The pig penis is a fibroelastic organ, meaning it achieves erection primarily by straightening rather than by swelling with blood the way a human penis does. It features an S-shaped bend called the sigmoid flexure, a corkscrew-shaped tip, and a tough collagen-rich sheath that has attracted significant interest from biomedical engineers. The anatomy matters in two quite different arenas: commercial pig breeding, where understanding the organ’s mechanics improves artificial insemination programs, and human reconstructive surgery, where processed porcine tissue now serves as a graft material for conditions ranging from Peyronie’s disease to urethral strictures.
Gross Anatomy and the Sigmoid Flexure
The boar penis is cylindrical, extending from the pelvic floor forward to roughly the level of the umbilicus when fully protruded. Anatomists divide it into root, body, and tip. The root is anchored by two crura and a structure called the bulbous urethrae, which helps propel urine and semen through the urethra. The body makes up most of the organ’s length and is round in cross-section through the middle, flattening toward the tip.1International Journal of Livestock Research. Gross Studies on Penis of The Boar (Sus scrofa domesticus)
The most immediately recognizable feature is the sigmoid flexure, an S-shaped loop in the body of the penis that sits just in front of the scrotum in the inguinal region.2International Journal of Livestock Research. Gross Studies on Penis of the Boar (Sus scrofa domesticus) This loop is not a defect; it is a storage mechanism. When the boar is not mating, paired retractor muscles hold the penis folded into this S-shape inside the body. During erection, those muscles relax, the flexure straightens, and the penis extends. This is fundamentally different from the human mechanism, where erection depends on massive blood engorgement of spongy tissue. In fibroelastic species like pigs, rams, and bulls, the organ is already fairly rigid at rest, and extension is largely a matter of unfolding.
The Corkscrew Glans
The distal tip of the boar penis spirals into a corkscrew shape, a feature that has no real parallel in human anatomy. This spiral glans is functionally essential: during mating, it locks into the spiral folds of the sow’s cervix, creating a seal that allows the boar to deposit semen directly into the uterine body rather than just the vaginal canal. A study comparing Landrace and Duroc boars found that the number of spiral turns was the same across both breeds, averaging one full turn.3Buletin Veteriner Udayana. Variation in the Size of the Corkscrew-Shaped or Spiral Glans Penis in Landrace and Duroc Boars Size variation existed between strains, but the basic architecture was consistent, suggesting the corkscrew design is under strong selective pressure.
The mechanical lock between the corkscrew glans and the cervix also explains why boar ejaculation takes so long compared to many other mammals. The process can last five to twenty minutes, with semen delivered in distinct waves. Artificial insemination catheters for sows are designed to mimic this cervical lock, using a spiraled foam tip that threads into the cervical folds in place of the glans.
What the Tissue Is Made Of
The structural backbone of the boar penis is the tunica albuginea, a dense fibrous sheath that surrounds the erectile tissue. A detailed proteomic analysis of porcine penile tunica albuginea found that collagen made up roughly half the dry weight of the tissue, with type I collagen alone accounting for about 95% of the total protein.4PubMed Central. Tissue anisotropy and collagenomics in porcine penile tunica albuginea: Implications for penile structure-function relationships and tissue engineering Types III, XII, and VI collagen were present in much smaller amounts. The high proportion of type I collagen gives the tissue its stiffness and tensile strength, which is why fibroelastic penises feel firm even when not erect.
Beneath the tunica albuginea sit the vascular compartments. The corpus cavernosum penis in the boar is threaded with large longitudinal cavernous spaces, while the corpus spongiosum consists of a single large vascular channel divided by internal struts called trabeculae. These trabeculae are packed more closely together on the ventral side and spaced farther apart dorsally.5PubMed. Impotence in the boar: angioarchitecture and venous drainage of the penis in normal boars A fine network of small vascular spaces around the urethral opening connects the corpus spongiosum to the glans. Although these spaces do fill with blood during erection, the degree of engorgement is modest compared to vascular-type penises; the organ’s architecture simply does not allow the kind of dramatic expansion you see in primates or horses.
This composition is remarkably similar to what researchers have found in other fibroelastic species. Histological analysis of the ram penis, for instance, confirmed a tunica albuginea composed primarily of collagen fibers arranged in two layers, outer longitudinal and inner circular, with elastic fibers interspersed between collagen bundles.6Pesquisa Veterinária Brasileira. Morphological analysis of the elastic and collagen fibers in the ram penis The shared blueprint across pigs, sheep, and cattle reflects a common evolutionary solution for species that mate quickly and rely on mechanical locking rather than prolonged intromission.
