Humans do not have a bone in the penis. The human penis achieves rigidity entirely through blood pressure, making it a hydraulic system rather than a skeletal one. This puts humans in a minority among mammals, because a surprisingly large number of species do carry a bone inside the penis, called the baculum. The story of why so many mammals have one and why we lost ours turns out to be one of the more colorful chapters in evolutionary biology.
How the Human Penis Works Without a Bone
The erect human penis is essentially a pressurized tube. The main erectile tissue, the corpus cavernosum, functions as what biologists call a hydrostatic skeleton, a structure that gets its stiffness from internal fluid pressure rather than from a rigid frame. The wall of the corpus cavernosum contains layers of highly organized collagen fibers arranged at right angles to each other. When the penis is flaccid, this tissue is folded and soft. During erection, blood fills the chambers, the collagen fibers straighten, and the whole structure becomes larger and resistant to bending, compression, and tensile forces.1PubMed. Penises as variable-volume hydrostatic skeletons
Even the tip of the penis has a structural workaround. Although there is no bone in the human glans, a strong fibrous band called the distal ligament runs through it, acting as a central supporting trunk.2PubMed. Hypothesis of human penile anatomy, erection hemodynamics and their clinical applications This ligament is made of collagen and shares some structural proteins with the baculum found in other species. Without it, the glans would be too flexible to withstand the buckling pressure generated during intercourse and too limp to serve as an effective channel for ejaculation.3PubMed. Distal ligament in human glans: a comparative study of penile architecture So the human body didn’t simply delete the structural support and walk away. It replaced bone with a dense ligament that does part of the same job.
The ligament varies between individuals. A study examining cadaveric specimens found that roughly half had distinct fibrous bands extending toward the tip of the glans, while others had less organized connective tissue. These structural differences likely contribute to the varying degrees of rigidity and flexibility different men experience.4PubMed Central. Ligamentous structures in human glans penis
What the Baculum Actually Is
The baculum, sometimes called the os penis, is a bone found within the penis of many mammals, including numerous primates, rodents, bats, carnivores, and some insectivores. It is not attached to the rest of the skeleton. Instead, it develops from connective tissue and sits at the far end of the penis, above the urethra.5Current Biology. Is There a Bone in Your Penis? What Science Says The bone forms after the rest of the skeleton has already developed, and its growth can depend on sexual maturity, with both standard bone-formation pathways involved in building it.6PubMed. Formation, structure, and function of extra-skeletal bones in mammals
The diversity in baculum shape and size is genuinely bizarre. Some species have a tiny sliver of bone. Others have an elaborate structure with tooth-like projections or digit-like prongs that protrude from the head of the penis. The walrus holds the record for sheer size, with a baculum that can exceed 60 centimeters in length.5Current Biology. Is There a Bone in Your Penis? What Science Says Baculum length scales only weakly with body mass, meaning a large animal doesn’t necessarily have a proportionally large penis bone.7PubMed. Sexual selection and genital evolution in mammals: a phylogenetic analysis of baculum length Something other than body size is driving the variation.
One consistent finding is that the baculum has lower mineral density and reduced stiffness compared to regular skeletal bones. A study of dog bacula found that baculum mineral density was lower than that of the radius in both castrated and uncastrated animals, resulting in much lower stiffness. This is probably a feature rather than a bug: a softer bone is less likely to snap under the unusual mechanical stresses of copulation.8Journal of Structural Biology. The canine baculum: The structure and mechanical properties of an unusual bone A similar pattern appears in seals, where the baculum varies more in size and shape than load-bearing bones like the humerus, suggesting it is under different evolutionary pressures than weight-bearing skeletal structures.9Wiley Online Library / PubMed Central. Microarchitecture of the penis bone (baculum) of a seal: A 3D morphometric examination using synchrotron and laboratory micro-computed tomography
Why Do So Many Mammals Have One
The baculum appears to serve multiple purposes depending on the species, but the most widely supported function is mechanical: it stiffens the penis enough to achieve penetration, especially during prolonged mating. In primates, an elongated baculum correlates with copulatory patterns involving prolonged penetration or maintenance of penetration after ejaculation.10American Journal of Primatology. Baculum length and copulatory behavior in primates Species that mate quickly and move on tend to have shorter bacula or none at all.
