the last 200 years, and that the genetic diversity of dogs has been reduced by selective breeding. This could significantly bias DNA analyses to support an earlier divergence date. The genetic evidence for the domestication event occurring in East Asia is also subject to violations of assumptions. These conclusions are based on the location of maximal genetic divergence, and assume that hybridization does not occur, and that breeds remain geographically localized. Although these assumptions hold for many species, there is good reason to believe that they do not hold for canines.
Genetic analyses indicate all dogs are likely descended from a handful of domestication events with a small number of founding females, although there is evidence that domesticated dogs interbred with local populations of wild wolves on several occasions. Data suggests that dogs first diverged from wolves in East Asia, and that these domesticated dogs then quickly migrated throughout the world, reaching the North American continent around 8000 B.C. The oldest groups of dogs, which show the greatest genetic variability and are the most similar to their wolf ancestors, are primarily Asian and African breeds, including the Basenji, Lhasa Apso, and Siberian Husky. Some breeds that were thought to be very old, such as the Pharaoh Hound, Ibizan Hound, and Norwegian Elkhound, are now known to have been created more recently.
There is a great deal of controversy surrounding the evolutionary framework for the domestication of dogs. Although it is widely claimed that "man domesticated the wolf," man may not have taken such a proactive role in the process. The nature of the interaction between man and wolf that led to domestication is unknown and controversial. At least three early species of the Homo genus began spreading out of Africa roughly 400,000 years ago, and thus lived for a considerable period in contact with canine species. Despite this, there is no evidence of any adaptation of canine species to the presence of the close relatives of modern man. If dogs were domesticated, as believed, roughly 15,000 years ago, the event (or events) would have coincided with a large expansion in human territory and the development of agriculture. This has led some biologists to suggest that one of the forces that led to the domestication of dogs was a shift in human lifestyle in the form of established human settlements. Permanent settlements would have coincided with a greater amount of disposable food and would have created a barrier between wild and anthropogenic canine populations.
Roles with humans
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Dogs have lived and worked with humans in so many roles that they have earned the unique nickname, "man's best friend", a phrase which is used in other languages as well. They have been bred for herding livestock, hunting (e.g. pointers and hounds), rodent control, guarding, helping fishermen with nets, and pulling loads, in addition to their roles as companions.
Service dogs such as guide dogs, utility dogs, assistance dogs, hearing dogs, and psychological therapy dogs provide assistance to individuals with physical or mental disabilities. Some dogs owned by epileptics have been shown to alert their handler when the handler shows signs of an impending seizure, sometimes well in advance of onset, allowing the owner to seek safety, medication, or medical care.
Sports and shows
In conformation shows, also referred to as breed shows, a judge familiar with the specific dog breed evaluates individual purebred dogs for conformity with their established breed type as described in the breed standard. As the breed standard only deals with the externally observable qualities of the dog (such as appearance, movement, and temperament), separately tested qualities (such as ability or health) are not part of the judging in conformation shows.
As a food source
Dog meat is consumed in some East Asian countries, including Korea, China, and Vietnam, a practice that dates back to antiquity. It is estimated that 13–16 million dogs are killed and consumed in Asia every year. The BBC claims that, in 1999, more than 6,000 restaurants served soups made from dog meat in South Korea. In Korea, the primary dog breed raised for meat, the nureongi (누렁이), differs from those breeds raised for pets which Koreans may keep in their homes. The most popular Korean dog dish is gaejang-guk (also called bosintang), a spicy stew meant to balance the body's heat during the summer months; followers of the custom claim this is done to ensure good health by balancing one's gi, or vital energy of the body. A 19th century version of gaejang-guk explains that the dish is prepared by boiling dog meat with scallions and chili powder. Variations of the dish contain chicken and bamboo shoots. While the dishes are still popular in Korea with a segment of the population, dog is not as widely consumed as beef, chicken, and pork.
Other cultures, such as Polynesia and Pre-Columbian Mexico, also consumed dog meat in their history. However, Western cultures generally regard dog meat as taboo. In some places, however, such as in rural areas of Poland, dog fat is believed to have medicinal properties - being good for the lungs for instance.
Health risks to humans
In the USA, dogs cause 88% of the 86,000 falls caused by pets.
Dog feces can cause a number of human diseases, including toxocariasis, which can cause blindness, and can also cause human hookworm.. In the United States, about 10,000 cases of Toxocara infection are reported in humans each year. Almost 14% of the US population is infected with Toxocara, a parasite of dogs and cats that can be passed from animals to humans.
The incidence of dog bites, and especially fatal dog bites, is extremely rare in America considering the number of pet dogs in the country. Fatalities from dog bites occur in America at the rate of one per four million dogs. A Colorado study found that bites in children were less severe than bites in adults. The incidence of dog bites in the US is 12.9 per 10,000 inhabitants, but for boys aged 5 to 9 the incidence rate is 60.7 per 10,000. Moreover, children have a much higher chance to be bitten in the face or neck. Sharp claws with powerful muscles behind them can lacerate flesh in a scratch that can lead to serious infections.
