The cutest dog in the world

Main article: Dog anatomy
Skeleton

A lateral view of a dog skeleton
All healthy dogs, regardless of their size and type, have an identical skeletal structure with the exception of the number of bones in the tail, although there is significant skeletal variation between dogs of different types.[27][28] The dog’s skeleton is well adapted for running; the vertebrae on the neck and back have extensions for powerful back muscles to connect to, the long ribs provide plenty of room for the heart and lungs, and the shoulders are unattached to the skeleton allowing great flexibility.

Compared to the dog's wolf-like ancestors, selective breeding since domestication has seen the dog’s skeleton greatly enhanced in size for larger types as mastiffs and miniaturised for smaller types such as terriers; dwarfism has been selectively utilised for some types where short legs are advantageous such as dachshunds and corgis.[28] Most dogs naturally have 26 vertebrae in their tails, but some with naturally short tails have as few as three.

The dog's skull has identical components regardless of breed type, but there is significant divergence in terms of skull shape between types.[28][29] The three basic skull shapes are the elongated dolichocephalic type as seen in sighthounds, the intermediate mesocephalic or mesaticephalic type, and the very short and broad brachycephalic type exemplified by mastiff type skulls.

Senses
Further information: Dog anatomy § Senses
A dog's senses include vision, hearing, smell, taste, touch, and sensitivity to Earth's magnetic field. Another study has suggested that dogs can see Earth's magnetic field.

Coat
Main article: Coat (dog)

Dogs display wide variation in coat type, density, length, color, and composition
The coats of domestic dogs are of two varieties: "double" being familiar with dogs (as well as wolves) originating from colder climates, made up of a coarse guard hair and a soft down hair, or "single," with the topcoat only. Breeds may have an occasional "blaze," stripe, or "star" of white fur on their chest or underside.[31] Premature graying can occur in dogs from as early as one year of age; this is associated with impulsive behaviors, anxiety behaviors, fear of noise, and fear of unfamiliar people or animals.

Tail
There are many different shapes for dog tails: straight, straight up, sickle, curled, or corkscrew. As with many canids, one of the primary functions of a dog's tail is to communicate their emotional state, which can be crucial in getting along with others. In some hunting dogs the tail is traditionally docked to avoid injuries.

Health
Main article: Dog health
Some breeds of dogs are prone to specific genetic ailments such as elbow and hip dysplasia, blindness, deafness, pulmonic stenosis, cleft palate, and trick knees. Two severe medical conditions significantly affecting dogs are pyometra, affecting unspayed females of all breeds and ages, and Gastric dilatation volvulus (bloat), which affects larger breeds or deep-chested dogs. Both of these are acute conditions and can kill rapidly. Dogs are also susceptible to parasites such as fleas, ticks, mites, hookworms, tapeworms, roundworms, and heartworms, which is a roundworm species that lives in the hearts of dogs.

Several human foods and household ingestibles are toxic to dogs, including chocolate solids, causing theobromine poisoning, onions and garlic, causing thiosulphate, sulfoxide or disulfide poisoning, grapes and raisins, macadamia nuts, and xylitol.[33] The nicotine in tobacco can also be dangerous to dogs. Signs of ingestion can include copious vomiting (e.g., from eating cigar butts) or diarrhea. Some other symptoms are abdominal pain, loss of coordination, collapse, or death.
needed]

Dogs are also vulnerable to some of the same health conditions as humans, including diabetes, dental and heart disease, epilepsy, cancer, hypothyroidism, and arthritis.[citation needed]

Lifespan
Further information: Aging in dogs
The typical lifespan of dogs varies widely among breeds, but for most, the median longevity (the age at which half the dogs in a population have died and half are still alive) ranges from 10 to 13 years.[35][36] The median longevity of mixed-breed dogs, taken as an average of all sizes, is one or more years longer than that of purebred dogs when all breeds are averaged.[35][36][37] For dogs in England, increased body weight has been found to be negatively correlated with longevity (i.e., the heavier the dog, the shorter its lifespan), and mixed-breed dogs live on average 1.2 years longer than purebred dogs.

Reproduction
Main article: Canine reproduction

A female dog nursing newborn puppies
In domestic dogs, sexual maturity happens around six months to one year for both males and females, although this can be delayed until up to two years of age for some large breeds, and 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 become estrous, mentally and physically receptive to copulation. Because the ova survive and can be fertilized for a week after ovulation, more than one male can sire the same litter.[12]

Fertilization typically occurs two to five days after ovulation; 14–16 days after ovulation, the embryo attaches to the uterus and after seven to eight more days, a heartbeat is detectable.[39][40]

Dogs bear their litters roughly 58 to 68 days after fertilization,[12][41] with an average of 63 days, although the length of gestation can vary. An average litter consists of about six puppies.[42]

Neutering
Neutering refers to the sterilization of animals, usually by removing the male's testicles or the female's ovaries and uterus, to eliminate the ability to procreate and reduce sex drive. Because of dogs' overpopulation in some countries, many animal control agencies, such as the American Society for the Prevention of Cruelty to Animals (ASPCA), advise that dogs not intended for further breeding should be neutered, so that they do not have undesired puppies that may later be euthanized.[43]

