In some animals, the male penis possesses a muscle enabling retraction into the prepuce.
The retractor penis muscle occurs in marsupials and carnivorans, but it is absent in humans.
[1] A stag's penis forms an S-shaped curve when it is not erect, and is retracted into its preputial sheath by the retractor penis muscle.
[3] In stallions, the retractor penis muscle is relatively underdeveloped.
[5] In bulls, protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the sigmoid flexure.