Dog anatomy

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Dog anatomy includes the same internal structures that are in humans. Details of structures vary tremendously from breed to breed, more than in any other animal species, wild or domesticated,^[1] as dogs vary from the tiny Chihuahua to the giant Irish Wolfhound.

External anatomy (topography) of a typical dog: 1.Stop 2.Muzzle 3.Dewlap (throat, neck skin) 4.Shoulder 5.Elbow 6.Forefeet 7.Croup 8.Leg (thigh and hip) 9.Hock 10.Hind feet 11.Withers 12.Stifle 13.Paws 14.Tail

Skeleton of a domestic dog

Skull of a dog

[edit] Physical characteristics

Like most predatory mammals, the dog has powerful muscles, a cardiovascular system that supports both sprinting and endurance, and teeth for catching, holding, and tearing.

The dog's ancestral skeleton provided the ability to run and leap. Their legs are designed to propel them forward rapidly, leaping as necessary, to chase and overcome prey. Consequently, they have small, tight feet, walking on their toes; their rear legs are fairly rigid and sturdy; the front legs are loose and flexible, with only muscle attaching them to the torso.

Although selective breeding has changed the appearance of many breeds, all dogs retain the basic ingredients from their distant ancestors. Dogs have disconnected shoulder bones (lacking the collar bone of the human skeleton) that allow a greater stride length for running and leaping. They walk on four toes, front and back, and have vestigial dewclaws (dog thumbs) on their front legs and sometimes on their rear legs. In some cases, these claws are missing due to surgery, the rear dewclaws sometimes being removed to prevent the possibility of their being ripped off, or catching on something and breaking, especially in dogs with loose dewclaws. This practice is illegal in some countries.

The dog's ancestor was about the size of a Dingo, and its skeleton took about 10 months to mature. Today's toy breeds have skeletons that mature in only a few months, while giant breeds such as the Mastiffs take 16 to 18 months for the skeleton to mature. Dwarfism has affected the proportions of some breeds' skeletons, as in the Basset Hound.

Knowledge of basic anatomy also helps when competing in dog shows or contests.

[edit] Size

Researchers have identified a particular piece of genetic material that is common to every small-dog breed and, in turn, is probably responsible for making them tiny. The study, published in 2007, found a regulatory sequence (not a gene) next to the gene IGF1; together the gene and regulatory sequence together are known as a haplotype that "is a major contributor to body size in all small dogs." Medium and large size dogs do not usually have the regulatory sequence, although the small-size sequence was found in the Rottweiler breed. The study included 3,241 dogs from 143 breeds. The researchers concluded the genetic instructions to make dogs small must be at least 12,000 years old, and it is not found in wolves.^[2] Another study has shown that lap dogs (small dogs) are among the oldest dog types.^[3]

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.

[edit] Sight

A Greyhound, one of many breeds of sighthound

Like most mammals, dogs are dichromats and have color vision equivalent to red-green color blindness in humans.^[4][5][6] Different breeds of dogs have different eye shapes and dimensions, and they also have different retina configurations.^[7] Dogs with long noses have a "visual streak" which 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 breeds, particularly the sighthounds, have a field of vision up to 270° (compared to 180° for humans), although broad-headed breeds with short noses have a much narrower field of vision, as low as 180°.^[4][5]

[edit] Hearing

According to hypertextbook.com, the frequency range of dog hearing is approximately 40 Hz to 60,000 Hz.^{[8][unreliable source?]} 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^[9] (compared to 13 to 20 kHz for humans),^[5][8] and in addition have a degree of ear mobility that helps them to rapidly pinpoint the exact location of a sound.^[10] 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.^[10] Those with more natural ear shapes, like those of wild canids like the fox, generally hear better than those with the floppier ears of many domesticated species.

[edit] Smell

Scent hounds, especially the Bloodhound, are bred for their keen sense of smell.^[11]

Dogs have nearly 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).^[12][13] According to nhm.org, dogs can sense odours at concentrations nearly 100 million times lower than humans can.^[14] According to Dummies.com, the percentage of the dog's brain that is devoted to analyzing smells is actually 40 times larger than that of a human.^[12] Some dog breeds have been selectively bred for excellence in detecting scents, even compared to their canine brethren.

