Rat Penis Anatomy: Baculum, Erection, and ED Research

The rat penis is one of the most intensively studied reproductive organs in biomedical science, and its anatomy differs from the human version in ways that matter for both evolutionary biology and medical research. It contains a bone called a baculum, relies on a dual-chamber erectile system with unusual vascular plumbing, and depends on testosterone so heavily that removing the hormone source triggers cell death in penile tissue within days. Because rats are the primary animal model for erectile dysfunction research, understanding how this organ is built and how it works has direct implications for human medicine.

Anatomy of the Rat Penis

At a glance, the rat penis looks simple, but its internal architecture is surprisingly complex. It contains two independent erectile tissues: the corpora cavernosa (a paired structure running along the top of the shaft) and the corpus spongiosum (which surrounds the urethra). Each has its own dedicated arterial supply and venous drainage, meaning each can fill and pressurize somewhat independently during erection.¹ The base of the penis, where the corpora cavernosa originate as paired crura, contains large vascular spaces packed with smooth muscle. These crura act as a kind of hydraulic regulator, controlling how much blood reaches the rest of the shaft.

A few vascular features stand out as unusual. The helicine arteries within the crura have muscular cushions built into their walls, something not found in arteries elsewhere in the penis. These cushions likely help control blood inflow during arousal. On the outflow side, the paired cavernous veins that drain the shaft split into smaller channels before joining a venous network beneath the tunica (the tough outer sheath). This splitting arrangement appears to be a built-in mechanism for trapping blood inside the penis during erection by increasing resistance to outflow.²

The Baculum, a Bone Inside the Penis

Perhaps the most striking anatomical difference from the human penis is the baculum, or os penis, a small bone embedded in the tip. Most mammals have one (humans are among the exceptions), and in the rat it serves a structural role during copulation. The baculum sits within the glans and is connected to the corpus cavernosum by a layer of fibrocartilage. During erection, the tip of the corpus cavernosum swells and wraps around the base of the baculum, locking the soft erectile tissue to the rigid bone.³ This interface allows the erect penis to transmit mechanical force to the female’s cervix during the rapid, repeated intromissions that characterize rat mating.

The baculum itself develops in two segments. The proximal (nearer-to-body) segment appears on radiographs as early as postnatal day five and grows rapidly over the first forty days of life, reaching roughly four to five millimeters. The distal segment does not show up until around day forty and continues growing until about day 180, adding another two to three millimeters.⁴ The two segments even form differently: the proximal half develops partly from membrane bone and partly from ossifying cartilage, while the distal segment develops from fibrocartilage.⁵

From an evolutionary standpoint, baculum length in rodents appears to track with the intensity of sperm competition within a species. Species where females mate with multiple males tend to have longer bacula, suggesting that the bone plays a role in competitive mating success beyond simple structural support.⁶

How Erection Works in the Rat

The basic mechanism of erection in rats follows the same broad principle as in humans: nitric oxide (NO) is released from nerve endings and blood-vessel linings in the erectile tissue, triggering smooth muscle relaxation, which allows blood to rush in and pressurize the corpora cavernosa. The enzyme nitric oxide synthase (NOS) is central to this process. What makes the rat interesting as a model is how cleanly researchers can isolate each piece of the machinery.

Castration, for example, cuts the erectile response to nerve stimulation roughly in half and correspondingly reduces NOS activity in penile tissue. Replacing testosterone restores both. The actual amount of NOS protein does not drop much after castration; rather, the enzyme’s activity falls, suggesting that testosterone keeps the existing machinery running rather than building new copies of it.⁷ The researchers who demonstrated this concluded that penile erection in rats is essentially fully androgen-dependent.

On the other side of the cycle, detumescence (the return to a flaccid state) depends on breaking down a signaling molecule called cGMP. The enzyme responsible, phosphodiesterase type 5 (PDE5), is the same target that drugs like sildenafil (Viagra) inhibit in humans. In rat erectile tissue, PDE5 exists in two forms, with one being the dominant isoform.⁸⁹ The fact that this enzyme system closely mirrors the human version is a big part of why rats became the go-to species for testing erectile dysfunction treatments.

Neural Wiring and Brain Control

Erection in the rat is not a single reflex but a layered response involving spinal circuits, peripheral nerves, and several brain regions. The dorsal nerve of the penis, a sensory nerve running along the top of the shaft, plays a key role. Electrical stimulation of this nerve in spinal rats (those with the brain disconnected from the spinal cord) can produce full tonic erections, along with a coordinated ejaculatory response involving the bulbospongiosus muscle.¹⁰ This tells researchers that a complete erection-to-ejaculation sequence can be generated at the spinal level alone, without input from the brain.

The sympathetic nervous system, meanwhile, acts as a brake. Sympathetic nerves keep the penis in a flaccid state by maintaining smooth muscle tone in the erectile tissue. Blocking sympathetic activity with chemical sympathectomy improves the erectile response, while pharmacological experiments have shown that both norepinephrine and at least one other unidentified co-transmitter participate in this inhibitory signaling.¹¹¹²

Higher up in the nervous system, the hypothalamus exerts considerable influence. Oxytocin, a hormone best known for its role in social bonding and childbirth, is a potent trigger of penile erection when it acts within the brain. The most sensitive site is the paraventricular nucleus of the hypothalamus, but oxytocin can also promote erection when delivered to the ventral tegmental area, parts of the hippocampus, the amygdala, and the thoraco-lumbar spinal cord. The mechanism appears to involve activating dopaminergic neurons, sometimes directly and sometimes by first releasing glutamate.¹³

Testosterone and Penile Tissue Maintenance

Testosterone does more than modulate NOS activity. It actively maintains the cellular structure of penile tissue. When rats are castrated, cells in the erectile tissue begin undergoing programmed cell death (apoptosis). Restoring testosterone reverses this process and induces new DNA synthesis, meaning new cells are being produced.¹⁴ This regenerative effect is not limited to a single cell type; smooth muscle cells, endothelial cells, and connective tissue cells all respond to testosterone replacement with proliferation.¹⁵

This finding has shaped how researchers think about the relationship between low testosterone and erectile dysfunction in aging men. If the same basic dependency holds across species, then long-term androgen deficiency does not just reduce a chemical signal, it physically remodels the organ.