Sexual selection also plays a role. In rodents and carnivores, baculum length and glans penis length vary with relative testis size, a common proxy for the intensity of sperm competition. When females mate with multiple males, males with larger testes and longer bacula may have a reproductive edge. The relationship doesn’t hold equally across all groups, though. In bats and primates, baculum length shows no clear link to testis size, suggesting different selective pressures are at work in different lineages.7PubMed. Sexual selection and genital evolution in mammals: a phylogenetic analysis of baculum length
Research on primates and carnivores specifically found that postcopulatory sexual selection, the competition that happens after mating via sperm competition and related mechanisms, influences baculum evolution. The ancestral primate and the ancestral carnivore both possessed a baculum, suggesting it was the default equipment for these lineages.11PubMed Central. Postcopulatory sexual selection influences baculum evolution in primates and carnivores
How Humans Lost It
Reconstructing the evolutionary timeline, researchers estimate that the ancestral mammal did not possess a baculum. The bone first appeared after placental and non-placental mammals split but before the common ancestor of primates and carnivores evolved. By that point, baculum presence was essentially universal in these lineages.11PubMed Central. Postcopulatory sexual selection influences baculum evolution in primates and carnivores Across all mammals, the baculum has been gained a minimum of nine times and lost a minimum of ten times, making it one of the more evolutionarily restless structures in the mammalian body. Groups that have a baculum also show higher rates of species diversification, which is intriguing even if the causal relationship isn’t fully understood.12PubMed Central. The Morphological Diversity of Intromittent Organs
So humans didn’t just never develop a baculum. Our primate ancestors had one, and at some point in hominin evolution, it disappeared. One hypothesis focuses on mating systems: because human copulation is relatively brief compared to many primates and doesn’t involve a prolonged post-ejaculatory “lock,” there may have been less selective pressure to maintain the bone. Another proposal is more dramatic. If the presence of a baculum made a flaccid penis more vulnerable to injury from blunt trauma, males who could recognize this vulnerability might have exploited it during fights. A boneless penis would be harder to permanently damage, giving mutant males without a baculum a fitness advantage in socially aggressive environments.13Mammal Review. The missing human baculum: a victim of conspecific aggression and budding self‐awareness? This idea is speculative but illustrates how evolutionary pressures on genital anatomy can come from unexpected directions.
Genomic evidence provides a complementary angle. A landmark study comparing the human and chimpanzee genomes identified specific chunks of regulatory DNA that humans lost. One of these deletions removed an enhancer from the androgen receptor gene that, in other species, drives the development of penile spines, small keratinized structures found on the penis of many primates. The loss of this enhancer is correlated with the anatomical disappearance of penile spines in humans.14Nature. Human-specific loss of regulatory DNA and the evolution of human-specific traits Penile spines are not the same as the baculum, but the loss of both structures in humans points to a broader pattern of genital simplification in our lineage, possibly linked to shifts toward pair-bonding and longer copulation without a need for mechanical aids.
The “Honest Signal” Theory
One interesting consequence of relying entirely on hydraulic erection is that the system is fragile. Erection depends on a complex chain of neurological, vascular, and hormonal signals all working correctly. Cardiovascular disease, diabetes, hormonal disruption, and psychological stress can all interfere. Some researchers have proposed that this fragility is actually the point, at least from an evolutionary perspective. If erection reliably reflects a male’s overall health, then a boneless penis becomes an honest signal of quality. A female selecting a mate who can achieve and maintain an erection is, in effect, selecting for cardiovascular fitness, neurological health, and hormonal balance all at once.15PubMed. Male reproductive physiology as a sexually selected handicap? Erectile dysfunction is correlated with general health and health prognosis and may have evolved as a marker of poor phenotypic quality
Under this framework, erectile dysfunction isn’t just a modern medical problem. It may be the specific consequence of an evolutionary process driven by the handicap principle, the idea that costly or risky traits persist because they honestly advertise the bearer’s genetic quality.16PubMed. Back to Stir It Up: Erectile Dysfunction in an Evolutionary, Developmental, and Clinical Perspective A baculum would shortcut this signal. If a bone guaranteed rigidity regardless of the male’s condition, females would lose a useful piece of information about mate quality. Whether this scenario actually drove baculum loss or is a downstream consequence of it remains debated, but it’s a compelling way to think about why human erection is so dependent on overall health.
When Bone Does Form in the Human Penis
Although healthy human penile tissue contains no bone, pathological ossification can occur. The most common context is Peyronie’s disease, a condition where fibrous plaque develops in the tunica albuginea, the tough sheath surrounding the erectile tissue. In a study of 71 surgical cases, about 61% showed dense fibrotic plaque alone, roughly 35% had plaque with patches of actual bone formation, and 4% were composed predominantly of bone.17PubMed. Peyronie disease: a clinicopathologic study of 71 cases with emphasis on histopathologic patterns and prevalent metaplastic ossification The calcification visible on imaging in Peyronie’s patients almost always represents genuine bone, not just calcium deposits.
