Via Bits & Pieces
Why do dogs pee on hydrants?:
Dogs—like all other canids—leave urine conspicuously splashed on all manner of object. Urine marking, as this method of communication is called, conveys a message. Every dog owner is familiar with the raised-leg marking of fire hydrants, lampposts, trees, bushes. Most marked spots are high or prominent: better to be seen, and better for the odor to be smelled.
From observations of the behavior of sniffing dogs, it appears that the chemicals in the urine give information about, for females, sexual readiness, and for males, their social confidence. The prevailing myth is that the message is “this is mine,” that dogs urinate to “mark territory.” But research has failed to bear this out as the exclusive, or even predominant, use of urine marking. Instead, marking seems to leave information about who the urinator is, how often he walks by this spot, his recent victories, and his interest in mating. In this way, the invisible pile of scents on the hydrant becomes a community bulletin board, with old, deteriorating announcements and requests peeking out from underneath more recent posts.
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