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Domestic dog Temporal range: Late Pleistocene – Recent
A Labrador Retriever
Conservation status
Domesticated
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Phylum:	Chordata
Subphylum:	Vertebrata
Class:	Mammalia
Subclass:	Theria
Order:	Carnivora
Suborder:	Caniformia
Family:	Canidae
Subfamily:	Caninae
Tribe:	Canini
Genus:	Canis
Species:	C. lupus
Subspecies:	C. l. familiaris
Trinomial name
Canis lupus familiaris (Linnaeus, 1758)
Synonyms
Canis familiaris Canis familiaris domesticus

The dog (Canis lupus familiaris, Template:Pron-en) is a domesticated subspecies of the gray wolf, a mammal of the Canidae family of the order Carnivora. The term can also be used more generally to encompass any member of the biological family Canidae, the subfamily Caninae or the genus Canis (also called "true dogs"). When referring to Canis lupus familiaris, the term is used to specify both feral and pet varieties. The domestic dog has been one of the most widely kept working and companion animals in human history.

The domestication of the gray wolf took place in a handful of events roughly 15,000 years ago in central Asia. The dog quickly became ubiquitous across culture in all parts of the world. Dogs became extremely valuable to early human settlements. For instance, it is believed that the successful emigration across the Bering Strait may not have been possible without sled dogs. As a result of the domestication process, the dog developed a sophisticated intelligence that includes unparalleled social cognition and a simple theory of mind that is important to their interaction with humans. These social skills have helped the dog to perform in myriad roles, such as hunting, herding, protection and more recently assisting handicapped individuals. Currently, there are estimated to be 400 million dogs in the world.

Over much of the 15,000 year span that the dog has been domesticated, the dog had diverged into only a handful of landraces. Environmental factors and functional roles shaped their morphology and behavior, but humans did not take an active, intentional role in this process. In the last few hundred years, as the modern understanding of genetics has developed, humans have intentionally bred 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. 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 very short to many centimeters long, from coarse hair to something akin to wool, straight or curly, or smooth. It is common for most breeds to shed this coat, but non-shedding breeds are also popular.

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 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 can be traced back to the Old English docga, a "powerful breed of canine". The term may derive from Proto-Germanic *dukkōn, represented in Old English finger-docce ("finger-muscle"). Due to the linguistically 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.

The English word hound is cognate to other Germanic terms, including German Hund, Dutch hond, common Scandinavian hund, and Icelandic hundur which, though referring to a specific breed group in English, means "dog" in general in the other Germanic languages. Hound itself is derived from the Proto-Indo-European *kwon-, which is also the direct root of the Greek κυων (kuōn) and the indirect root of the Latin canis through the variant form *kani-.

In breeding circles, a male canine is referred to as a dog, while a female is called a bitch. 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. A group of offspring is a litter. The process of birth is whelping. Many terms are used for dogs that are not purebred.

Taxonomy and evolution

The domestic dog was originally classified as Canis familiaris and Canis familiarus domesticus by Linnaeus in 1758. and is currently classified 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.

The current consensus amongst 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. however most believe domestication to have occurred much earlier. The evidence comes from two primary sources: mtDNA studies and archaeological findings, neither of which are conclusive. DNA studies have provided a wide range of possible divergence dates, including 15,000 - 40,000 years and 100,000 - 140,000 years. This evidence depends on a number of assumptions that are likely violated (see discussion below). Thus, the strongest evidence comes from archaeological discoveries, which demonstrate that the domestication of dogs must have occurred prior to 15,000 years ago. This is only a lower limit, as it is possible that dogs were domesticated earlier but showed no immediate morphological differences from wolves.

The archaeological evidence converges with some genetic evidence to indicate 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. The earliest dog fossils, two large crania from Russia and a mandible from Germany, date from roughly 14,000 years ago. Their likely ancestor is the large northern Holarctic wolf (or Eurasian wolf, Canis lupus lupus). Remains of smaller dogs from Mesolithic (Natufian) cave deposits in the Middle East, has been dated to around 10,000 to 12,000 years ago. At this site, the remains of a human was found with its hand resting on the remains of a puppy, suggesting an affectionate relationship. There is a great deal of archealogical evidence for dogs around this period throughout Europe and Asia during the next two thousand years (roughly 8,000 to 10,000 years ago), with fossils being uncovered in Germany, the French Alps, and Iraq, and cave paintings being uncovered in Turkey.

