Experts estimate that due to their long association with humans, dogs have gained the ability to thrive on a starch-rich diet that would be inadequate for other canids.

They have the same number of bones (with the exception of the tail), powerful jaws that house around 42 teeth, and well-developed senses of smell, hearing, and sight.

Compared to humans, dogs have an inferior visual acuity, a superior sense of smell, and a relatively large olfactory cortex.

They perform many roles for humans, such as hunting, herding, pulling loads, protection, companionship, therapy, aiding disabled people, and assisting police and the military.

[2] Linnaeus considered the dog to be a separate species from the wolf because of its upturning tail (cauda recurvata in Latin term), which is not found in any other canid.

[16] In 2019, a workshop hosted by the IUCN/Species Survival Commission's Canid Specialist Group considered the dingo and the New Guinea singing dog to be feral Canis familiaris.

[20] Genetic studies suggest a domestication process commencing over 25,000 years ago, in one or several wolf populations in either Europe, the high Arctic, or eastern Asia.

As the differences in these genes could also be found in ancient dog fossils, these were regarded as being the result of the initial domestication and not from recent breed formation.

The skull, body, and limb proportions between breeds display more phenotypic diversity than can be found within the entire order of carnivores.

These breeds possess distinct traits related to morphology, which include body size, skull shape, tail phenotype, fur type, and colour.

[39] They have difficulty differentiating between red and green,[40] and much like other mammals, the dog's eye is composed of two types of cone cells compared to the human's three.

[47] Dogs have stiff, deeply embedded hairs known as whiskers that sense atmospheric changes, vibrations, and objects not visible in low light conditions.

The lower most part of whiskers hold more receptor cells than other hair types, which help in alerting dogs of objects that could collide with the nose, ears, and jaw.

[63] Common dog parasites are lice, fleas, fly larvae, ticks, mites, cestodes, nematodes, and coccidia.

This is the time at which female dogs will have their first estrous cycle, characterized by their vulvas swelling and producing discharges, usually lasting between 4 and 20 days.

[89] However, neutering increases the risk of urinary incontinence in bitches,[90] prostate cancer in dogs,[91] and osteosarcoma, hemangiosarcoma, cruciate ligament rupture, pyometra, obesity, and diabetes mellitus in either sex.

[93] The American Society for the Prevention of Cruelty to Animals (ASPCA) advises that dogs not intended for further breeding should be neutered so that they do not have undesired puppies that may later be euthanized.

[94][95] However, the Society for Theriogenology and the American College of Theriogenologists made a joint statement that opposes mandatory neutering; they said that the cause of overpopulation in the US is cultural.

[108][105] Traits of high sociability and lack of fear in dogs may include genetic modifications related to Williams-Beuren syndrome in humans, which cause hypersociability at the expense of problem-solving ability.

[119] The female dog may produce colostrum, a type of milk high in nutrients and antibodies, 1–7 days before giving birth.

With their acute sense of hearing, dogs rely on the auditory aspect of communication for understanding and responding to various cues, including the distinctive barking patterns that convey different messages.

[82][165][166] Compared to wolves, dogs from agricultural societies have extra copies of amylase and other genes involved in starch digestion that contribute to an increased ability to thrive on a starch-rich diet.

[169] Of the twenty-one amino acids common to all life forms (including selenocysteine), dogs cannot synthesize ten: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Starting practice in December 1993, dogs were later outlawed by the Protocol on Environmental Protection to the Antarctic Treaty international agreement due to the possible risk of spreading infections.

[190][191] Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and the military, companionship, and aiding disabled individuals.

[7] In 2018, the South Korean government passed a bill banning restaurants that sell dog meat from doing so during that year's Winter Olympics.

[262] Earlier studies have shown that pet-dog or -cat guardians make fewer hospital visits and are less likely to be on medication for heart problems and sleeping difficulties than non-guardians.

A 2005 paper by the British Medical Journal states:[259] 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.

For example, when in a pet dog's presence, people show reductions in cardiovascular, behavioral, and psychological indicators of anxiety[266] and are exposed to immune-stimulating microorganisms, which can protect against allergies and autoimmune diseases (according to the hygiene hypothesis).

[275] In Hindu mythology, Yama, the god of death, owns two watchdogs named Shyama and Sharvara, which each have four eyes—they are said to watch over the gates of Naraka.