Domestic dog Temporal range: 0.015–0 Ma PreЄ Є O S D C P T J K Pg N ↓ Pleistocene – Recent
Yellow Labrador Retriever, the most registered breed of 2009 with the AKC More images of dogs.
Conservation status
Domesticated
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Carnivora
Family:	Canidae
Genus:	Canis
Species:	C. lupus
Subspecies:	C. l. familiaris[1]
Trinomial name
Canis lupus familiaris[2]
Synonyms
Species synonymy aegyptius Linnaeus, 1758 alco C. E. H. Smith, 1839 americanus Gmelin, 1792 anglicus Gmelin, 1792 antarcticus Gmelin, 1792 aprinus Gmelin, 1792 aquaticus Linnaeus, 1758 aquatilis Gmelin, 1792 avicularis Gmelin, 1792 borealis C. E. H. Smith, 1839 brevipilis Gmelin, 1792 cursorius Gmelin, 1792 domesticus Linnaeus, 1758 extrarius Gmelin, 1792 ferus C. E. H. Smith, 1839 fricator Gmelin, 1792 fricatrix Linnaeus, 1758 fuillus Gmelin, 1792 gallicus Gmelin, 1792 glaucus C. E. H. Smith, 1839 graius Linnaeus, 1758 grajus Gmelin, 1792 hagenbecki Krumbiegel, 1950 haitensis C. E. H. Smith, 1839 hibernicus Gmelin, 1792 hirsutus Gmelin, 1792 hybridus Gmelin, 1792 islandicus Gmelin, 1792 italicus Gmelin, 1792 laniarius Gmelin, 1792 leoninus Gmelin, 1792 leporarius C. E. H. Smith, 1839 major Gmelin, 1792 mastinus Linnaeus, 1758 melitacus Gmelin, 1792 melitaeus Linnaeus, 1758 minor Gmelin, 1792 molossus Gmelin, 1792 mustelinus Linnaeus, 1758 obesus Gmelin, 1792 orientalis Gmelin, 1792 pacificus C. E. H. Smith, 1839 plancus Gmelin, 1792 pomeranus Gmelin, 1792 sagaces C. E. H. Smith, 1839 sanguinarius C. E. H. Smith, 1839 sagax Linnaeus, 1758 scoticus Gmelin, 1792 sibiricus Gmelin, 1792 suillus C. E. H. Smith, 1839

The domestic dog (Canis lupus familiaris),^[2][3] is a subspecies of the gray wolf (Canis lupus), a member of the Canidae family of the mammilian order Carnivora. The term "domestic dog" is generally used for both domesticated and feral varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in human history. The word "dog" may also mean the male of a canine species,^[4] as opposed to the word "bitch" for the female of the species.

The present lineage of dogs was domesticated from gray wolves about 15,000 years ago.^[5] Remains of domesticated dogs have been found in Siberia and Belgium from about 33,000 years ago. None of these early domestication lineages seem to have survived the Last Glacial Maximum. Although mDNA suggest a split between dogs and wolves around 100,000 years ago no specimens predate 33,000 years ago that are clearly morphologically domesticated dog.^[6][7][8]

Dogs' value to early human hunter-gatherers led to them quickly becoming ubiquitous across world cultures. Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and military, companionship, and, more recently, aiding handicapped individuals. This impact on human society has given them the nickname "Man's Best Friend" in the Western world. In some cultures, dogs are also source of meat.^[9][10] In 2001, there were estimated to be 400 million dogs in the world.^[11]

Most breeds of dogs are at most a few hundred years old, having been artificially selected for particular morphologies and behaviors by people for specific functional roles. Through this selective breeding, the dog has developed into hundreds of varied breeds, and shows more behavioral and morphological variation than any other land mammal.^[12] For example, height measured to the withers ranges from a 2 inches (51 mm) in the Chihuahua to a 2 feet (0.61 m) 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.^[13] It is common for most breeds to shed this coat.