Genetic analyses indicate all dogs are likely descended from a handful of domestication events including a small number of founding females, although there is evidence that domesticated dogs interbred with local populations of wild wolves on several occasions(a process known in genetics as introgression). These data suggest that the first dogs first diverged from wolves in East Asia, and that these domesticated dogs then quickly migrated throughout the world. Dogs that are native to America also stem from this domestication in East Asia, and first appear in fossil records roughly 10,000 years ago. The oldest group 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, Saluki, Afghan, Tibetan Terrier, Lhasa Apso, Chow Chow, Pekingese, Shar-Pei, Shi Tzu, Akita, Shiba Inu, Alaskan Malamute and Siberian Husky and Samoyed. 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 recreated more recently.

There is a great deal of controversy surrounding the evolutionary framework for the domestication of dogs. 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 these 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.

As noted above, care should be taken when considering the genetic evidence for the origins of dogs and dog breeds. These studies depend on a number of assumptions that are likely to be violated. Genetic studies are based in comparisons of genetic diversity between species, and depend on a calibration date, such as the wolf-coyote divergence date, which is estimated to be roughly 1 million years ago. If this divergence date is closer to 750,000 or 2 million years ago, then genetic analyses 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 bias DNA analyses to support an earlier divergence. 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 assumes 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.

Biology

Domestic dogs come in many shapes and sizes because they have been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes. Their morphology, although highly modified, is based on that of their wild ancestors, gray wolves. Modern dog breeds show more variation in size, appearance, and behavior than any other domestic animal. Within the range of extremes, dogs generally share attributes with their wild ancestors, the wolves. Dogs are predators and scavengers, possessing sharp teeth and strong jaws for attacking, holding, and tearing their food. Although selective breeding has changed the appearance of many breeds, all dogs retain basic traits from their distant ancestors. Like many other predatory mammals, the dog has powerful muscles, fused wristbones, 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 dog was a Yorkshire Terrier, who stood only 6.3 cm (2.5 in) at the shoulder, 9.5 cm (3.75 in) in length along the head-and-body, and weighed only 113 grams (4 ounces). The largest known dog was an English Mastiff which weighed 155.6 kg (343 lbs) and was 250 cm (8.2 feet) from the snout to the tail. The tallest dog is a Great Dane that stands 106.7 cm (42.2 in) at the shoulder

Senses

The dog's visual system is engineered to serve the purposes of a hunter. Although difficult to measure, the visual acuity of poodles have been estimated to translate to a Snellen rating of 20/75. Visual discrimination, however, is greatly increased for moving objects. Dogs have been shown to be able to discriminate between humans (i.e., identifying their owner) from distances up to a mile. As crepuscular hunters, dogs rely on their vision in low light situations. To aid their low light vision, dogs have very large pupils, a greater density of rods in the fovea, an increased flicker rate, and a reflective tapetum. The tapetum is a reflective surface behind the retina that reflects light back to give the photoreceptors a second chance to catch the photons. Although these adaptations serve to improve low-light vision, they also serve to impair the dog's visual acuity. Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans. Different breeds of dogs have different eye shapes and dimensions, and they also have different retina configurations. Many 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, while those with short noses 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 dolichocephalic breeds, particularly the sighthounds, have a field of vision up to 270° (compared to 180° for humans), although the brachycephalic, broad-headed breeds with short noses have a much narrower field of vision, as low as 180°. Some breeds also show a genetic predisposition for myopic vision. Although most breeds are memmetropic, one out of two Rottweilers have been found to be myopic.

The frequency range of dog hearing is approximately 40 Hz to 60,000 Hz. Dogs detect sounds as low as the 16 to 20 Hz frequency range (compared to 20 to 70 Hz for humans) and above 45 kHz (compared to 13 to 20 kHz for humans), and in addition have a degree of ear mobility that helps them to rapidly pinpoint the exact location of a sound. Eighteen or more muscles can tilt, rotate, and raise or lower a dog's ear. Additionally, a dog can identify a sound's location much faster than a human can, as well as hear sounds up to four times the distance that humans are able to. Those with more natural ear shapes, like those of wild canids, generally hear better than those with the floppier ears of many domesticated species.

Whereas the human brain is dominated by a large visual cortex, the dog brain is largely 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). Depending on the breed, dogs have between 125 and 220 million smell-sensitive cells over an area about the size of a pocket handkerchief (compared to 5 million over an area the size of a postage stamp for humans). The bloodhound is the exception with nearly 300 million smell-sensitive receptors. Dogs can discriminate odors at concentrations nearly 100 million times lower than humans can.

Physical Characteristics

Domestic dogs, like the wolf, have a coat, a layer of pelage that covers its body. A dog coat may be a double coat, made up of a soft undercoat and a coarse topcoat. Unlike the wolf, dogs may instead have a single coat, without an undercoat. Dogs with a double coat, like the wolf, are adapted to survive in colder temperatures, and tend to originate from colder climates.