Nerve Supply
The boar penis has a complex nerve supply involving at least eleven different neurotransmitters and neuropeptides. Immunohistochemical mapping in the pig identified nerves carrying tyrosine hydroxylase, dopamine beta-hydroxylase, vasoactive intestinal polypeptide, and somatostatin as the most numerous, with nitric oxide synthase, neuropeptide Y, substance P, and calcitonin gene-related peptide present at lower densities.7PubMed. Innervation of the fibro-elastic type of the penis: an immunohistochemical study in the male pig The densest concentration of nerve fibers was found in a specific region around the cavernous tissue and retractor muscles, with the glans penis carrying somewhat fewer fibers than the internal structures.
This matters for understanding how erection is regulated in fibroelastic species. Nitric oxide is the primary signal for smooth muscle relaxation in vascular-type erections, and it plays a role here too, but the balance of signals is weighted more toward the mechanical components: retractor muscle relaxation, slight cavernous engorgement, and sigmoid flexure straightening all need to be coordinated. The rich peptidergic innervation suggests the process is more finely tuned than the simple “relax and fill” model might imply.
Ejaculation and Semen Fractions
Boar ejaculation is unusually voluminous and structurally complex. A single ejaculate typically measures in the range of 200 to 300 milliliters, delivered over the course of several minutes in distinct phases.8Scientific Reports. Impact of inclusion of post-spermatic ejaculate fraction in boar seminal doses on sperm metabolism, quality, and interaction with uterine fluid The ejaculate comes out in four recognized fractions: a pre-spermatic fraction that is mostly glandular secretions without sperm, a sperm-rich fraction containing the bulk of the sperm cells, an intermediate fraction with fewer sperm, and a post-spermatic fraction that is mostly seminal plasma from the seminal vesicles and prostate.
In commercial artificial insemination centers, the sperm-rich fraction has traditionally been the only part collected and used to prepare insemination doses.9PubMed. Artificial insemination of all ejaculated sperm fractions accelerates embryo development and increases the uterine vascularity in the pig The later fractions were discarded as low-value dilution. More recent research has challenged that practice. A study comparing insemination doses prepared from the sperm-rich fraction alone versus doses incorporating all ejaculate fractions found no significant differences in pregnancy rates, farrowing rates, litter size, or offspring health, while the inclusive approach yielded meaningful cost savings by producing more doses per collection.10PubMed Central. Should All Fractions of the Boar Ejaculate Be Prepared for Insemination Rather Than Using the Sperm Rich Only?
During natural mating, the cervical lock formed by the corkscrew glans means semen is deposited at or near the cervix, but the sheer volume flushes much of it directly into the uterine body.11Theriogenology. Uterine activity, sperm transport, and the role of boar stimuli around insemination in sows This is why semen deposition in the sow is often described as intrauterine even though the anatomical site of delivery is technically intracervical.
Veterinary Conditions Affecting the Boar Penis
One of the more common congenital problems in young boars is persistence of the preputial frenulum, a band of connective tissue that tethers the penis to the prepuce and normally breaks down before sexual maturity. When it persists, the boar physically cannot extend or use the penis for mating. A review covering three decades of cases found persistent frenulum in 49 young boars, and surgical removal of the connective bands restored breeding ability in about 71% of cases.12PubMed. Persistence of the preputial frenulum in boars The remaining cases likely involved additional structural abnormalities that simple frenulum removal could not fix.
Traumatic penile hematoma is better documented in bulls than in boars, but the underlying mechanics apply across fibroelastic species. The tunica albuginea can rupture at the distal bend of the sigmoid flexure during a misdirected mating thrust. Research on bull penises identified an intrinsic weak point at that location where the tunica is thinner and the cavernous spaces are relatively large.13American Association of Bovine Practitioners Conference Proceedings. Surgical Correction of Hematoma of the Penis in Bulls Normal mating pressures inside the corpus cavernosum can be substantial, and when the tunica gives way, the resulting jet of blood tears through surrounding elastic tissue, creating the massive swelling that gives the condition its common name. The same vulnerability exists wherever a fibroelastic penis has a sigmoid flexure and high intracavernous pressure.
Porcine Tissue as a Graft Material in Peyronie’s Disease
Peyronie’s disease causes fibrous plaques to form inside the human tunica albuginea, bending the penis and often making intercourse painful or impossible. When the curvature is severe, surgery to cut out or incise the plaque and replace the defect with a graft is one of the standard treatments. The choice of graft material has been an open question for decades, and processed porcine tissue has emerged as one of the leading options.