Etymology and related terminology[link]

Dog is the common use term that refers to members of the subspecies Canis lupus familiaris (canis, "dog"; lupus, "wolf"; familiaris, "of a household" or "domestic"). The term can also be used to refer to a wider range of related species, such as the members of the genus Canis, or "true dogs", including the wolf, coyote, and jackals; or it can refer to the members of the tribe Canini, which would also include the African wild dog; or it can be used to refer to any member of the family Canidae, which would also include the foxes, bush dog, raccoon dog, and others.^[14] 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.

The English word dog comes from Middle English dogge, from Old English docga, a "powerful dog breed".^[15] The term may derive from Proto-Germanic *dukkōn, represented in Old English finger-docce ("finger-muscle").^[16] The word also shows the familiar petname diminutive -ga also seen in frogga "frog", picga "pig", stagga "stag", wicga "beetle, worm", among others.^[17] 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.^[18]

In 14th-century England, hound (from Old English: 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 this "dog" type of "hound" was so common it eventually became the prototype of the category “hound”.^[19] By the 16th century, dog had become the general word, and hound had begun to refer only to types used for hunting.^[20] 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.^[21]

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, in general, 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 valpa, Icelandic hvelpur).^[22]

Taxonomy[link]

In 1753, the father of modern biological taxonomy, Carl Linnaeus, listed among the types of quadruped familiar to him, the Latin word for dog, “Canis.” Among the species within this genus, Linnaeus listed the fox, as "Canis vulpes", wolves (Canis lupus), and the domestic dog, (Canis canis; see File:Linnaeus - Regnum Animale (1735).png).

In later editions, Linnaeus dropped "Canis canis" and greatly expanded his list of the Canis genus of quadrupeds, and by 1758 included alongside the foxes, wolves, and jackels and many more terms that are now listed as synonyms for domestic dog, including ‘’aegyptius” (hairless dog), ‘’aquaticus’’, (water dog), and ‘’mustelinus’’ (literally “badger dog).” Among these were two that later experts have been widely used for domestic dogs as a species: ‘’Canis domesticus’’ and, most predominantly, ‘’Canis familiaris”, the “common” or “familiar” dog.^[23]

The domestic dog was accepted as a species in its own right until 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.^[24][25][26] In recognition of this fact, the domestic dog 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. Canis lupus familiaris is listed as the name for the taxon that is broadly used in the scientific community and recommended by ITIS although Canis familiaris, however, is a recognised synonym.^[27]

Since that time, domesticus and all taxa referring to domestic dogs or subspecies of dog listed by Linnaeus, Johann Friedrich Gmelin in 1792, and Christian Smith in 1839, lost their subspecies status and have been listed as taxonomic synonyms for Canis lupus familiaris ^[28]

History and evolution[link]

Ancient Greek rhyton in the shape of a dog's head, made by Brygos, early 5th century BC. Jérôme Carcopino Museum, Department of Archaeology, Aleria

Domestic dogs inherited complex behaviors from their wolf ancestors, which would have been pack hunters with complex body language. 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 given dogs a relationship with humans that has enabled them to become one of the most successful species on the planet today.^[24]

Although experts largely disagree over the details of dog domestication, it is agreed that human interaction played a significant role in shaping the subspecies.^[29] Domestication may have occurred initially in separate areas particularly Siberia and Europe. Currently it is thought domestication of our current lineage of dog occurred sometime as early as 15,000 years ago and arguably as late as 8500 years ago. Shortly after the latest 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^[who?] suggest the use of sled dogs may have been critical to the success of the waves that entered North America roughly 12,000 years ago,^{[citation needed]} although the earliest archaeological evidence of dog-like canids in North America dates from about 9,000 years ago.^[30] 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.^{[31][page needed]}

The current consensus among biologists and archaeologists is that the dating of first domestication is indeterminate,^[29][31] although more recent evidence shows isolated domestication events as early as 33,000 years ago.^[32][33] There is conclusive evidence the present lineage of dogs genetically diverged from their wolf ancestors at least 15,000 years ago,^[5][34][35] but some believe domestication to have occurred earlier.^[29] Evidence is accruing that there were previous domestication events, but that those lineages died out.^[7]

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. For example, it is hypothesized that some wolves gathered around the campsites of paleolithic camps to scavenge refuse, and associated evolutionary pressure developed that favored those who were less frightened by, and keener in approaching, humans.