Domestic dogs often display the remnants of counter-shading, 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 predators can learn to watch for. A countershaded animal will have dark coloring on its upper surfaces and light coloring below. This reduces the general visibility of the animal. One reminder of this pattern is that many breeds will have the occasional "blaze", stripe, or "star" of white fur on their chest or undersides.

There are many different shapes for dog tails: straight, straight up, sickle, curled, cork-screw. In some breeds, the tail is traditionally docked to avoid injuries (especially for hunting dogs). It can happen that some puppies are born with a short tail or no tail in some breeds. This occurs more frequently for certain breeds, especially those breeds that are frequently docked and thus have no breed standard regarding the tail.

Types and Breeds

All dogs are genetically very similar. However, natural selection and selective breeding has reinforced certain characteristics in specific 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 possesses 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 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 type (e.g., Malamute and Shar-Pei), Mastiff type (e.g., Labrador Retriever), herding type (e.g., St. Bernard), and all others (also called modern or hunting type).

Health

Dogs are susceptible to various diseases, ailments, and poisons, some of which affect humans in the same way, others of which are unique to dogs. Dogs, like all mammals, are also susceptible to heat exhaustion when dealing with high levels of humidity and/or extreme temperatures.

Disorders and Diseases

Some breeds of dogs are also prone to certain genetic ailments, such as hip dysplasia, luxating patellas, cleft palate, blindness, or deafness. Dogs are also susceptible to the same ailments that humans are, including diabetes, epilepsy, cancer, and arthritis. Gastric torsion and bloat is a dangerous problem in some large-chested breeds.

Infectious diseases commonly associated with dogs include rabies (hydrophobia), canine parvovirus, and canine distemper. Inherited diseases of dogs can include a wide range from elbow or hip dysplasia and medial patellar luxation to epilepsy and pulmonic stenosis. Dogs may also be affected by many of the same conditions as humans, such as hypothyroidism, cancer, dental disease, and heart disease. To defend against many common diseases, dogs are often vaccinated.

Two serious medical conditions particularly affecting dogs are pyometra, affecting unspayed females of all types and ages, and bloat, which affects the larger breeds or deep chested dogs. Both of these are acute conditions, and can kill rapidly; owners of dogs which may be at risk should learn about such conditions as part of good animal care.

Common external parasites are various species of fleas, ticks, and mites. Internal parasites include hookworms, tapeworms, roundworms, and heartworms. See also CVBD (Canine Vector-Borne Diseases).

Mortality

The typical lifespan of dogs varies widely among breeds. The median longevity of most dog breeds is between 10 and 13 years. The breed with the dubious distinction of the shortest lifespan (among breeds for which there is a questionnaire survey with a reasonable sample size) is the Dogue de Bordeaux with a median longevity of about 5.2 years, but several breeds, including Miniature Bull Terrier, Bulldog, Nova Scotia Duck-Tolling Retriever, Bloodhound, Irish Wolfhound, Greater Swiss Mountain Dog, Great Dane, and Mastiff, are nearly as short-lived, with median longevities between 6 and 7 years. On the other end of the spectrum, the longest-lived breeds, including Toy Poodle, Border Terrier, Miniature Dachshund, Miniature Poodle, and Tibetan Spaniel, have median longevities between 14 and 15 years. The median longevity of mixed breed dogs (average of all sizes) is one or more years longer than that of purebred dogs (all breeds averaged). As a rule of thumb, small breeds are longer-lived than large breeds, but some of the longest lived large breeds have median longevities nearly as long as those of the shortest lived small breeds, and some of the breeds with the shortest longevities are medium-sized.

"Median longevity" refers to the age at which half the dogs in a population have died and half are still alive. Individual dogs, even in breeds with low median longevities, may live well beyond the median. The dog widely reported to be the longest-lived on record is "Bluey," purportedly born in 1910 in Australia. He died in 1939 at the age of 29.5 years. Bluey is usually identified as an Australian Cattle Dog, but the first Australian Cattle Dog breed standard was written in 1902, only eight years before Bluey's birth. It is unclear how closely Bluey was related to the breed as it exists today. The Bluey record is anecdotal and unverified. The longest verified records are of dogs living to 24 years.

