The human penis does not contain a bone. While this might seem counterintuitive given the rigidity achieved during an erection, the mechanism is entirely hydraulic. Many other mammals, however, possess a bone within their penis, known as a baculum or os penis. This skeletal element varies greatly in size and shape across species where it is present.
The Human Erectile Mechanism
The human penis achieves rigidity through a complex physiological process involving blood flow and specialized tissues. An erection begins with sensory or mental stimulation, which prompts the brain to send signals through nerves to the penis. These signals trigger the relaxation of smooth muscles within the erectile tissues.
The primary erectile tissues are the two corpora cavernosa, parallel, sponge-like structures running the length of the penis. The corpus spongiosum, a third column containing the urethra, also fills with blood. As smooth muscles relax, arteries within these tissues dilate, allowing a rapid influx of blood. This fills the corpora cavernosa, causing them to expand.
The expanding corpora cavernosa compress the veins that would normally drain blood. This compression, aided by muscles like the ischiocavernosus and bulbospongiosus, traps blood under high pressure, leading to firmness. The tunica albuginea, a fibrous membrane, helps maintain this pressure. The erection subsides when muscles contract, allowing blood to flow out and the penis to return to a flaccid state.
The Baculum in Other Mammals
The baculum is a bone found within the penis of many male mammals. This skeletal element is composed of spongy bone and can vary significantly in shape, from a simple rod or splint to more complex, curved forms. Its presence provides structural support during copulation, assisting in maintaining penile stiffness.
Approximately 80% of mammalian species possess a baculum, including most primates, rodents, carnivores, and bats. Its size and shape vary greatly between species. The baculum’s function is to facilitate prolonged intromission, which can be advantageous in certain mating strategies.
Evolutionary Absence in Humans
The absence of a baculum in humans, despite its widespread presence in other mammals, including many primates, has been a subject of evolutionary inquiry. One prominent hypothesis links its loss to changes in human mating systems. Research suggests that the baculum evolved to support prolonged intromission, defined as penetration lasting longer than three minutes, which is often seen in species with high levels of postcopulatory sexual competition.
In mating systems where multiple males compete for access to females, a longer copulation time, facilitated by a baculum, could help a male prevent others from fertilizing the female. Humans exhibit a relatively short intromission duration and tend towards monogamous or polygynous mating systems, without intense male-male competition for individual mating events. This reduced postcopulatory competition may have diminished the selective pressure for retaining a baculum.
Another theory proposes that achieving an erection solely through vascular control might have served as an indicator of male health or fitness. A male’s capacity for a strong, blood-engorged erection could signal his well-being. The human lineage is thought to have lost the baculum roughly six million years ago, as human mating patterns shifted towards reduced male-male sexual competition.