A comparative study of 63 patients undergoing plaque excision and grafting found that porcine dermal grafts and bovine pericardium grafts performed similarly on most outcomes: rates of erectile dysfunction, penile shortening, sensitivity changes, complications, and penile straightening were statistically equivalent. The one difference was that patients who received bovine pericardium grafts were significantly more likely to develop palpable penile nodules at follow-up, at about 21% compared with none in the porcine dermal group.14PubMed. Long-term outcomes after plaque incision and grafting for Peyronie’s disease: comparison of porcine dermal and bovine pericardium grafts Separately, a large case series using a human-derived acellular dermal matrix (FlexHD Pliable) reported low complication rates and concluded that it was a practical option, though it noted the absence of a gold-standard grafting material in the field.15The Journal of Sexual Medicine. The Use of an Acellular Dermal Matrix for Peyronie’s Plaque Excision and Grafting The takeaway is that no single graft material has won definitively, but porcine dermal grafts hold up well against the alternatives and may cause fewer palpable irregularities than bovine tissue.
Porcine Small Intestinal Submucosa in Urethral Repair
Porcine small intestinal submucosa, commonly abbreviated SIS, is a thin sheet of processed pig intestine that acts as a biological scaffold. When sewn into a urethral defect, the idea is that the patient’s own cells gradually migrate into the scaffold and replace it with native tissue. The material has been tested for repairing urethral strictures, which are scar-tissue narrowings that can block urine flow.
Results have been genuinely mixed. One series found that SIS grafts worked well for strictures in the bulbar and bulbopenile urethra, with midterm outcomes comparable to traditional skin flap and mucosal graft repairs.16PubMed. Porcine small intestinal submucosa graft for repair of anterior urethral strictures Another group described SIS as versatile and potentially useful in selected patients, while cautioning that longer follow-up was needed before recommending widespread adoption.17PubMed. Small intestinal submucosa (SIS) graft urethroplasty: short-term results But a third series had poor enough results that the surgeons stopped using SIS entirely for open urethral stricture repair.18PubMed. Small intestine submucosa in urethral stricture repair in a consecutive series
The inconsistency likely reflects differences in stricture length, location, and the degree of surrounding tissue scarring, all of which affect whether the scaffold gets enough blood supply and cellular ingrowth to succeed. SIS remains a niche option rather than a go-to material, used primarily when other graft sources are unavailable or when the surgeon has specific experience with the technique.
Penile Girth Augmentation with Porcine Grafts
Processed porcine dermal tissue has also been tested for cosmetic penile augmentation, with strikingly different results depending on the surgical technique. A 69-patient series using porcine dermal acellular grafts reported an average girth increase of about 3 centimeters in the flaccid state at one year, with no major complications and beneficial effects on patient satisfaction and sexual function.19PubMed. Original technique for penile girth augmentation through porcine dermal acellular grafts: results in a 69-patient series Sexual activity resumed within one to two months after surgery.
A smaller pilot study using a different porcine collagen product called Pelvicol told a much less encouraging story. Among 18 patients, complications were common in both surgical technique groups, including severe penile swelling and ischemic skin ulcers. Four patients required graft removal, and overall satisfaction was poor. The investigators concluded that Pelvicol was not an ideal option for enhancing penile girth.20PubMed Central. The use of an acellular collagen matrix in penile augmentation: A pilot study in Saudi Arabia
The contrast highlights something important about porcine biomaterials in general: “porcine-derived” is not a single product category. The processing method, the source tissue (dermis versus intestine versus pericardium), the thickness, and the surgical placement all matter enormously. A graft that works well in one configuration can fail badly in another, which is why the field has not converged on a single standardized approach.
The Pig as a Preclinical Model for Erectile Tissue Engineering
Beyond providing raw graft material, pigs serve as one of the primary animal models for testing experimental treatments aimed at restoring erectile function. The fibroelastic pig penis is not a perfect stand-in for the vascular human organ, but the size match is closer than what smaller laboratory animals can offer, and the tissue composition of the tunica albuginea is similar enough to make the pig useful for testing scaffold integration and mechanical performance.
Recent work in bioengineered erectile tissue has used pigs alongside rabbits to test three-dimensionally printed cavernous tissue constructs. In these experiments, animals with induced penile deformities received implants of bioengineered tissue and regained erectile function within weeks, eventually mating and reproducing successfully. The research is still at the proof-of-concept stage, but it illustrates why the porcine model keeps showing up in urological engineering: the pig’s penile tissue is large enough to work with surgically, structurally characterized in detail, and available at scale from the commercial meat industry.
The detailed proteomic and histological maps of porcine penile tissue that researchers have assembled over the past decade serve a dual purpose. They inform veterinary reproductive science and breeding programs, but they also provide the reference data that tissue engineers need when they try to build, or rebuild, functional erectile structures from scratch. That crossover between agricultural anatomy and cutting-edge biomedicine is part of what makes the porcine penis a more scientifically active topic than its barnyard origins might suggest.