Predation

Although wild dogs, like wolves, are apex predators, they can be killed in territory disputes with wild animals. Furthermore, in areas where dogs and other large predators are sympatric, dogs can be a major food source for big cats or canines. Reports from Croatia indicate that dogs are killed more frequently than sheep. Wolves in Russia apparently limit feral dog populations. In Wisconsin, more compensation has been paid for dog losses than livestock. Some wolf pairs have been reported to prey on dogs by having one wolf lure the dog out into heavy brush where the second animal waits in ambush. In some instances, wolves have displayed an uncharacteristic fearlessness of humans and buildings when attacking dogs, to the extent that they have to be beaten off or killed. Coyotes have also been known to attack dogs. Big cats have been recorded to kill dogs. Leopards in particular are known to have a predilection for dogs, and have been recorded to kill and consume them regardless of the dog's size or ferocity. Unlike sympatric leopards, tigers in India seldom prey on dogs, though in Manchuria, Indochina, Indonesia, and Malaysia, tigers are reputed to kill dogs with the same vigour as leopards. Additionally, Striped Hyenas are major predators of village dogs in Turkmenistan, India, and the Caucasus.

Diet

Nutrition

Despite descending from wolves, the domestic dog is classified as an omnivore. The classification in the Order Carnivora does not necessarily mean that a dog's diet must be restricted to meat; unlike an obligate carnivore, such as the cat family with its shorter small intestine, a dog is neither dependent on meat-specific protein nor a very high level of protein in order to fulfill its basic dietary requirements. Dogs are able to healthily digest a variety of foods including vegetables and grains, and in fact dogs can consume a large proportion of these in their diet. In the wild, canines often eat available plants and fruits.

A dog's diet should consist of balanced proportions of proteins, carbohydrates, fats, and water. The average size dog requires about 30 calories per pound of body weight per day, whereas larger breeds may require only 20 calories per pound of weight and smaller breeds require 40. Puppies have greater nutritional demands than older dogs. They need twice as much protein and 50% more calories per pound of body weight. Dogs can survive a long time without food, and are capable of losing up to 40% of their body weight without dying. In contrast, a 15% water loss could be fatal. All-meat diets may not be recommended for dogs as they lack vital components of a healthy diet.

Domestic dogs can survive healthily on a reasonable and carefully designed vegetarian diet, particularly if eggs and milk products are included, although some sources suggest that a dog fed on a strict vegetarian diet without L-carnitine may develop dilated cardiomyopathy, However, L-carnitine is found in many nuts, seeds, beans, vegetables, fruits, and whole grains. In the wild, dogs can survive on a vegetarian diet when animal prey is not available. Observation of extremely stressful conditions such as the Iditarod Trail Sled Dog Race, and scientific studies of similar conditions has shown that high-protein (approximately 40%) diets including meat help prevent damage to muscle tissue in dogs and some other mammals. This level of protein corresponds to the percentage of protein found in the wild dog's diet when prey is abundant; higher levels of protein seem to confer no added benefit.

Dangerous substances

Some foods commonly enjoyed by humans are dangerous to dogs, including chocolate (Theobromine poisoning), onions, grapes and raisins, some types of gum, certain sweeteners, and Macadamia nuts.

• Chocolate can contain high amounts of fat and caffeine-like stimulants known as methylxanthines which, ingested in significant amounts, can potentially produce clinical effects in dogs ranging from vomiting and diarrhea to panting, excessive thirst and urination, hyperactivity, abnormal heart rhythm, tremors, seizures, and even death in severe cases. Typically, the darker the chocolate, the higher the potential for clinical problems from methylxanthine poisoning. As little as 20 oz (570 g) of milk chocolate—or only 2 oz (57 g) of baking chocolate—can cause serious problems in a 10-pound (4.5 kg) dog. White chocolate may not have the same potential as darker forms to cause a methylxanthine poisoning, but the high fat content of lighter chocolates could still lead to vomiting and diarrhea, as well as the possible development of life-threatening pancreatitis.
• The acute danger from grapes and raisins was discovered around 2000, and has slowly been publicized since then. The cause is not known. Small quantities will induce acute renal failure. Sultanas and currants may also be dangerous.
• Xylitol, a sugar substitute found in a variety of sugar-free and dietetic cookies, mints, and chewing gum is proving highly toxic, even fatal, to dogs.
• A toxic dose of roasted macadamia nuts may be as little as one nut per kilogram of body weight in the dog.
• Alcoholic beverages pose comparable hazards to dogs as they do to humans, but due to low body weight and lack of alcohol tolerance they are toxic in much smaller portions. Signs of alcohol intoxication in pets may include vomiting, wobbly gait, depression, disorientation, and/or hypothermia. High doses may result in heart arrhythmias, seizures, tremors, and even death.

Plants such as caladium, dieffenbachia, and philodendron will cause throat irritations that will burn the throat going down as well as coming up. Hops are particularly dangerous and even small quantities can lead to malignant hyperthermia. Amaryllis, daffodil, English ivy, iris, and tulip (especially the bulbs) causes gastric irritation and sometimes central nervous system excitement followed by coma, and, in severe cases, even death. Ingesting foxglove, Lily of the Valley, larkspur, and oleander can be life threatening because the cardiovascular system is affected. Yew is very dangerous because it affects the nervous system. Immediate veterinary treatment is required for dogs that ingest these.

