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.[2]
In Tandonia, the retractor penis inserts at the boundary between the penis and epiphallus.[3]
In stallions, the retractor penis muscle is relatively underdeveloped.[4] The retractor muscle contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath.[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.