The baculum (also penis bone), is a bone found in the penis of many male mammals. It is absent in the human penis, but present in the penises of our close relatives the apes, such as the gorilla and chimpanzee. The bone is located along the top of the penis and it aids in sex by keeping it stiff for a long time during the sex act.
A bone in the penis allows a male to mate for a much longer time with a female. This can be an advantage as it prevents a rival male from having sex with the female for some time.
Male animals without a penis bone tend to form close emotional bonds with a female and so ensure they are the father of her offspring.