Many household cleaners such as ammonia, bleach, disinfectants, drain cleaner, soaps, detergents and other cleaners, mothballs, and matches are dangerous to dogs, as are cosmetics such as deodorants, hair coloring, nail polish and remover, home permanent lotion, and suntan lotion. Dogs find some poisons attractive, such as antifreeze, slug and snail bait, insect bait, and rodent poisons. Antifreeze is insidious to dogs, either puddled or even partly cleaned residue, because of its sweet taste. A dog may pick up antifreeze on its fur and then lick it off.

Dogs occasionally eat their own feces, or the feces of other dogs and other species if available, such as cats, deer, cows, or horses. This is known as coprophagia. Some dogs develop preferences for one type over another. There is no definitive reason known, although boredom, hunger, and nutritional needs have been suggested. Eating cat feces is common, possibly because of the high protein content of cat food. Dogs eating cat feces from a litter box may lead to Toxoplasmosis. Dogs seem to have different preferences in relation to eating feces. Some are attracted to the stools of deer, cows, or horses.

Human medications may be toxic to dogs, for example paracetamol/acetaminophen (Tylenol). Zinc toxicity, mostly in the form of the ingestion of US cents minted after 1982, is commonly fatal in dogs where it causes a severe hemolytic anemia. Some wet dog and cat food was recalled by Menu Foods in 2007 because it contained a dangerous substance.

Reproduction

In domestic dogs, sexual maturity (puberty) begins to happen around age 6 to 12 months for both males and females, although this can be delayed until up to two years old for some large breeds. Adolescence for most domestic dogs is around 12 to 15 months, beyond which they are for the most part more adult than puppy. As with other domesticated species, domestication has selectively bred for higher libido and earlier and more frequent breeding cycles in dogs, than in their wild ancestors. Dogs remain reproductively active until old age.

Most female dogs have their first estrous cycle between 6 and 12 months, although some larger breeds delay until as late as 2 years. Females experience estrous cycles biannually, during which her body prepares for pregnancy, and at the peak she will come into estrus, during which time she will be mentally and physically receptive to copulation. Because the ova survive and are capable of being fertilized for a week after ovulation, it is possible for a female to mate with more that one male.

Dogs bear their litters roughly 56 to 72 days after fertilization, with an average of 63 days, although the length of gestation can vary. An average litter consists of about six puppies, though this number may vary widely based on the breed of dog. Toy dogs generally produce from one to four puppies in each litter, while much larger breeds may average as many as 12 pups in each litter.

Some dog breeds have acquired traits through selective breeding that interfere with reproduction. Male French Bulldogs, for instance, are incapable of mounting the female. For many dogs of this breed, the female must be artificially inseminated in order to reproduce.

Spaying and neutering

Neutering (spaying females and castrating males) refers to the sterilization of animals, usually by removal of the male's testicles or the female's ovaries and uterus, in order to eliminate the ability to procreate and reduce sex drive. Because of the overpopulation of dogs in some countries, animal control agencies, such as the ASPCA, advise that dogs not intended for further breeding should be neutered, so that they do not have undesired puppies that may have to be destroyed later.

Neutering has benefits in addition to eliminating the ability to procreate. Neutering reduces problems caused by hyper sexuality, especially in male dogs. Spayed female dogs are less likely to develop some forms of cancer, affecting mammary glands, ovaries and other reproductive organs.

Neutering can also have undesirable medical effects on the animal. Neutering increases the risk of urinary incontinence in females and prostate cancer in males, as well as osteosarcoma, hemangiosarcoma, cruciate ligament rupture, obesity, and diabetes mellitus in either gender. The hormonal changes involved with sterilization are likely to somewhat change the animal's personality, along with metabolism. Recent studies proved that spayed and neutered dogs in general are more aggressive towards people and other dogs, as well as more fearful and sensitive to touch than dogs than had not been sterilized, though individual effects may vary.

Spaying or neutering very young animals, also known as early-age spay, can result in increased health concerns later on in life for both sexes. Incontinence in female dogs is made worse by spaying too early. In both males and females, alteration causes changes in hormones during development. This inhibits the natural signals needed for proper body development, leading to larger animals with greater risk for hip dysplasia, osteoporosis and other joint disorders. However other studies have shown that early-age neutering of male dogs is associated with no major risks compared to neutering at the more traditional age of six months. These findings suggest that although male dogs can be altered even before three months, female dogs should be spayed closer to six months.