What Happens When the Rat Penis Ages

Aging in the rat penis mirrors some of what happens in humans, which is part of why it is so useful as a model. Aged rats show disordered blood sinuses, thinning and discontinuous smooth muscle layers, and a shift in the ratio of smooth muscle to collagen in favor of collagen (fibrosis).¹⁶ Elastin content and the length of the largest collagen fibers also decline. The functional result is that the corpora cavernosa can no longer trap blood effectively, a condition called corporal veno-occlusive dysfunction, which is the primary mechanical cause of age-related erectile dysfunction in both rats and humans.¹⁷

Diabetes accelerates these changes. In rat models of diabetes, erection frequency drops, the structural organization of the corpora cavernosa deteriorates, and oxidative stress markers rise in penile tissue.¹⁸ These diabetic rat models have been indispensable for testing candidate drugs aimed at protecting erectile tissue from metabolic damage.

The Rat as a Model for Erectile Dysfunction Research

One of the most productive uses of the rat penis in biomedical science is the cavernous nerve injury model. During prostate removal surgery in men (radical prostatectomy), the cavernous nerves that supply the penis are frequently damaged, often leading to lasting erectile dysfunction. Researchers replicate this by crushing or cutting the cavernous nerves in rats and then testing whether various therapies can restore function.¹⁹

Recent work using this model has tested regenerative approaches including stem cells and bioactive materials. One group implanted a fibrous membrane containing nerve growth factor around the crushed nerves in rats and found that it promoted nerve regeneration, increased the number of smooth muscle cells and nitric oxide synthase levels in penile tissue, and reversed the fibrosis that normally follows nerve injury.²⁰ Another study tested human amniotic fluid stem cells in the same bilateral nerve crush model, exploring whether stem cell therapy during the early phase after injury could accelerate recovery.²¹

These studies are still a long way from human clinical use, but they illustrate why the rat model is considered essential. The rat’s penile physiology is close enough to the human version, particularly in its dependence on nitric oxide signaling and its vulnerability to nerve damage, that findings reliably identify promising leads for human therapies. The consistent results from this model over decades have made it the standard starting point for any experimental erectile dysfunction treatment.

Developmental Vulnerability and Environmental Chemicals

The development of the rat penis is exquisitely sensitive to hormonal disruption, which makes it valuable for studying how environmental chemicals can interfere with male reproductive development. Phthalates, a class of chemicals used in plastics and personal care products, have been a major focus. When pregnant rats are exposed to di-n-butyl phthalate (DBP) during a critical window of gestation, male offspring develop hypospadias, a birth defect in which the urethral opening forms on the underside of the penis rather than at the tip. In one study, maternal exposure to a high dose of DBP from gestation day 14 to 18 produced hypospadias in about half of male pups, along with reduced body weight and shorter anogenital distance (a marker of androgen exposure in utero).²²

The defect is dose-dependent: higher phthalate exposures produce more cases of hypospadias and cryptorchidism (undescended testes), and the effects appear to stem from disruption of androgen-dependent gene expression at specific developmental stages.²³ A systematic review of rodent studies on phthalate-induced hypospadias catalogued urethral abnormalities across nineteen studies, consistently finding misplaced urethral openings and incomplete fusion of the ventral foreskin.²⁴

These rat findings have been central to the broader scientific case that certain industrial chemicals can act as anti-androgens during fetal development. While the doses used in rat studies are typically much higher than what humans encounter, the consistency and specificity of the effects, hitting androgen-dependent structures during narrow time windows, have helped shape regulatory risk assessments for phthalates in consumer products.

Copulatory Plugs and Postcopulatory Behavior

Rat reproduction involves a detail that surprises most people unfamiliar with rodent biology: the copulatory plug. After ejaculation, secretions from the seminal vesicles and coagulating gland solidify into a rubbery plug that fills the female’s vagina. This plug is not incidental; it is required for sperm to reach the uterus. When male rats were treated with finasteride (a drug that blocks conversion of testosterone to the more potent dihydrotestosterone), their seminal vesicles and prostate shrank, and they either failed to form a plug or formed one that was too small and poorly positioned. The result was dramatically reduced fertility, even though the males were still mounting and ejaculating normally.²⁵

After mating, male rats engage in conspicuous genital grooming, licking the penis and surrounding area extensively. This behavior turns out to be more than fastidiousness. When researchers prevented postcopulatory grooming, male rats were more likely to acquire a bacterial infection (Staphylococcus aureus) that had been experimentally placed in the female’s vagina beforehand. Interestingly, rat saliva itself showed almost no ability to kill the bacteria, suggesting that the protective effect comes from the physical washing action rather than any antimicrobial properties in the saliva.²⁶ It is a practical, mechanical hygiene strategy that probably evolved under the pressure of sexually transmitted infections.