This ossified tissue bears a real resemblance to skeletal bone. Detailed imaging of mineralized Peyronie’s plaques has revealed highly porous material with features like lamellae and structures resembling the tiny cavities where bone cells normally live.18Acta Biomaterialia. Mineralized Peyronie’s plaque has a phenotypic resemblance to bone In extreme cases, ossification can extend along the entire penile shaft, though the condition remains rare overall, with fewer than 40 published case reports in the medical literature.19PubMed Central. Penile ossification of the entire penile shaft found incidentally on pelvic x-ray
This isn’t a vestigial baculum reasserting itself. Peyronie’s ossification is a pathological process, essentially the body’s wound-healing machinery going awry and laying down bone in soft tissue. But the fact that human penile tissue retains the molecular capacity to form bone is a reminder that the developmental toolkit for building a baculum hasn’t been entirely dismantled, just turned off under normal circumstances.
Penile Fracture and Structural Vulnerability
A common follow-up question is whether the lack of a bone makes human penises more vulnerable to injury. The answer is mixed. There’s no bone to break, but the tunica albuginea can rupture during vigorous intercourse if bent sharply while fully erect, an injury colloquially called a “penile fracture.” A study of patients who experienced this kind of rupture found that five out of six had pre-existing structural abnormalities in the tunica albuginea, including fibrous scarring and inflammatory cell infiltration, which could weaken the tissue and lower the pressure threshold at which a tear occurs.20PubMed. Traumatic rupture of the corpora cavernosa: new physiopathologic acquisitions In other words, penile fracture isn’t purely a matter of bad luck or extreme force. Underlying tissue quality plays a major role.
Species with a baculum face their own version of this problem. The reduced mineral density that makes bacula less likely to snap also means they are not invulnerable. Baculum fracture does occur during copulation in some species and can compromise reproductive success. Whether the hydraulic human system or the bony mammalian system is more “fragile” overall is hard to say; they fail in different ways under different conditions.
The Female Equivalent
The baculum has a lesser-known counterpart in females called the baubellum, or os clitoridis, a small bone found in the clitoris of many mammals. The clitoris and penis develop from the same embryonic tissue, so the existence of a female version makes anatomical sense. A comparative study across 163 species found that in most cases, males and females matched: if males had a baculum, females had a baubellum, and vice versa. But in 29 of those species, a baculum was present while a well-developed baubellum was not.21PubMed Central. The baubellum is more developmentally and evolutionarily labile than the baculum
The baubellum has undergone significantly more evolutionary transitions than the baculum, especially losses. It also shows more developmental variability even within species where it is present. These patterns suggest the baubellum may be largely nonfunctional, persisting as a developmental echo of the baculum rather than serving its own independent role. In contrast to the baculum, which has received extensive study, the baubellum has gotten remarkably little research attention. A 2024 paper described it for the first time in the spectacled bear, underscoring just how much basic descriptive work remains undone.22PubMed Central. First description of the baubellum in the spectacled bear Tremarctos ornatus (Mammalia: Ursidae)
What Fossil Bacula Tell Us About Extinct Species
Because the baculum is one of the few parts of the male reproductive system that fossilizes, paleontologists can use it to infer the mating behavior of species that have been dead for millions of years. The size and shape of fossil bacula correlate with several aspects of reproductive biology in living mammals, including mating duration, mating system type, and population density.23PLOS ONE. Fossil bacula of five species of Borophaginae (Family: Canidae): Implications for their reproductive biology
A study of fossil bacula from the extinct bear Indarctos arctoides used baculum size and morphology to infer that this species probably had prolonged copulation, a multi-male mating system, and large home ranges or low population density.24PLOS ONE. On the Socio-Sexual Behaviour of the Extinct Ursid Indarctos arctoides: An Approach Based on Its Baculum Size and Morphology It’s a remarkable example of how a single bone can open a window into the private lives of animals that no human ever observed alive. Similar work on extinct dog relatives has used fossil bacula to reconstruct mating behavior across the canid family tree, revealing how reproductive strategies shifted as species diversified and competed in different ecological niches.
Scaling Patterns Across the Dog Family
How baculum size relates to body size varies dramatically depending on which group of mammals you look at. In most mammalian lineages, larger species tend to have disproportionately shorter bacula relative to their body mass, a pattern called negative allometry. Canids break this trend. A study of 26 living species in the dog subfamily found that baculum length scales roughly in proportion to body size, neither getting relatively larger nor smaller as the animal gets bigger.25Biological Journal of the Linnean Society. Evolutionary allometry of the canid baculum (Carnivora: Mammalia) This pattern held even after accounting for the shared evolutionary history among canid species, suggesting something about canid mating biology or genital mechanics maintains a consistent proportional relationship that other groups don’t share. Whether this reflects a constraint on function, a particular intensity of sexual selection, or something else entirely remains an open question and a reminder that even within the world of penis bones, different lineages play by different rules.