Overpopulation

According to the Humane Society of the United States, 3–4 million dogs and cats are put down each year in the United States and many more are confined to cages in shelters because there are many more animals than there are homes. Spaying or castrating dogs helps keep overpopulation down. Local humane societies, SPCAs and other animal protection organizations urge people to neuter their pets and to adopt animals from shelters instead of purchasing them. Several notable public figures have spoken out against animal overpopulation, including Bob Barker. On his game show, The Price Is Right, Barker stressed the problem at the end of every episode, saying: "Help control the pet population. Have your pets spayed or neutered." The current host, Drew Carey, makes a similar plea at the conclusion of each episode.

Behavior and intelligence

Although dogs have been the subject of a great deal of Behaviorist psychology (e.g., Pavlov's Dog), they do not enter the world with a psychological "blank slate". Rather, dog behavior is affected by genetic factors as well as environmental factors. Domestic dogs exhibit a number of behaviors and predispositions that were inherited from wolves. The grey wolf is a social animal that has evolved a sophisticated means of communication and social structure. The domestic dog has inherited some of these predispositions, but many of the salient characteristics in dog behavior have been largely shaped by selective breeding by humans. Thus, some of these characteristics, such as the dog's highly developed social cognition, are found only in primitive forms in grey wolves.

Intelligence

Intelligence is an umbrella term that encompasses the faculties involved in a wide range of mental tasks, such as learning, problem-solving, communication. The domestic dog has a predisposition to exhibit a social intelligence that is uncommon in the animal world. Dogs, like humans, are capable of learning in a number of ways. In addition to simple reinforcement learning (e.g., classical or operant conditioning), dogs are capable of learning by observation.

For instance, handlers of working dogs such as herding dogs and sled dogs have long known that pups learn behaviors quickly by following examples set by experienced dogs. Many of these behaviors are allelomimetic in that they depend on an genetic predisposition to learn and imitate behaviors of other dogs. This form of intelligence is not peculiar to those tasks dogs have been bred to perform, but can be generalized to myriad abstract problems. Adler & Alder demonstrated this by giving Dachshund puppies the task of learning to pull a cart by tugging on an attached piece of ribbon in order to get a reward in the cart. Puppies that watched an experienced dog successfully retrieve the reward in this way learned the task fifteen times faster than those who were left to solve the problem on their own. In addition to learning by example from other dogs, dogs have also been shown to learn by mimicking human behaviors. In another study, puppies were presented with a box, and shown that when a handler pressed a lever, a ball would roll out of the box. The handler then allowed the puppy to play with the ball, making it an intrinsic reward. The pups were then allowed to interact with the box. Roughly three quarters of puppies subsequently touched the lever, and over half successfully released the ball. This is compared to only 6 percent in a control group that did not watch the human manipulate the lever. Furthermore, the ability for dogs to learn by example has been shown to be as effective operant conditioning. McKinley and Young have demonstrated that dogs show equivalent accuracy and learning times when taught to identify an object by operant conditioning as they do when taught by human example. In this study, it was found that handing an object between experimenters who then use its name in a sentence successfully taught an observing dog each objects name, allowing them to subsequently retrieve the item.

Studies have shown that the learning to mimic other dogs or humans is not limited to classical conditioning. Rather, dogs demonstrate a sophisticated social cognition, by associating behavioral cues with abstract meanings. One such class of cognition that involves the understanding that others are conscious agents, often referred to as theory of mind, is an area where dogs excel. Research has shown that dogs are capable of interpreting subtle social cues, and appear to recognize when a human or dog's attention is focused on them. For instance, researchers devised a task in which a reward was hidden under one of two buckets. The experimenter then attempted to communicate with the dog to indicate the location of the reward by using a wide range of signals: tapping the bucket, pointing to the bucket, nodding to the bucket or simply saccading to the bucket. Results showed that domestic dogs were better than chimpanzees, wolves and even human infants at this task, even when the experimenter indicated the location of the reward with only their eyes. Further, even young puppies with limited exposure to humans performed well on this task. These results demonstrate that the social cognition of dogs can exceed that of even our closest genetic relatives, and that this capacity is a recent genetic acquisition that distinguishes the dog from its ancestor, the wolf. Studies have also investigated whether dogs engaged in dyadic play change their behavior depending on the attentional state of their partner. These studies show that play signals were only sent when the dog was holding the attention of its partner. If the partner was distracted, the dog instead engaged in attention-getting behavior before sending a play signal. Dr. Stanley Coren, an expert on the subject of dog psychology, has argued that dogs demonstrate a sophisticated theory of mind by engaging in deception. Due to the difficulty in producing deceptive behavior in the lab, Coren supports this position with a number of anecdotes, including one example where a dog hid a stolen treat by sitting on it until the rightful owner of the treat left the room. Although this could have been accidental, Coren suggests that the thief understood that the treat's owner would be unable to find the treat if it were out of view. Together, the empirical data and anecdotal evidence points to dogs possessing at least a limited form of theory of mind.