Tesem, an old Egyptian sighthound-like dog.

The bulk of the scientific evidence for the evolution of the domestic dog stems from morphological studies of archaeological findings and mitochondrial DNA studies. The divergence date of roughly 15,000 years ago is based in part on archaeological evidence that demonstrates the domestication of dogs occurred more than 15,000 years ago,^[24][31] and some genetic evidence indicates 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.^[36] But there is a wide range of other, contradictory findings that make this issue controversial.^{[citation needed]} There are findings beginning currently at 33,000 years ago distinctly placing them as domesticated dogs evidenced not only by shortening of the muzzle but widening as well as crowding of teeth.

Archaeological evidence suggests the latest dogs could have diverged from wolves was roughly 15,000 years ago, although it is possible they diverged much earlier.^[24] In 2008, a team of international scientists released findings from an excavation at Goyet Cave in Belgium declaring a large, toothy canine existed 31,700 years ago and ate a diet of horse, musk ox and reindeer.^[37]

Prior to this Belgian discovery, the earliest dog fossils were two large skulls from Russia and a mandible from Germany dated from roughly 14,000 years ago.^[5][24] 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.^[5][38] 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.^[24] The oldest remains of a domesticated dog in the Americas were found in Texas and have been dated to about 9,400 years ago.^[39]

DNA studies[link]

DNA studies have provided a wide range of possible divergence dates, from 15,000 to 40,000 years ago,^[5] to as much as 100,000 to 140,000 years ago.^[40] These results depend on a number of assumptions.^[24] Genetic studies are based on comparisons of genetic diversity between species, and depend on a calibration date. Some estimates of divergence dates from DNA evidence use an estimated wolf-coyote divergence date of roughly 700,000 years ago as a calibration.^[41] If this estimate is incorrect, and the actual wolf-coyote divergence is closer to one or two million years ago, or more,^[42] then the DNA evidence that supports specific dog-wolf divergence dates would be interpreted very differently.

Furthermore, it is believed 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 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.^[24]

Genetic analyses indicate all dogs are likely descended from a handful of domestication events with a small number of founding females,^[24][36] although there is evidence domesticated dogs interbred with local populations of wild wolves on several occasions.^[5] Data suggest dogs first diverged from wolves in East Asia, and these domesticated dogs then quickly migrated throughout the world, reaching the North American continent around 8000 BC.^[5] 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.^[43] Some breeds thought to be very old, such as the Pharaoh Hound, Ibizan Hound, and Norwegian Elkhound, are now known to have been created more recently.^[43]

There is a great deal of controversy surrounding the evolutionary framework for the domestication of dogs.^[24] Although it is widely claimed that "man domesticated the wolf,"^[44] man may not have taken such a proactive role in the process.^[24] 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 time in contact with canine species.^[24]

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 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.^[24]

Roles with humans[link]

A Siberian Husky used as a pack animal

Early roles[link]

Wolves, and their dog descendants, would have derived significant benefits from living in human camps—more safety, more reliable food, lesser caloric needs, and more chance to breed.^[45] They would have benefited from humans’ upright gait that gives them larger range over which to see potential predators and prey, as well as color vision that, at least by day, gives humans better visual discrimination.^[45] Camp dogs would also have benefitted from human tool use, as in bringing down larger prey and controlling fire for a range of purposes.^[45]

Humans would also have derived enormous benefit from the dogs associated with their camps.^[46] For instance, dogs would have improved sanitation by cleaning up food scraps.^[46] Dogs may have provided warmth, as referred to in the Australian Aboriginal expression “three dog night” (an exceptionally cold night), and they would have alerted the camp to the presence of predators or strangers, using their acute hearing to provide an early warning.^[46]

Anthropologists believe the most significant benefit would have been the use of dogs' sensitive sense of smell to assist with the hunt.^[46] The relationship between the presence of a dog and success in the hunt is often mentioned as a primary reason for the domestication of the wolf, and a 2004 study of hunter groups with and without a dog gives quantitative support to the hypothesis that the benefits of cooperative hunting was an important factor in wolf domestication.^[47]

The cohabitation of dogs and humans would have greatly improved the chances of survival for early human groups, and the domestication of dogs may have been one of the key forces that led to human success.^[48]

A British Bulldog shares a day at the park.