According to the Humane Society of the United States, three to four million dogs and cats are euthanized each year.[44] 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.[45]

Neutering reduces problems caused by hypersexuality, especially in male dogs.[46] Spayed female dogs are less likely to develop cancers affecting the mammary glands, ovaries, and other reproductive organs.[47][page needed] However, neutering increases the risk of urinary incontinence in female dogs[48] and prostate cancer in males[49] and osteosarcoma, hemangiosarcoma, cruciate ligament rupture, obesity, and diabetes mellitus in either sex.[50]

Inbreeding depression
A common breeding practice for pet dogs is mating between close relatives (e.g., between half and full siblings).[51] Inbreeding depression is considered to be due mainly to the expression of homozygous deleterious recessive mutations.[52] Outcrossing between unrelated individuals, including dogs of different breeds, results in the beneficial masking of deleterious recessive mutations in progeny.[53]

In a study of seven dog breeds (the Bernese Mountain Dog, Basset Hound, Cairn Terrier, Brittany, German Shepherd Dog, Leonberger, and West Highland White Terrier), it was found that inbreeding decreases litter size and survival.[54] 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.[55] In a study of Boxer litters, 22% of puppies died before reaching 7 weeks of age. Stillbirth was the most frequent cause of death, followed by infection. Mortality due to infection increased significantly with increases in inbreeding.[56]

Intelligence, behavior and communication
Intelligence
Main article: Dog intelligence
Dog intelligence is the dog's ability to perceive information and retain it as knowledge for applying to solve problems. Studies of two dogs suggest that dogs can learn by inference and have advanced memory skills. A study with Rico, a Border Collie, showed that he knew the labels of over 200 different items. He inferred the names of novel things by exclusion learning and correctly retrieved those new items immediately and four weeks after the initial exposure. A study of another Border Collie, "Chaser," documented his learning and memory capabilities. He had learned the names and could associate by verbal command over 1,000 words.[57] Dogs can read and react appropriately to human body language such as gesturing and pointing and human voice commands.

One study of canine cognitive abilities found that dogs' capabilities are no more exceptional than those of other animals, such as horses, chimpanzees, or cats.[58] One limited study of 18 household dogs found that they lacked spatial memory, and were more focussed on the "what" of a task rather than the "where".[59]

Dogs demonstrate a theory of mind by engaging in deception.[60] An experimental study showed compelling evidence that Australian dingos can outperform domestic dogs in non-social problem-solving, indicating that domestic dogs may have lost much of their original problem-solving abilities once they joined up with humans.[61] Another study revealed that after undergoing training to solve a simple manipulation task, dogs faced with an insoluble version of the same problem look at the human, while socialized wolves do not.[62]

Behavior
Main article: Dog behavior
See also: Dog behavior § Behavior compared with other canids
Dog behavior is the internally coordinated responses (actions or inactions) of the domestic dog (individuals or groups) to internal and external stimuli.[63] As the oldest domesticated species, dogs' minds inevitably have been shaped by millennia of contact with humans. As a result of this physical and social evolution, dogs have acquired the ability to understand and communicate with humans more than any other species and they are uniquely attuned to human behaviors.[9] Behavioral scientists have uncovered a surprising set of social-cognitive abilities in domestic dogs. These abilities are not possessed by the dog's closest canine relatives or other highly intelligent mammals, such as great apes, but rather parallel to children's social-cognitive skills.[64]

Unlike other domestic species selected for production-related traits, dogs were initially selected for their behaviors.[65][66] In 2016, a study found that only 11 fixed genes showed variation between wolves and dogs. These gene variations were unlikely to have been the result of natural evolution and indicate selection on both morphology and behavior during dog domestication. These genes have been shown to affect the catecholamine synthesis pathway, with the majority of the genes affecting the fight-or-flight response[66][67] (i.e., selection for tameness) and emotional processing.[66] Dogs generally show reduced fear and aggression compared with wolves.[66][68] Some of these genes have been associated with aggression in some dog breeds, indicating their importance in both the initial domestication and later in breed formation.[66] 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.

Communication
Main article: Dog communication
Dog communication is how dogs convey information to other dogs, understand messages from humans and translate the information that dogs are transmitting.[70]: xii  Communication behaviors of dogs include eye gaze, facial expression, vocalization, body posture (including movements of bodies and limbs), and gustatory communication (scents, pheromones, and taste). Humans communicate to dogs by using vocalization, hand signals, and body posture.

Ecology
Population
The dog is probably the most widely abundant large carnivoran living in the human environment.[71][72] In 2013, an estimated global dog population was between 700 million[73] and 987 million.[74] Although it is said that the "dog is man's best friend,"[75] this refers mainly to the ~20% of dogs that live as pets in developed countries.[76] In the developing world, dogs are more commonly feral or communally owned, with pet dogs uncommon. Most of these dogs live their lives as scavengers and have never been owned by humans, with one study showing their most common response when approached by strangers is to run away (52%) or respond aggressively (11%).[77] Little is known about these dogs, or the dogs in developed countries that are feral, strays, or are in shelters because the great majority of modern research on dog cognition has focused on pet dogs living in human homes.