Biology of dog

Anatomy

Domestic dogs have been selectively bred for millennia for various behaviors, sensory
capabilities, and physical attributes. Modern dog breeds show more variation in size, appearance,
and behavior than any other domestic animal. Dogs are predators and scavengers; like many
other predatory mammals, the dog has powerful muscles, large and sharp claws and teeth, fused
wrist bones, a cardiovascular system that supports both sprinting and endurance, and teeth for
catching and tearing.

Size and weight

Dogs are highly variable in height and weight. The smallest known adult dog was a Yorkshire
Terrier, that stood only 6.3 centimetres (2 1⁄2 inches) at the shoulder, 9.5 cm (3 3⁄4 in) in length
along the head-and-body, and weighed only 113 grams (4 ounces). The most massive known dog
was a Saint Bernard, which weighed 167.6 kg (369 1⁄2 lb) and was 250 cm (8 ft 2 in) from the
snout to the tail. The tallest dog is a Great Dane that stands 106.7 cm (3 ft 6 in) at the shoulder.

Senses

The dog's senses include vision, hearing, smell, taste, touch and sensitivity to the earth's
magnetic field. Another study suggested that dogs can see the earth's magnetic field.

Coat

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.The coat can be maintained or affected by multiple nutrients
present in the diet; see Coat (dog) for more information.

Premature graying can occur in dogs from as early as one year of age; this is shown to be
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 cork-screw.
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,
however, the tail is traditionally docked to avoid injuries. In some breeds, such as the Braque du
Bourbonnais, puppies can be born with a short tail or no tail at all.

Differences from wolves

Despite their close genetic relationship and interbreedability, there are several diagnostic features
to distinguish the gray wolves from domestic dogs. Domesticated dogs are distinguishable from
wolves by starch gel electrophoresis of red blood cell acid phosphatase. The tympanic bullae are
large, convex, and almost spherical in gray wolves, while the bullae of dogs are smaller,
compressed, and slightly crumpled. Compared with equally sized wolves, dogs tend to have 20%
smaller skulls and 30% smaller brains. The teeth of gray wolves are also proportionately larger
than those of dogs. Dogs have a more domed forehead and a distinctive "stop" between the
forehead and nose. The temporalis muscle that closes the jaws is more robust in wolves. Wolves
do not have dewclaws on their back legs unless there has been admixture with dogs that had
them. Most dogs lack a functioning pre-caudal gland and enter estrus twice yearly, unlike gray
wolves, which only do so once a year. So-called primitive dogs such as dingoes and Basenjis
retain the yearly estrus cycle.Dogs generally have brown eyes, and wolves almost always have
amber or light-colored eyes. Domestic dogs' skin 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. The paws of a dog are half the size of a wolf, and their tails tend to curl
upwards, another trait not found in wolves. The dog has developed into hundreds of varied
breeds and shows more behavioral and morphological variation than any other land mammal.

Health

Many household plants are poisonous to dogs (and other mammals), including Begonia,
Poinsettia, and Aloe vera.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 types and ages, and gastric dilatation volvulus (bloat), which affects the 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 (roundworm species that lives in the heart of dogs).

Several human foods and household ingestibles are toxic to dogs, including chocolate solids
(theobromine poisoning), onion and garlic (thiosulphate, sulfoxide or disulfide poisoning),
grapes and raisins, macadamia nuts, xylitol, as well as various plants and other potentially
ingested materials. The nicotine in tobacco can also be dangerous. Dogs can be exposed to the
substance by scavenging through garbage bins or ashtrays and eating cigars and cigarettes. Signs
can be vomiting of large amounts (e.g., from eating cigar butts) or diarrhea. Some other
symptoms are abdominal pain, loss of coordination, collapse, or death. Dogs are susceptible to
theobromine poisoning, typically from the ingestion of chocolate. Theobromine is toxic to dogs
because, although the dog's metabolism is capable of breaking down the chemical, the process is
so slow that for some dogs, even small amounts of chocolate can be fatal, mostly dark chocolate.

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

Lifespan
In 2013, a study found that mixed breed dogs live on average 1.2 years longer than purebred
dogs. Increasing body-weight was negatively correlated with longevity (i.e., the heavier the dog,
the shorter its lifespan).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. Individual dogs may live well beyond the median of their
breed.

The breed with the shortest lifespan (among breeds for which there is a questionnaire survey
with reasonable sample size) is the Dogue de Bordeaux, with a median longevity of about 5.2
years. Still, several breeds, including miniature bull terriers, bloodhounds, and Irish wolfhounds
are nearly as short-lived, with median longevities of 6 to 7 years.The longest-lived breeds,
including toy poodles, Japanese spitz, Border terriers, and Tibetan spaniels, have median
longevities of 14 to 15 years. 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.
The longest-lived dog was "Bluey," an Australian Cattle Dog who died in 1939 at 29.5 years of
age.

Reproduction

In domestic dogs, sexual maturity happens around six to twelve months of age for both males
and females, although this can be delayed until up to two years old 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 estrus, 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.Fertilization typically occurs 2–5 days after ovulation; 14–16 days after ovulation, the
embryo attaches to the uterus, and after 7-8 more days, the heartbeat is detectable.Dogs bear their
litters roughly 58 to 68 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 dog breed. In general, toy dogs produce from one to four puppies in each litter,
while much larger breeds may average as many as twelve.

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 to reproduce.

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. That way, they do not have undesired puppies that may later
be euthanized.According to the Humane Society of the United States, 3–4 million dogs and cats
are euthanized each year. 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 adopt animals from shelters instead of purchasing them.

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

Inbreeding depression

A common breeding practice for pet dogs is mating between close relatives (e.g., between half-
and full siblings). Inbreeding depression is considered to be due mainly to the expression of
homozygous deleterious recessive mutations. Outcrossing between unrelated individuals,
including dogs of different breeds, results in the beneficial masking of deleterious recessive
mutations in progeny.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. 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. In a study of boxer litters, 22% of puppies died before reaching seven weeks of age.
Stillbirth was the most frequent cause of death, followed by infection. Mortality due to infection
increased significantly with increases in inbreeding.