Male dogs lift a single leg to pee because they do it with a purpose other than just relieving themselves. Male dogs use their urine to communicate; when they lift a leg to pee, they can more accurately deposit it on the surface of their choosing. Like a business card pinned to a public bulletin board, the urine is readily noticeable to other dogs.
A male dog's urine is laced with communicative hormones, so when he pees, he isn't just going to the bathroom -- he's leaving a message. For an intact dog, that message is communicating his virility to females in the neighborhood. For a neutered dog, the purpose is a more generalized form of territorial marking. In either case, the male prefers to urinate on vertical surfaces like walls, telephone poles and fire hydrants. Sometimes females engage in urine-marking behavior, lifting a rear leg to pee, as well.
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