The baculum, or penile bone, is an intriguing anatomical feature present in many male mammals. Its widespread occurrence across diverse species often leads to a curious question: why do humans not possess this bone? The absence of a baculum in humans stands out as a unique characteristic within the mammalian kingdom, prompting scientific inquiry into its evolutionary history and the factors that led to its disappearance in our lineage.
Understanding the Baculum
The baculum, also known as the os penis, is a bone located within the penis of numerous placental mammals. This bone provides structural support, helping to maintain rigidity during copulation. It is an extra-skeletal bone, meaning it is not directly connected to the main skeleton, and typically lies above the urethra.
The size and shape of the baculum vary significantly among species, often reflecting differences in mating behaviors and reproductive strategies. This diversity highlights its functional importance for sexual reproduction in the animals that possess it.
Evolutionary Context in Mammals
The presence of a baculum is common across a wide range of mammalian groups, including most primates, carnivores, rodents, and bats. This broad distribution initially suggested it might be an ancient, ancestral trait. However, research indicates that the baculum has evolved independently on at least nine separate occasions.
Conversely, this bone has also been lost independently in at least ten different mammalian lineages. While the ancestral mammal likely did not possess a baculum, it was present in the common ancestors of primates and carnivores. The widespread, yet variable, occurrence of the baculum makes its absence in humans a particularly interesting evolutionary puzzle.
Leading Hypotheses for its Absence
One prominent hypothesis for the baculum’s absence in humans centers on the evolution of distinct mating systems. Many species with a baculum engage in prolonged intromission, defined as penetration lasting longer than three minutes. This extended duration, facilitated by the baculum’s structural support, can be a strategy for males to guard females from other potential mates and increase paternity certainty in highly competitive mating environments.
Human mating patterns, however, typically involve relatively short intromission durations, often less than two minutes. This brevity, combined with a shift towards more monogamous or pair-bonded relationships in human evolutionary history, may have reduced the selective pressure for a penile bone. If males were more consistently paired with a single female, the need for prolonged copulation to deter rivals would diminish.
The transition to a mating system with reduced postcopulatory sexual competition, possibly emerging around 1.9 to 2 million years ago with Homo erectus, could have rendered the baculum obsolete. In such a system, frequent, shorter matings could effectively ensure paternity without requiring the sustained rigidity provided by a bone. The energetic cost of developing and maintaining a baculum, though minor, might also have contributed to its loss if its benefits no longer outweighed this investment.
Another theory suggests that a boneless penis offers greater flexibility, potentially allowing for a wider range of copulatory positions and increased tactile stimulation for the female. This “tactile stimulation hypothesis” proposes that female selection might have favored this flexibility over the skeletal support. Additionally, an “honest advertising theory” posits that a hydraulic erection, reliant solely on blood flow, could serve as a more reliable indicator of a male’s overall health.
Biological and Behavioral Implications
The absence of a baculum means that human penile erection relies entirely on hydraulic pressure generated by blood flow. During arousal, neural signals cause the smooth muscles within the penis to relax, allowing blood to rapidly fill two spongy chambers called the corpora cavernosa and, to a lesser extent, the corpus spongiosum. This influx of blood engorges the tissues, and a surrounding fibrous sheath, the tunica albuginea, helps to trap the blood, leading to rigidity.
Once stimulation ceases or ejaculation occurs, the muscles contract, blood flows out, and the penis returns to its flaccid state. This reliance on a purely hydraulic mechanism distinguishes human erection from that of many other mammals. Despite this anatomical difference, human reproductive strategies and success have been shaped by a combination of biological adaptations and complex social behaviors.