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Dog anatomy varies more within its species than in any other animal. Despite these differences, dogs all share identical bone and muscle structure. They also all have a circulatory system that transports blood, a respiratory system that takes in oxygen, and a digestive tract that digests nutrients. A dog also has powerful muscles, a strong cardiovascular system enabling speed and endurance and sharp teeth typical of all predators.

The domestic dog, as we know it today, is the result of thousands of years of selective breeding. But despite huge differences in physical appearance, all dogs are the same.

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Body Size

Dogs come in a variety of sizes. Canis (lupus) familiaris ranges in size from the miniature chihuahua to the imposing great dane, which can be the size of a lioness. The largest dog on record is a great dane that measures 7’4’’ in length and 155 pounds in weight. The smallest is a Chihuahua that stands 4 inches in height and weighs 1.5 pounds. The giant breeds like great danes, Newfoundlands and St.Bernards typically can weigh as much as 200 pounds. Miniature breeds such as chihuahuas, Yorkshire terriers, and Shih Tzus usually weigh between 5 and 10 pounds.

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Skull

The dog is the only animal whose skull has three different shapes. Generations of breeding practices have led to enormous differences in physique and morphology among dog breeds. The skull is composed of 2 parts, the neurocranium and the facial skeleton, which are also composed of several bones. The neurocranium surrounds and protects the dog's brain, and the facial skeleton encases the dog's face. Brachycephalic dogs feature a broad, compact head that is found in breeds like bulldogs, Shih Tzus, and Boston terriers. While dolichocephalic breeds like Salukis, Afghan hounds and collies have long, narrow bone structures.

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Sense of Smell

While humans navigate their world using sight, dogs are guided by smell. A dog's sense of smell is a million times more sensitive than a human's, and their nasal cavities contain 220 million scent receptors, compared to 5 million in humans. Dogs, accordingly, have a large proportion of their brain capacity devoted to analyzing smells. This area is 40 times larger than a comparable section in the human brain. A dog's sense of smell is so acute and sensitive that it is able to detect scents in parts per trillion.

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Physical Characteristics

The domestic dog is a direct descendant of the grey wolf. Its skeletal structure consists of 319 bones which provide the support necessary for running, sprinting, and jumping. Like most predators, a dog has powerful muscles and small, compact feet, capable of propelling him forward when pursuing prey. For this reason, their hind legs are firm and relatively rigid, while their front legs are flexible and loosely attached to the torso. Dogs also have disconnected shoulder bones enabling them to take longer strides.

Beagle dog scratching body on green grass outdoor in the park on sunny day.
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Temperature Regulation

Dogs don't perspire through their skins as humans do. Their coats provide an insulating barrier between them and extreme temperatures that keep them warm in winter and cool in summer. However, dogs are far better at conserving heat than they are at cooling themselves. Counter to popular belief; dogs do have sweat glands which are situated in the pads of their feet. But dogs don't rely on sweating to regulate temperature. Panting is a dog's primary means of lowering body temperature. He does so by taking short, rapid breaths which evaporate water through his mucous membranes.

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Hair

A dog's coat can be one of two kinds. A double coat which consists of a topcoat made up of stiff, water repellant guard hairs that protect the dog's undercoat. And an undercoat that is soft and downy with shorter fur. Dogs with a single coat have only one layer of hair: the topcoat. Dogs' coats are also often counter-shaded. This is a form of natural camouflage that conceals animals. A counter-shaded animal will have dark coloring above and light coloring below.

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Metabolism

Dogs have a faster metabolism than humans. Their basic bodily processes occur at a heightened rate, resulting in faster breathing, faster blood circulation, faster maturation, and a raised resting body temperature. This high metabolism, however, includes a shorter lifespan. As a general rule, a dog year is equivalent to 10 human years for the first two years and a to 4 human years after that. Researchers at the University of Gottingen have finally come up with a formula to calculate a dog's age. They have discovered that for every 4.4-pound increase in weight, a dog's lifespan decreases by one month.

Catahoula Leopard Dog puppy playing with stick at beach
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Puppy Dog Eyes

Recent research on dogs and wolves has revealed that a dog's facial anatomy has changed to appeal to humans. Researchers found that both species shared the same facial musculature, except above the eyes. Dogs have a powerful muscle above the eye that enables them to raise their eyebrows intensely. They have also been found to use this expression largely among humans, suggesting that the behavior is intentional and for human benefit. It is thought that this imploring expression has held such favor with humans over thousands of years of selective breeding, that it has encouraged dogs to evolve a more human facial anatomy.

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Whiskers

In humans, sense of touch is mostly connected to hands and fingers. A dog feels his way around his world with his snout. His whiskers are able to detect even the smallest changes in air currents, providing information on objects in the vicinity. This assists dogs in avoiding danger in the dark. The end of each whisker contains a large number of touch-sensitive nerves that send sensory messages to the brain, at even the slightest motion.

dog, whiskers, snout, nerves amandafoundation.org / Getty Images
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Vision

It is a common myth that dogs see only in black and white. Dogs do have color vision, just not the same kind as humans. A dog's eyes are only able to process the colors blue and yellow, and they cannot distinguish between red and green. In bright light, their vision is slightly blurred, but their superior night vision can compensate for this. They are also able to see a wide 250-degree view of their immediate environment. Which, together with their ability to see in the dark, makes their eyes perfectly suited for hunting prey.

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