Every year, between 6 and 8 million dogs and cats enter US animal shelters. The Humane Society of the United States (HSUS) estimates that approximately 3 to 4 million dogs and cats are euthanized yearly in shelters across the United States.
Domestic dogs have been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes. Modern dog breeds show more variation in size, appearance, and behavior than any other domestic animal. Nevertheless, their morphology is based on that of their wild ancestors, gray wolves. Dogs are predators and scavengers, and like many other predatory mammals, the dog has powerful muscles, fused wrist bones, a cardiovascular system that supports both sprinting and endurance, and teeth for catching and tearing. Dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire Terrier, that stood only 6.3 centimetres (2.5 in) at the shoulder, 9.5 cm (3.7 in) in length along the head-and-body, and weighed only 113 grams (4.0 oz). The largest known dog was an English Mastiff which weighed 155.6 kilograms (343 lb) and was 250 cm (98 in) from the snout to the tail. The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder.
The dog's visual system has evolved to aid proficient hunting. While a dog's visual acuity is poor (that of a poodle's has been estimated to translate to a Snellen rating of 20/75), their visual discrimination for moving objects is very high; dogs have been shown to be able to discriminate between humans (e;g. identifying their owner) from distances up to a mile. As crepuscular hunters, dogs often rely on their vision in low light situations: they have very large pupils, a high density of rods in the fovea, an increased flicker rate, and a tapetum lucidum. The tapetum is a reflective surface behind the retina that reflects light back to give the photoreceptors a second chance to catch the photons.
The eyes of different breeds of dogs have different shapes, dimensions, and retina configurations. Many long-nosed breeds have a "visual streak" – a wide foveal region that runs across the width of the retina and gives them a very wide field of excellent vision. Some long-muzzled breeds, particularly the sighthounds, have a field of vision up to 270° (compared to 180° for humans). Short-nosed breeds, on the other hand, have an "area centralis": a central patch with up to three times the density of nerve endings as the visual streak, giving them detailed sight much more like a human's. Some broad-headed breeds with short noses have a field of vision similar to that of humans. Most breeds have good vision, but some show a genetic predisposition for myopia – such as Rottweilers, where one out of every two has been found to be myopic.
The frequency range of dog hearing is approximately 40 Hz to 60,000 Hz, which means that dogs can detect sounds far beyond the upper limit of the human auditory spectrum. Additionally, dogs have ear mobility which allows them to rapidly pinpoint the exact location of a sound. Eighteen or more muscles can tilt, rotate, raise, or lower a dog's ear. A dog can identify a sound's location much faster than a human can, as well as hear sounds at four times the distance.
While the human brain is dominated by a large visual cortex, the dog brain is dominated by an olfactory cortex. The olfactory bulb in dogs is roughly forty times bigger than the olfactory bulb in humans, relative to total brain size, with 125 to 220 million smell-sensitive receptors. The bloodhound exceeds this standard with nearly 300 million receptors. Dogs can discriminate odors at concentrations nearly 100 million times lower than humans can. The wet nose is essential for determining the direction of the air current containing the smell. Cold receptors in the skin are sensitive to the cooling of the skin by evaporation of the moisture by air currents.
The coats of domestic dogs are either "double", made up of a coarse guard hair and a soft down hair, like a wolf, or "single", with the topcoat only. Dogs with double coats tend to originate in colder climates.
Domestic dogs often display the remnants of countershading, a common natural camouflage pattern. The general theory of countershading is that an animal that is lit from above will appear lighter on its upper half and darker on its lower half, where it will usually be in its own shade. This is a pattern that prey can learn to watch for. A countershaded animal will have dark coloring on its upper surfaces and light coloring below, which reduces its general visibility. Thus many breeds will have an occasional "blaze", stripe, or "star" of white fur on their chest or underside.
There are many different shapes for dog tails: straight, straight up, sickle, curled, or cork-screw. In some breeds, the tail is traditionally docked to avoid injuries (especially for hunting dogs). In some breeds, puppies can be born with a short tail or no tail at all. This occurs more frequently in those breeds that are frequently docked and thus have no breed standard regarding the tail.
Types and breeds
While all dogs are genetically very similar, natural selection and selective breeding have reinforced certain characteristics in certain populations of dogs, giving rise to dog types and dog breeds. Dog types are broad categories based on function, genetics, or characteristics.
Dog breeds are groups of animals that possess a set of inherited characteristics that distinguishes them from other animals within the same species. Modern dog breeds are non-scientific classifications of dogs kept by modern kennel clubs. Purebred dogs of one breed are genetically distinguishable from purebred dogs of other breeds, but the means by which kennel clubs classify dogs is unsystematic. Systematic analyses of the dog genome has revealed only four major types of dogs that can be said to be statistically distinct. These include the "old world dogs" (e.g., Malamute and Shar Pei), "Mastiff"-type (e.g., Labrador Retriever), "herding"-type (e.g., View current version