A dog will shake himself several times a day, in different situations. It's an inherent canine behavior that reverts to the days before dogs were domesticated. Every shake has a reason and a purpose.
Shake Dynamics
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A dog starts the shake at his head, sending waves through the rest of his body, which follow from the shoulders to his tail. A dog's head can rotate further than his body and gives the body impulsion to propel the shake. A dog's skin twists as his body shakes, providing an effective method to rid himself of dirt, bugs and debris. Dogs frequently shake when they awake from sleeping. This behavior originated from canines in the wild, who lived and slept outdoors, as a way to keep their coats clean.
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Doggie Spin Cycle
When your dog comes in from the rain, or after you've given him a bath, you can bet you'll get wet if you're close. A dog can shake off about 70 percent of the water in his coat in roughly 4 seconds. It's instinctive for dogs to shake when wet to dry themselves. Before they had humans to help them dry with towels, shaking was the only way to dry off in bad weather.
Fixing the Fur
Spend an hour brushing and grooming your dog and as soon as you're finished, he'll shake. It may seem that he's trying to put his fur back in place after you've disturbed it, but it's really because dogs react to extensive handling or touching by shaking. It's the canine version of "shaking it off." You'll find he may react the same way to a lengthy scratch, even though he enjoys it.
Reacting to Discomfort
A dog will shake if he's itchy from irritated skin, ear infections, dental problems or other discomforts. If your dog shakes frequently for no reason, part his fur and check for parasites such as fleas or redness. Consult your vet if you can't find a reason for extensive shaking or if you notice red, irritated skin. If your dog is just shaking his head frequently, it may indicate a medical problem with his ears or teeth.