Development

Like human children, dogs go through a series of stages of cognitive development. For instance, dogs, like humans, are not born with an understanding of object permanence, the understanding that objects that are not being actively perceived remain in existence. In both species, an understanding of object permanence occurs during as the infant is learning the coordination of secondary circular reactions, as described by Jean Piaget. That is, as the infants learn to interact intentionally with objects around it. For dogs, this occurs at roughly 8 weeks of age (as compared to 18 months in human children)

Interactions with humans

Domestic dogs inherited a complex social hierarchy and behaviors from their ancestor, the wolf. Dogs are pack animals with a complex set of behaviors related to determining each dog's position in the social hierarchy. Dogs 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. 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. For many groups, the dog was the only domesticated animal. Dogs were used by Athabascan emmigrants again 1,400 years ago, when they carried much of the load in the migration of the Apache and Navajo tribes. Use of dogs as pack animals in these cultures often persisted after the introduction of the horse to North America.

Dogs have lived and worked with humans in so many roles that they have earned the unique sobriquet "man's best friend", a term which is used in other languages as well.

Work and sport

Dogs have traditionally been used for a variety of tasks since their domestication by early man. Dogs have been bred for herding livestock, different kinds of hunting (e.g., pointers, hounds), keeping living spaces clear of rats, guarding, helping fishermen with nets, and pulling loads in addition to their roles as companions.

More recently, many dogs have taken on a number of roles under the general classification of service dogs. Service dogs provide assistance to individuals with disabilities (either physical or mental). These roles include guide dogs, utility dogs, assistance dogs, hearing dogs and psychological therapy dogs. Some dogs have even been shown to alert their handler when the handler shows signs of an impending seizure. Some can achieve this well in advance of the onset of the seizure, allowing the owner to seek safety, medication or medical care.

Owners of dogs often enter them in competitions, whether show (breed conformation shows) or sports, including dog racing and dog sledding.

Conformation shows, also referred to as breed shows, are a kind of dog show in which a judge familiar with a specific dog breed evaluates individual purebred dogs for how well the dogs conform to the established breed type for their breed, as described in a breed's individual breed standard. As the breed standard has only to do with the externally observable qualities of the dog such as appearance, movement, and temperament, separately tested for qualities such as tests for ability in specific work or dog sports, tests for genetic health, tests for general health or specific tests for inherited disease, or any other specific tests for characteristics that cannot be directly observed, are not part of the judging in conformation shows.

Differences from wolves

Physical characteristics

Compared to equally sized wolves, dogs tend to have 20% smaller skulls and 10% smaller brains, as well as proportionately smaller teeth than other canid species. Dogs require fewer calories to function than wolves. Their diet of human refuse in antiquity made the large brains and jaw muscles needed for hunting unnecessary. It is thought by certain experts that the dog's limp ears are a result of atrophy of the jaw muscles. The skin of domestic dogs tends to be thicker than that of wolves, with some Inuit tribes favoring the former for use as clothing due to its greater resistance to wear and tear in harsh weather. Unlike wolves, but like coyotes, domestic dogs have sweat glands on their paw pads. The paws of a dog are half the size of those of a wolf, and their tails tend to curl upwards, another trait not found in wolves.

Behavior

Dogs tend to be poorer than wolves and coyotes at observational learning, being more responsive to instrumental conditioning. Feral dogs show little of the complex social structure or dominance hierarchy present in wolf packs. For dogs, other members of their kind are of no help in locating food items, and are more like competitors. Feral dogs are primarily scavengers, with studies showing that unlike their wild cousins, they are poor ungulate hunters, having little impact on wildlife populations where they are sympatric. However, feral dogs have been reported to be effective hunters of reptiles in the Galapagos islands, and free ranging pet dogs are more prone to predatory behavior toward wild animals.

Despite common belief, domestic dogs can be monogamous. Breeding in feral packs can be, but does not have to be restricted to a dominant alpha pair (despite common belief, such things also occur in wolf packs). Male dogs are unusual among canids by the fact that they mostly seem to play no role in raising their puppies, and do not kill the young of other females to increase their own reproductive success. Some sources say that dogs differ from wolves and most other large canid species by the fact that they do not regurgitate food for their young, nor the young of other dogs in the same territory. However, this difference was not observed in all domestic dogs. Regurgitating of food by the females for the young as well as care for the young by the males has been observed in domestic dogs, dingos as well as in other feral or semi-feral dogs. Regurgitating of food by the females and direct choosing of only one mate has been observed even in those semi-feral dogs of direct domestic dog ancestry. Also regurgitating of food by males has been observed in free-ranging domestic dogs.

