The human penis does not contain a bone, a fact that often surprises people as many other mammals do possess a penile bone. This article explores the unique anatomy of the human penis, the presence of a penile bone in other species, and the evolutionary reasons for its absence in humans.
The Human Penis: Structure and Function
The human penis is primarily composed of specialized erectile tissue, not bone or muscle. Its internal structure includes three cylindrical columns: two larger corpora cavernosa located dorsally and a single corpus spongiosum positioned ventrally. These columns are encased by a fibrous connective tissue layer known as the tunica albuginea. The urethra, which serves as a passage for both urine and semen, runs through the corpus spongiosum.
An erection occurs through a hydraulic mechanism involving significant blood flow. Upon sexual stimulation, nerve signals trigger the relaxation of smooth muscle cells within the penile arteries, allowing a rapid influx of blood into the spongy tissues of the corpora cavernosa. As these chambers engorge with blood, they expand, compressing veins against the tunica albuginea and trapping the blood within the penis. This increased pressure causes the penis to become firm and enlarged.
The Baculum: A Bone for Other Mammals
A structure called the baculum, or os penis, is present in the penises of many placental mammals. This bone varies greatly in size and shape across species. Examples include various primates (though not humans), canids like dogs, felids like cats, rodents, bears, seals, and walruses.
The baculum is not attached to the main skeleton; instead, it is an extra-skeletal bone embedded in the soft tissues of the penis. Its functions in these animals can include providing structural support during copulation, which may facilitate prolonged intromission. This extended duration can be advantageous in certain mating strategies, such as when males face high levels of postcopulatory competition, allowing a male to guard a female from other potential mates. In some species, the baculum may also protect the urethra from compression during mating or help stimulate the female reproductive tract.
Why Humans Don’t Have a Penis Bone
The absence of a baculum in humans has been a subject of scientific inquiry. One prominent theory suggests it correlates with human mating strategies. Unlike many mammals with bacula, humans typically have shorter copulation times, often less than two minutes from penetration to ejaculation. Species with prolonged intromission, defined as lasting over three minutes, tend to have longer bacula.
Evolutionary pressures related to human pair bonding and reduced male-on-male competition for mates may have contributed to the baculum’s loss. In mating systems where a female primarily mates with one male, the need for a prolonged intromission to secure paternity against competitors decreases. This reduction in postcopulatory sexual competition could have rendered the baculum less advantageous over evolutionary time.