Horses do not possess a baculum, which is the scientific name for the penis bone (os penis). This extra-skeletal bone is found in the male anatomy of many placental mammals, but it is absent in all members of the order Perissodactyla, including horses, zebras, and donkeys. The absence of this skeletal element means the horse’s reproductive system relies on a completely different biological mechanism to achieve rigidity during copulation.
Defining the Baculum
The baculum (os penis) is a heterotopic bone, meaning it is not connected to the main skeleton. This rod-shaped element develops within the soft tissues of the penis, specifically in the distal portion, influenced by male hormones. Its primary function is to provide continuous structural support and rigidity during copulation. This skeletal element allows the male to maintain penetration, often enabling prolonged intromission.
Mammalian Groups That Possess the Bone
The baculum is widely distributed across placental mammals. It is commonly found in the following orders:
- Carnivora (dogs, bears, and seals)
- Rodentia (mice, rats, and squirrels)
- Chiroptera (bats)
- Primates, though its size is highly variable within this order
The bone is notably absent in several major groups, including ungulates, lagomorphs (rabbits and hares), and cetaceans (whales and dolphins).
Equine Anatomy and the Vascular Alternative
The horse’s reproductive system uses a different mechanical principle known as the musculocavernous type of penis. This structure contains a large amount of erectile tissue, allowing the organ to be highly flexible when not in use. The stallion’s erection mechanism is entirely dependent on a massive influx of blood and resulting hydrostatic pressure, rather than skeletal support.
Mechanism of Rigidity
The penis contains two large, cylindrical chambers of erectile tissue: the paired corpus cavernosum and the single corpus spongiosum, which surrounds the urethra. During arousal, parasympathetic nerve signals cause arteries to dilate, rapidly filling these chambers with blood. The corpus cavernosum is the primary tissue that engorges, causing a significant increase in the organ’s length and diameter.
A dense layer of connective tissue called the tunica albuginea surrounds the corpus cavernosum. This sheath traps the blood within the erectile tissue by compressing the veins that would otherwise drain it, a process known as the veno-occlusive mechanism. Contraction of muscles at the root of the penis, such as the ischiocavernosus, forces blood distally and further enhances rigidity. This allows the stallion’s penis to approximately double in length and thickness when erect.