Almost all mammal penises have foreskins, although in non-human cases the foreskin is usually a sheath (sometimes called the preputial sheath or penile sheath) into which the whole penis is retracted. In koalas, the foreskin contains naturally occurring bacteria that play an important role in fertilization. Only monotremes (the platypus and the echidna) lack foreskins. During musth, a male elephant may urinate with the penis still in the sheath, which causes the urine to spray on the hind legs. Male dogs have a conspicuous penis sheath. In stallions, the retractor penis muscle contracts to retract the stallion’s penis into the sheath and relaxes to allow the penis to extend from the sheath. The penis sheath of a male axis deer is elongated and urine-stained. When rubbing trees with their horns, chital stags sometimes move the penis back and forth rapidly inside its sheath. Male bison and fallow deer have tufts of fur at the end of their penis sheaths.
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