Trainability

Dogs display much greater tractability than tame wolves, and are generally much more responsive to coercive techniques involving fear, aversive stimuli and force than wolves, which are most responsive toward positive conditioning and rewards. Unlike tame wolves, dogs tend to respond more to voice than hand signals. Although they are less difficult to control than wolves, they can be comparatively more difficult to teach than a motivated wolf.

See also

• Bark (dog)
• Dog king – Scandinavian tradition
• Dog licence
• Dog odor
• Dog paddle – basic swimming stroke
• Dog park
• Fear of dogs
• List of dog breeds
• List of dogs
• List of fictional dogs
• List of most popular dog breeds
• Subspecies of Canis lupus
• Wolf-dog hybrid

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Further reading

• Abrantes, Roger (1999). Dogs Home Alone. Wakan Tanka, 46 pages. ISBN 0-9660484-2-3 (paperback).
• A&E Television Networks (1998). Big Dogs, Little Dogs: The companion volume to the A&E special presentation, A Lookout Book, GT Publishing. ISBN 1-57719-353-9 (hardcover).
• Alderton, David (1984). The Dog, Chartwell Books. ISBN 0-89009-786-0.
• Bloch, Günther. Die Pizza-Hunde (in German), 2007, Franckh-Kosmos-Verlags GmbH & Co. KG, Stuttgart, ISBN 9783440109861
• Brewer, Douglas J. (2002) Dogs in Antiquity: Anubis to Cerberus: The Origins of the Domestic Dog, Aris & Phillips ISBN 0-85668-704-9
• Coppinger, Raymond and Lorna Coppinger (2002). Dogs: A New Understanding of Canine Origin, Behavior and Evolution, University of Chicago Press ISBN 0-226-11563-1
• Cunliffe, Juliette (2004). The Encyclopedia of Dog Breeds. Parragon Publishing. ISBN 0-7525-8276-3.
• Derr, Mark (2004). Dog's Best Friend: Annals of the Dog-Human Relationship. University of Chicago Press. ISBN 0-226-14280-9
• Donaldson, Jean (1997). The Culture Clash. James & Kenneth Publishers. ISBN 1-888047-05-4 (paperback).
• Fogle, Bruce, DVM (2000). The New Encyclopedia of the Dog. Doring Kindersley (DK). ISBN 0-7894-6130-7.
• Grenier, Roger (2000). The Difficulty of Being a Dog. University of Chicago Press. ISBN 0-226-30828-6
• Milani, Myrna M. (1986). The Body Language and Emotion of Dogs: A practical guide to the Physical and Behavioral Displays Owners and Dogs Exchange and How to Use Them to Create a Lasting Bond, William Morrow, 283 pages. ISBN 0-688-12841-6 (trade paperback).
• Pfaffenberger, Clare (1971). New Knowledge of Dog Behavior. Wiley, ISBN 0-87605-704-0 (hardcover); Dogwise Publications, 2001, 208 pages, ISBN 1-929242-04-2 (paperback).
• Shook, Larry (1995). "Breeders Can Hazardous to Health", The Puppy Report: How to Select a Healthy, Happy Dog, Chapter Two, pp. 13–34. Ballantine, 130 pages, ISBN 0-345-38439-3 (mass market paperback); Globe Pequot, 1992, ISBN 1-55821-140-3 (hardcover; this is much cheaper should you buy).
• Shook, Larry (1995). The Puppy Report: How to Select a Healthy, Happy Dog, Chapter Four, "Hereditary Problems in Purebred Dogs", pp. 57–72. Ballantine, 130 pages, ISBN 0-345-38439-3 (mass market paperback); Globe Pequot, 1992, ISBN 1-55821-140-3 (hardcover; this is much cheaper should you buy).
• Thomas, Elizabeth Marshall (1993). The Hidden Life of Dogs (hardcover), A Peter Davison Book, Houghton Mifflin. ISBN 0-395-66958-8.
• Trumler, Eberhard. Mit dem Hund auf du; Zum Verständnis seines Wesens und Verhaltens (in German); 4. Auflage Januar 1996; R. Piper GmbH & Co. KG, München
• Small animal internal medicine, RW Nelson, Couto page 107

External links

• American Kennel Club
• Australian National Kennel Club
• Biodiversity Heritage Library bibliography for Canis lupus familiaris
• Canadian Kennel Club
• Fédération Cynologique Internationale (FCI) - World Canine Organisation
• New Zealand Kennel Club
• The Kennel Club (UK)

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• Domesticated animals
• Dogs
• Cosmopolitan species
• Scavengers