Puppies don't have to do anything but sit there to look cute, but their funny little shenanigans make them all the more adorable. For most dogs, tail-chasing is simply a rite of puppyhood. When it continues into adulthood, though, it can be a serious problem.
Puppies love to explore, which can lead to them falling clumsily into food bowls and getting stuck under the couch. They explore not only the world around them, but also their own bodies. That quick shadow a puppy spies as she whips around might appear like a new toy, when it's actually her tail. Chasing it -- and catching it, if she can -- helps her learn the shape and size of her body and gives her the ability to distinguish her wagging tail from a toy in the future.
Littermates spend much of their time playing; when they're weaned and adopted out, individual puppies can become lonely and bored if they're left without stimulation. The tail offers a built-in that goes everywhere she goes. In cases, a flick of the tail seen peripherally can spark the prey drive. If your puppy chases her tail after she's home with you, and it's not an on-cue trick, the dog's likely lacking attention. Give your puppy lots of exercise, love and toys to reduce the urge to chase her tail.
Scratching that Itch
There's a spot right in the middle of your back that seems impossible to scratch. Puppies have the same problem, but it extends along most of the tail. Not yet coordinated enough to sit and reach for her tail with her mouth, your puppy might just be trying to scratch an itch she can't easily reach. If she seems frantic rather than playful as she's chasing her tail, scratch it for her to see if that helps, and examine her for fleas or other pests that might be causing the itch. It could also be dry skin, but check with your vet if you're concerned.
When It's a Problem
It's normal for puppies to chase their tails, but not so much for adult dogs. As your dog matures, she learns to distinguish her body parts by sight and loses much of her puppy playfulness. Some adult dogs might still chase their tails as a way to get your attention, but tail-chasing in adult dogs tends to be a sign of a bigger problem. If your dog seems obsessive about chasing her tail, running in circles for long periods of time without being easily distracted from the behavior, check with your vet about a possible obsessive-compulsive disorder. Extreme cases can cause unhealthy weight loss, as your dog skips meals to spin, as well as exhaustion from constant activity and painful sores from biting her tail when she catches it. Your vet can help you find the best way to work with your dog to alleviate the behavior, which is likely to include antianxiety medications and behavioral therapy.