As pets[link]

"The most widespread form of interspecies bonding occurs between humans and dogs"^[46] and the keeping of dogs as companions, particularly by elites, has a long history.^[49] However, pet dog populations grew significantly after World War II as suburbanization increased.^[49] In the 1950s and 1960s, dogs were kept outside more often than they tend to be today ^[50] (using the expression “in the doghouse” to describe exclusion from the group signifies the distance between the doghouse and the home) and were still primarily functional, acting as a guard, children’s playmate, or walking companion. From the 1980s, there have been changes in the role of the pet dog, such as the increased role of dogs in the emotional support of their owners.^[51] People and dogs have become increasingly integrated and implicated in each other’s lives,^[52] to the point where pet dogs actively shape the way a family and home are experienced.^[53]

There have been two major trends in the changing status of pet dogs. The first has been the ‘commodification’ of the dog, shaping it to conform to human expectations of personality and behaviour.^[53] The second has been the broadening of the concept of the family and the home to include dogs-as-dogs within everyday routines and practices.^[53]

There are a vast range of commodity forms available to transform a pet dog into an ideal companion.^[54] The list of goods, services and places available is enormous: from dog perfumes, couture, furniture and housing, to dog groomers, therapists, trainers and care-takers, dog cafes, spas, parks and beaches, and dog hotels, airlines and cemeteries.^[54] While dog training as an organized activity can be traced back to the 18th century, in the last decades of the 20th century it became a high profile issue as many normal dog behaviors such as barking, jumping up, digging, rolling in dung, fighting, and urine marking became increasingly incompatible with the new role of a pet dog.^[55] Dog training books, classes and television programs proliferated as the process of commodifying the pet dog continued.^[56]

A pet dog taking part in Christmas traditions

The majority of contemporary dog owners describe their dog as part of the family,^[53] although some ambivalence about the relationship is evident in the popular reconceptualisation of the dog-human family as a pack.^[53] A dominance model of dog-human relationships has been promoted by some dog trainers, such as on the television program Dog Whisperer. However it has been disputed that "trying to achieve status" is characteristic of dog–human interactions.^[57] Pet dogs play an active role in family life; for example, a study of conversations in dog-human families showed how family members use the dog as a resource, talking to the dog, or talking through the dog, to mediate their interactions with each other.^[58]

Another study of dogs’ roles in families showed many dogs have set tasks or routines undertaken as family members, the most common of which was helping with the washing-up by licking the plates in the dishwasher, and bringing in the newspaper from the lawn.^[53] Increasingly, human family members are engaging in activities centred on the perceived needs and interests of the dog, or in which the dog is an integral partner, such as Dog Dancing and Doga.^[54]

According to the statistics published by the American Pet Products Manufacturers Association in the National Pet Owner Survey in 2009–2010, it is estimated there are 77.5 million dog owners in the United States.^[59] The same survey shows nearly 40% of American households own at least one dog, of which 67% own just one dog, 25% two dogs and nearly 9% more than two dogs. There does not seem to be any gender preference among dogs as pets, as the statistical data reveal an equal number of female and male dog pets. Yet, although several programs are undergoing to promote pet adoption, less than a fifth of the owned dogs come from a shelter.

Work[link]

Dogs have lived and worked with humans in so many roles that they have earned the unique nickname, "man's best friend",^[60] a phrase used in other languages as well. They have been bred for herding livestock,^[61] hunting (e.g. pointers and hounds),^[62] rodent control,^[3] guarding, helping fishermen with nets, detection dogs, and pulling loads, in addition to their roles as companions.^[3]

Book of the Hunt, Gaston III, Count of Foix, 1387–88.

