The dog (Canis lupus familiaris,[1]pronounced /ˈkeɪnɨs ˈluːpəs fəˈmɪli.ɛərɨs/) 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. The word "dog" may also mean the male of a canine species,[2] as opposed to the word "bitch" for the female of the species.[3]

The dog quickly became ubiquitous across culture across 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.[4] Dogs perform many roles for people, such as hunting, herding, protection, assisting police and military, companionship, and, more recently, aiding handicapped individuals. This versatility, more than almost any other known animal, has given them the nickname "Man's best friend" in the western world. Currently, there are estimated to be 400 million dogs in the world.[5]

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.[6] 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.[7] It is common for most breeds to shed this coat.

1-Etymology and related terminology

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 some belonging to the family Canidae, which includes foxes, jackals, Bush Dog, the African Wild Dog and coyotes and many others; or it can be used to refer to the subfamily of Caninae, or the genus Canis, also often called the "true dogs," which genus includes only the wolf, jackal, coyote, and dog.[8] 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".[9] The term may derive from Proto-Germanic *dukkōn, represented in Old English finger-docce ("finger-muscle").[10] The word also shows the familiar petname diminutive -ga also seen in frogga "frog", picga "pig", stagga "stag", wicga "beetle, worm", among others.[11] 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.[12]

In 14th century England, hound (from German: 'hund') was the general word for all domestic canines, and dog referred to a subtype of hound, a group including the mastiff. It is believed that this "dog" type of "hound" was so common that it eventually became the prototype of the category “hound”.[13] By the 16th century, dog had become the general word, and hound had begun to refer only to types used for hunting.[14] Hound, cognate to German Hund, Dutch hond, common Scandinavian hund, and Icelandic hundur, is ultimately derived from the Proto-Indo-European *kwon- "dog", found in Welsh ci (plural cwn), Latin canis, Greek kýōn, Lithuanian šuõ.[15]

In breeding circles, a male canine is referred to as a dog, while a female is called a bitch (Middle English bicche, from Old English bicce, ultimately from Old Norse bikkja). 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, from French poupée, until they are about a year old. The process of birth is whelping, from the Old English word hwelp, (cf. German Welpe, Dutch welp, Swedish valp, Icelandic hvelpur) .[16]

2-Taxonomy

The domestic dog was originally classified as Canis familiaris and Canis familiarus domesticus by Carolus Linnaeus in 1758,[17][18] 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;[4][19] however, the timeframe and mechanisms by which dogs diverged are controversial.[4] Canis lupus familiaris is listed as

the name for the taxon that is broadly used in the scientific community and recommended by ITIS, however Canis familiaris is a recognised synomym.[20] It should further be noted, that while the current edition of Mammal Species of the World uses the names Canis lupus familiaris and Canis lupus dingo, it still classifies the dingo as a domestic dog.[21]

3-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.[1] 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.[4]

Although experts largely disagree over the details of dog domestication, it is agreed that human interaction played a significant role in shaping the subspecies.[4] 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.[22]

The current consensus among biologists and archaeologists is that the dating of first domestication is indeterminate.[4][22] There is conclusive evidence that dogs genetically diverged from their wolf ancestors at least 15,000 years ago,[23][24][25] but some believe domestication to have occurred earlier.[4] It is not known whether humans domesticated the wolf as such to initiate dog's divergence from its ancestors, or whether dog's evolutionary path had already taken a different course prior to domestication. The latter view has gained proponents, such as biologists Raymond and Lorna Coppinger;[5] they theorize that some wolves gathered around the campsites of the paleolithical man to scavenge refuse, and that associated 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,[4][22] 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.[26] But there is a wide range of other, contradictory findings that make this issue controversial.

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.[4] 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.[27] 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.[4][25] 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.[25][28] 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

DNA studies have provided a wider range of possible divergence dates, from 15,000 to 40,000 years ago,[25] to as much as 100,000 to 140,000 years ago.[29] This evidence depends on a number of assumptions that may be violated.[4] 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.[4]

Genetic analyses indicate all dogs are likely descended from a handful of domestication events with a small number of founding females,[4][26] although there is evidence that domesticated dogs interbred with local populations of wild wolves on several occasions.[25] 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.[25] 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.[30] 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.[30]

There is a great deal of controversy surrounding the evolutionary framework for the domestication of dogs.[4] Although it is widely claimed that "man domesticated the wolf,"[31] man may not have taken such a proactive role in the process.[4] 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

4-Biology

Domestic dogs have been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes.[1] 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.[1] 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.[85] The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder

4-1 Senses

Sight

Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans (deuteranopia).[87][88][89][90]

The dog's visual system has evolved to aid proficient hunting.[87] 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[87]), 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.[87] 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.[87] 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.[91] 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.[88][89] 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

Hearing

The frequency range of dog hearing is approximately 40 Hz to 60,000 Hz,[92] which means that dogs can detect sounds far beyond the upper limit of the human auditory spectrum.[89][92][93] Additionally, dogs have ear mobility which allows them to rapidly pinpoint the exact location of a sound.[94] 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

Smell

While the human brain is dominated by a large visual cortex, the dog brain is dominated by an olfactory cortex.[87] 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.[87] The bloodhound exceeds this standard with nearly 300 million receptors.[87] Dogs can discriminate odors at concentrations nearly 100 million times lower than humans can.[95] 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

4-2 Physical characteristics

Coat

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. A countershaded animal will have dark coloring on its upper surfaces and light coloring below,[97] which reduces its general visibility. Thus many breeds will have an occasional "blaze", stripe, or "star" of white fur on their chest or underside.[98]

Tail

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).[99] In some breeds, puppies can be born with a short tail or no tail at all.[100] This occurs more frequently in those breeds that are frequently docked and thus have no breed standard regarding the tail