The human penis does not contain a bone. Its rigidity relies on a sophisticated hydraulic system, a contrast to many other mammals that possess a bone within their penis, known as a baculum.
Anatomy of the Human Penis
The human penis achieves its rigidity through a complex physiological process involving blood flow and specialized erectile tissues. It consists primarily of three cylindrical chambers: two larger structures called the corpora cavernosa, located on the dorsal side, and a smaller one, the corpus spongiosum, positioned ventrally around the urethra. These chambers are composed of a sponge-like network of blood vessels and smooth muscle fibers.
During sexual arousal, nerve signals from the brain trigger the relaxation of smooth muscles within the penile arteries, causing them to dilate and rapidly increase blood flow into the corpora cavernosa. As these chambers fill with blood, they expand, creating hydraulic pressure. This engorgement compresses the veins that typically drain blood from the penis, effectively trapping the blood within the erectile tissues. The tunica albuginea, a fibrous sheath surrounding the corpora cavernosa, helps maintain this pressure, ensuring the penis remains rigid for penetration. The corpus spongiosum also fills with blood, though to a lesser extent, which helps keep the urethra open for ejaculation.
The Baculum in the Animal Kingdom
Many male mammals, unlike humans, possess a bone within their penis called the baculum, or os penis. This heterotopic bone is not connected to the main skeleton but develops within the soft tissues of the penis, typically above the urethra. Its presence and form vary greatly among species, making it one of the most diverse bones in the animal kingdom.
The baculum provides structural support during copulation, aiding in prolonged intromission. This extended duration can be advantageous for reproductive success, as it may help prevent other males from mating with the female, thereby increasing the male’s chances of siring offspring. Examples of mammals with a baculum include most carnivores like bears, dogs, and raccoons, as well as many rodents, bats, and some primates, such as gorillas and chimpanzees. For instance, walruses have notably long bacula, sometimes exceeding 60 centimeters.
The evolutionary history of the baculum is complex, with evidence suggesting it evolved independently at least nine times across different mammalian lineages and was subsequently lost in at least ten separate lineages. In humans, the absence of a baculum is theorized to be linked to changes in mating systems and copulation duration. Research indicates that species with intromission periods longer than three minutes tend to have a baculum, while human copulation typically lasts less than two minutes. The shift towards more monogamous mating strategies in human ancestors, around 1.9 million years ago, may have reduced the evolutionary pressure for prolonged copulation and, consequently, the need for a penile bone.