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.^[63][64] 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.^[65]

Dogs included in human activities in terms of helping out humans are usually called working dogs. Dogs of several breeds are considered working dogs. Some working dog breeds include Akita, Alaskan Malamute, Anatolian Shepherd Dog, Bernese Mountain Dog, Black Russian Terrier, Boxer, Bullmastiff, Doberman Pinscher, Dogue de Bordeaux, German Pinscher, German Shepherd,^[66] Giant Schnauzer, Great Dane, Great Pyrenees, Great Swiss Mountain Dog, Komondor, Kuvasz, Mastiff, Neapolitan Mastiff, Newfoundland, Portuguese Water Dog, Rottweiler, Saint Bernard, Samoyed, Siberian Husky, Standard Schnauzer, and Tibetan Mastiff.

Sports and shows[link]

Owners of dogs often enter them in competitions^[67] such as breed conformation shows or sports, including racing and sledding.

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[link]

Fried dog meat offered as speciality, among lamb and fish. Shaoyang, Hunan Province, China

Dog meat is consumed in some East Asian countries, including Korea, China, and Vietnam, a practice that dates back to antiquity.^[68] It is estimated that 13–16 million dogs are killed and consumed in Asia every year.^[69] The BBC claims that, in 1999, more than 6,000 restaurants served soups made from dog meat in South Korea.^[70] In Korea, the primary dog breed raised for meat, the nureongi (누렁이), differs from those breeds raised for pets that Koreans may keep in their homes.^[71]

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.^[72]

Other cultures, such as Polynesia and pre-Columbian Mexico, also consumed dog meat in their history. However, Western, South Asian, African, and Middle Eastern cultures, in general, regard consumption of 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.^[73]

A CNN report in China dated March 2010 interviews a dog meat vendor who states that most of the dogs that are available for selling to restaurant are raised in special farms but that there is always a chance that a sold dog is someone's lost pet, although dog pet breeds are not considered edible.^[74]

Health risks to humans[link]

It is estimated that two percent of the US population, 4.7 million people, are bitten each year.^[75] In the 1980s and 1990s the US averaged 17 fatalities per year, while in the 2000s this has increased to 26.^[76] 77% of dog bites are from the pet of family or friends, and 50% of attacks occur on the dog owner's property.^[76]

A Colorado study found bites in children were less severe than bites in adults.^[77] The incidence of dog bites in the U.S. 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.^[78] Sharp claws with powerful muscles behind them can lacerate flesh in a scratch that can lead to serious infections.^[79]

In the UK between 2003 and 2004, there were 5,868 dog attacks on humans, resulting in 5,770 working days lost in sick leave.^[80]

In the United States, cats and dogs are a factor in more than 86,000 falls each year.^[81] It has been estimated around 2% of dog-related injuries treated in UK hospitals are domestic accidents. The same study found that while dog involvement in road traffic accidents was difficult to quantify, dog-associated road accidents involving injury more commonly involved two-wheeled vehicles.^[82]

Toxocara canis (dog roundworm) eggs in dog feces can cause toxocariasis. In the United States, about 10,000 cases of Toxocara infection are reported in humans each year, and almost 14% of the U.S. population is infected.^[83] In Great Britain, 24% of soil samples taken from public parks contained T. canis eggs.^[84] Untreated toxocariasis can cause retinal damage and decreased vision.^[84] Dog feces can also contain hookworms that cause cutaneous larva migrans in humans.^{[85][86][87][88]}

Health benefits for humans[link]

A human cuddles a Doberman puppy.

A growing body of research indicates the companionship of a dog can enhance human physical health and psychological wellbeing.^[89] Dog and cat owners have been shown to have better mental and physical health than nonowners, making fewer visits to the doctor and being less likely to be on medication than nonowners.^[90] In one study, new pet owners reported a highly significant reduction in minor health problems during the first month following pet acquisition, and this effect was sustained in dog owners through to the end of the study.^[91]

In addition, dog owners took considerably more physical exercise than cat owners and people without pets. The group without pets exhibited no statistically significant changes in health or behaviour. The results provide evidence that pet acquisition may have positive effects on human health and behaviour, and that for dog owners these effects are relatively long term.^[91] Pet ownership has also been associated with increased coronary artery disease survival, with dog owners being significantly less likely to die within one year of an acute myocardial infarction than those who did not own dogs.^[92]

Gunnar Kaasen and Balto, the lead dog on the last relay team of the 1925 serum run to Nome.

