In a study of seven breeds of dogs (Bernese mountain dog, basset hound, Cairn terrier, Epagneul Breton, German Shepherd dog, Leonberger, and West Highland white terrier) it was found that inbreeding decreases litter size and survival.[84] Another analysis of data on 42,855 dachshund litters found that as the inbreeding coefficient increased, litter size decreased and the percentage of stillborn puppies increased, thus indicating inbreeding depression.[85] In a study of boxer litters, 22% of puppies died before reaching 7 weeks of age.[86] Stillbirth was the most frequent cause of death, followed by infection. Mortality due to infection increased significantly with increases in inbreeding.[86]
In 1999, a study of mitochondrial DNA indicated that the domestic dog may have originated from multiple grey wolf populations, with the dingo and New Guinea singing dog "breeds" having developed at a time when human populations were more isolated from each other.[23] In the third edition of Mammal Species of the World published in 2005, the mammalogist W. Christopher Wozencraft listed under the wolf Canis lupus its wild subspecies, and proposed two additional subspecies: "familiaris Linneaus, 1758 [domestic dog]" and "dingo Meyer, 1793 [domestic dog]". Wozencraft included hallstromi – the New Guinea singing dog – as a taxonomic synonym for the dingo. Wozencraft referred to the mDNA study as one of the guides in forming his decision.[1] The inclusion of familiaris and dingo under a "domestic dog" clade has been noted by other mammalogists.[24] This classification by Wozencraft is debated among zoologists.[25]
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.[174]
In domestic dogs, sexual maturity happens around six to twelve months of age for both males and females,[17][68] although this can be delayed until up to two years old for some large breeds. This is the time at which female dogs will have their first estrous cycle. They will experience subsequent estrous cycles semiannually, during which the body prepares for pregnancy. At the peak of the cycle, females will come into estrus, being mentally and physically receptive to copulation.[17] Because the ova survive and are capable of being fertilized for a week after ovulation, it is possible for more than one male to sire the same litter.[17]

It has been suggested that the most significant benefit would have been the use of dogs' robust sense of smell to assist with the hunt.[136] 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.[137]
A 2005 paper states "recent research has failed to support earlier findings that pet ownership is associated with a reduced risk of cardiovascular disease, a reduced use of general practitioner services, or any psychological or physical benefits on health for community dwelling older people. Research has, however, pointed to significantly less absenteeism from school through sickness among children who live with pets."[191] In one study, new guardians reported a highly significant reduction in minor health problems during the first month following pet acquisition, and this effect was sustained in those with dogs through to the end of the study.[195]
The domestic dog is the first species, and the only large carnivore, known to have been domesticated. Especially over the past 200 years, dogs have undergone rapid phenotypic change and were formed into today's modern dog breeds due to artificial selection by humans. These breeds can vary in size and weight from a 0.46 kg (1.0 lb) teacup poodle to a 90 kg (200 lb) giant mastiff. Phenotypic variation can include height measured to the withers ranging from 15.2 centimetres (6.0 in) in the Chihuahua to 76 cm (30 in) 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.[131] The skull, body, and limb proportions vary significantly between breeds, with dogs displaying more phenotypic diversity than can be found within the entire order of carnivores. Some breeds demonstrate outstanding skills in herding, retrieving, scent detection, and guarding, which demonstrates the functional and behavioral diversity of dogs. The first dogs were domesticated from shared ancestors of modern wolves, however the phenotypic changes that coincided with the dog–wolf genetic divergence are not known.[26]
In mythology, dogs often serve as pets or as watchdogs.[208] Stories of dogs guarding the gates of the underworld recur throughout Indo-European mythologies[210][211] and may originate from Proto-Indo-European religion.[210][211] In Greek mythology, Cerberus is a three-headed watchdog who guards the gates of Hades.[208] In Norse mythology, a bloody, four-eyed dog called Garmr guards Helheim.[208] In Persian mythology, two four-eyed dogs guard the Chinvat Bridge.[208] In Welsh mythology, Annwn is guarded by Cŵn Annwn.[208] In Hindu mythology, Yama, the god of death, owns two watch dogs who have four eyes. They are said to watch over the gates of Naraka.[212]
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.[201] Animal-assisted intervention research has shown that animal-assisted therapy with a dog can increase social behaviors, such as smiling and laughing, among people with Alzheimer's disease.[202] 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 with those who were not in an animal-assisted program.[203]
Dogs bear their litters roughly 58 to 68 days after fertilization,[17][71] with an average of 63 days, although the length of gestation can vary. An average litter consists of about six puppies,[72] though this number may vary widely based on the breed of dog. In general, toy dogs produce from one to four puppies in each litter, while much larger breeds may average as many as twelve.
Experienced pet stylists hired from outside the company must complete a personal skill evaluation by a Petco Certified Pet Stylist or Grooming Salon Leader, as well as four weeks of safety and pet grooming operations training. They must also complete and pass a technical skills assessment within 14 days of hire. If the candidate does not pass the technical evaluations, they must complete Petco's 12-week Pet Stylist Apprentice & Certification program and retake the skills assessment.