Because they're there, of course. Watch a dog as it chases its tail, and you can almost hear it thinking, "What is this thing that keeps following me around, and why can't I catch it?"
Quantifying canine intelligence is a very tricky business; there are dogs that display great
aptitude for obedience work yet don't have the sense to come in out of the rain, and dogs that are seemingly untrainable yet exhibit astonishing reasoning and decision-making powers.

It's a pretty safe bet, though, that a dog that persists in chasing its tail wasn't at the front of the line when the brains were being distributed—although, in fairness to the dog, prolonged and repeated tail chasing can also be symptomatic of seizures brought on by a serious epileptic-type disorder. If your pet exhibits such behavior, get it to a veterinarian, pronto.

Tom Davis

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