The health benefits of dogs can result from contact with dogs, not just from dog ownership. For example, when in the presence of a pet dog, people show reductions in cardiovascular, behavioral, and psychological indicators of anxiety.^[93] The benefits of contact with a dog also include social support, as dogs are able to not only provide companionship and social support themselves, but also to act as facilitators of social interactions between humans.^[94] One study indicated that wheelchair users experience more positive social interactions with strangers when they are accompanied by a dog than when they are not.^[95]

The practice of using dogs and other animals as a part of therapy dates back to the late 18th century, when animals were introduced into mental institutions to help socialize patients with mental disorders.^[96] Animal-assisted intervention research has shown that animal-assisted therapy with a dog can increase a person with Alzheimer’s disease’s social behaviours, such as smiling and laughing.^[97] One study demonstrated that children with ADHD and conduct disorders who participated in an education program with dogs and other animals showed increased attendance, increased knowledge and skill objectives, and decreased antisocial and violent behavior compared to those who were not in an animal-assisted program.^[98]

Shelters[link]

Every year, between 6 and 8 million dogs and cats enter US animal shelters.^[99] 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.^[100] However, the percentage of dogs in US animal shelters that are eventually adopted and removed from the shelters by their new owners has increased since the mid 1990s from around 25% up to around 60–75% in the mid first decade of the 21st century.^[101]

Pets entering the shelters are euthanized in countries all over the world because of the lack of financial provisions to take care of these animals. Most shelters complain of not having enough resources to feed the pets and by being constrained to kill them, as the likelihood for all of them to find an owner is very small. In poor countries, euthanasia is usually violent.

Biology[link]

Lateral view of a dog's bone structure.

Domestic dogs have been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes.^[3] 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.^[3] 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.^[102] The tallest dog is a Great Dane that stands 106.7 cm (42.0 in) at the shoulder.^[103]

Senses[link]

Vision[link]

Dog's visual colour perception compared with humans.

Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans (deuteranopia).^{[104][105][106][107]} Dogs are less sensitive to differences in grey shades than humans and also can detect brightness at about half the accuracy of humans.^[108]

The dog's visual system has evolved to aid proficient hunting.^[104] 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^[104]), 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) at a range of between 800 and 900 m, however this range decreases to 500–600 m if the object is stationary.^[104]

Dogs have a temporal resolution of between 60 and 70 Hz, which explains why many dogs struggle to watch television, as most such modern screens are optimized for humans at 50–60 Hz.^[108] Dogs can detect a change in movement that exists in a single diopter of space within their eye. Humans, by comparison, require a change of between 10 and 20 diopters to detect movement.^[109][110]

The physiology of a dog ear.

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.^[104] The tapetum is a reflective surface behind the retina that reflects light to give the photoreceptors a second chance to catch the photons. There is also a relationship between body size and overall diameter of the eye. A range of 9.5 and 11.6 mm can be found between various breeds of dogs. This 20% variance can be substantial and is associated as an adaptation toward superior night vision.^[111]

The eyes of different breeds of dogs have different shapes, dimensions, and retina configurations.^[112] 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, in particular, 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.^[105][106]

Most breeds have good vision, but some show a genetic predisposition for myopia – such as Rottweilers, with which one out of every two has been found to be myopic.^[104] Dogs also have a greater divergence of the eye axis than humans, enabling them to rotate their pupils farther in any direction. The divergence of the eye axis of dogs ranges from 12–25° depending on the breed.^[109]

Experimentation has proven that dogs can distinguish between complex visual images such as that of a cube or a prism. Dogs also show attraction to static visual images such as the silhouette of a dog on a screen, their own reflections, or videos of dogs; however, their interest declines sharply once they are unable to make social contact with the image.^[113]

Hearing[link]

The frequency range of dog hearing is approximately 40 Hz to 60,000 Hz,^[114] which means that dogs can detect sounds far beyond the upper limit of the human auditory spectrum.^{[106][114][115]} In addition, dogs have ear mobility, which allows them to rapidly pinpoint the exact location of a sound.^[116] 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.^[116]