Baculum: More to the Penis than Meets the Eye!
Male walruses have an extra bone to help support copulation. It's called a baculum. Human males don't have one, but the human penis is longer than many of us are aware, suggests Dr. Abraham Morgentaler in The Male Body (Fireside, 1993).
The penis doesn't end when it disappears under the pubic bone. You just don't see the part that extends under the scrotum and splits into a Y. The branches of this Y attach to the pair of bones that join the pubic bone and the bone at the base of the buttocks. The penis is thus anchored so that it can follow the movements of the pelvis during intercourse.