The highly sensitive nose of a dog.

[edit] Coat

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.^[15][16] 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.^[15] 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.^[16]

Dogs diverged from a now-extinct Asian wolf between 12,000 and 15,000 years ago, according to recent DNA studies. In that time, the long nose and heavy grey-colored double coat of the wolf has changed into the wide variety of dog shapes and coats and colors seen today. The change was due at first to genetic changes that occurred as the original dogs learned to tolerate the presence of humans, as shown in the research on foxes by Dmitri Belyaev in his Farm-Fox Experiment. The research found that a genetic change to tameness brought along other unexpected changes as well; one notable change was in the coats, changed from a typical fox coat to a spotted coat resembling a dog's coat. As ancient dogs learned to live near humans and became less like wolves, their appearance changed as well, long before any selective breeding was done by people.^[17]

A Stanford University School of Medicine study published in Science in October, 2007 found the genetics that explain coat colors in other mammals such as in horse coats and in cat coats, did not apply to dogs.^[18] The project took samples from 38 different breeds to find the gene (a beta defensin gene) responsible for dog coat color. One version produces yellow dogs, and a mutation produces black. All dog coat colors are modifications of black or yellow.^[19] For example, the white in white miniature schnauzers is a cream color, not albinism (a genotype of e/e at MC1R.)

Modern dog breeds exhibit a diverse array of fur coats, including dogs without fur, such as the Mexican Hairless Dog. Dog coats vary in texture, color, and markings, and a specialized vocabulary has evolved to describe each characteristic.^[20]

[edit] Tail

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).^[21] It can happen that some puppies are born with a short tail or no tail in some breeds.^[22]

[edit] Puppy characteristics

This probably 15 weeks old German shepherd mongrel already shows an upward erection trend of the ears, with varying grades of erection during the day

Puppies often have characteristics that do not last beyond early puppyhood. Eye color often changes from blue to its adult color as the puppy matures. The coat color may change: Kerry Blue Terrier puppies have black coats at birth and change to blue with maturity, and Dalmatians are white and gain their spots with age. The ear shape will also often change, especially with erect-eared breeds such as the German Shepherd Dog which have soft ears at birth, but the cartilage strengthens with age. Blue Lacy Dogs and Labrador Retrievers, however, stay very much the same through life.

In some countries it is not uncommon for puppies to have their ears cropped or straightened and tails docked. Dogs intended for dog fighting often have their ears severely trimmed. Many countries have banned these practices as animal cruelty.

[edit] Temperature regulation

A common misconception is that dogs do not sweat. Primarily, dogs regulate their body temperature in a completely different way, through panting^{[citation needed]}. That is why, on a hot day or after a dog has been running, its mouth will be seen wide open with the tongue hanging out. This form of cooling maximizes heat loss while conserving moisture, because it carries heat from the hottest part of the body, the interior core of the thorax. Perspiration cools the body by the evaporation of moisture from the skin. This is much less efficient, because the skin is already the coolest part of the body—or in more scientific terms, this higher efficiency of thermal loss relative to moisture conservation arises because heat flow is proportional to temperature gradient. Dogs sweat through the pads of their feet, since they are not furred. On a warm day and after exercise, a dog's naturally wet footprints might be visible on a smooth floor.

Dogs possess a rete mirabile, a complex of intermingled small arteries and veins, in the carotid sinus at the base of their neck. This acts as a heat exchanger to thermally isolate the head, containing the brain, the most temperature-sensitive organ, from the body, containing the muscles, where most of the heat is generated. The result is that dogs can sustain intense physical exertion over a prolonged time in a hot environment, compared to animals which lack this apparatus; thus, a dog chasing a jackrabbit through the desert may not be able to outrun the rabbit, but it can continue the chase until the rabbit literally drops dead from overheating.

[edit] See also

• Canine terminology
• Conformation (dog)
• Temperament
• Grooming
• Junior Showmanship

[edit] References

[edit] External links

• Information and facts on dog anatomy