Dogs possess a bone in their penis, known as the baculum or os penis. This bone is a normal part of canine anatomy, found in many animal species and serving particular biological functions.
Understanding the Baculum
The baculum is a slender bone located within the penis. Unlike bones that form part of the main skeleton, it is a heterotopic bone, meaning it develops in soft tissue. Its size and shape vary among individual dogs, with average lengths in adult canines ranging from 83 to 128 millimeters.
The canine baculum has a ventral groove along its length, through which the urethra passes, providing a protective channel. The bone is composed of outer cortical plates and an inner cancellous bone. Studies indicate that the baculum has a lower mineral density compared to skeletal bones, which contributes to its flexibility and reduces the risk of fracture during its biological function.
The primary role of the baculum in canine reproduction is to provide rigidity to the penis, aiding in copulation. It allows for successful penetration even before full erection is achieved, as the bone provides initial structural support. This rigidity is particularly important during the “tie” phase of mating, where the male and female remain physically coupled for an extended period, facilitating successful fertilization.
Presence in Other Animals
The baculum is not exclusive to dogs; it is found in the penises of many other placental mammals, illustrating its widespread evolutionary presence. Examples include various carnivores such as bears, raccoons, seals, ferrets, and otters, as well as some primates like gorillas and chimpanzees. Rodents and bats also possess this bone.
Conversely, the baculum is absent in several mammalian lineages. Humans, for instance, do not have a penis bone. Horses and most other hoofed mammals (ungulates), as well as lagomorphs like rabbits and hares, also lack this structure. Evolutionary studies suggest that the baculum has evolved independently multiple times across different mammalian groups and has also been lost in various lineages throughout evolutionary history.
Common Health Considerations
While robust, the baculum can be subject to certain health issues in dogs. Although rare, fractures of the os penis can occur, often resulting from trauma, such as accidents, fights, or forceful separation during mating. Signs of a fractured baculum may include visible pain, swelling of the penis, difficulty or inability to urinate, and sometimes a noticeable deviation of the penis or discharge containing blood. Prompt veterinary attention is important if a fracture is suspected, as complications like urethral obstruction can arise from fibrous tissue or callus formation around the fracture site.
The close association between the baculum and the urethra also makes dogs susceptible to specific urinary problems. Since the urethra runs through a groove in the bone, damage to the baculum or conditions affecting the surrounding area can impact urinary flow. Urethral calculi, or urinary stones, are a common concern, as they frequently lodge in the urethra just behind the os penis, causing partial or complete obstruction of urine passage. Symptoms of urethral obstruction include straining to urinate, frequent attempts to urinate with little or no success, blood in the urine, and signs of discomfort or pain.
In some cases, non-neoplastic bone hyperplasia, an overgrowth of bone tissue, can occur around the baculum, leading to a narrowing of the urethra and subsequent difficulty urinating. Another condition, paraphimosis, where the penis cannot be retracted into its protective sheath, can also be indirectly related to trauma that might affect the baculum. This condition causes the exposed penis to swell and become painful, necessitating immediate veterinary intervention to prevent tissue damage and restore normal function.