The baculum, often called the penis bone, is a structure found in many male mammals. The common misconception that it is composed of ivory is incorrect; this skeletal element is made entirely of bone. Bone is a living tissue fundamentally different from ivory, which is a form of dentin. The confusion likely stems from the historical use and trade of both bone and ivory from certain large mammals.
Defining the Baculum and Its Function
The baculum, or os penis, is a bone that develops within the soft tissue of the penis in a broad range of placental mammals. This extra-skeletal bone is found in nearly all carnivores (such as bears, dogs, and walruses), rodents, bats, and most primates, though humans are a notable exception. Its presence and morphology vary dramatically, leading to its description as one of the most diverse bones in the mammalian skeleton.
The primary function of the baculum is to provide structural rigidity and support during copulation. By ensuring the stiffness of the male organ, the bone aids in successful penetration and can also help to prolong the duration of copulation. This prolonged intromission is a reproductive strategy that may help a male guard a female from competitors. The size and shape of the baculum are often linked to the species’ mating system.
Bone Versus Ivory: A Structural Comparison
The baculum is a true bone, meaning its structure and composition align with other skeletal bones in the body. Bone is a dynamic, living tissue composed primarily of an organic matrix of the protein collagen, which provides flexibility, and an inorganic component of calcium phosphate (hydroxyapatite), which provides hardness. This structure is highly organized, featuring microscopic channels called Haversian systems that housed blood vessels and nerves.
Ivory, by contrast, is not bone but a specialized form of dentin, the material that makes up the bulk of teeth and tusks. Although it shares a similar chemical makeup to bone, consisting of collagen and calcium phosphate, ivory is significantly denser and acellular. Unlike bone, ivory does not contain the complex network of Haversian canals, which contributes to its uniform, smoother appearance and greater density when dried.
The fundamental difference lies in their biological origins and structure: bone is part of the skeleton and is constantly remodeled, while ivory is a tooth product. Bone is porous and was once fed by a blood supply, while ivory is a much harder, non-vascularized material.
Why the Confusion About Ivory Exists
The misconception likely stems from the historical and cultural significance of bacula from large marine mammals, particularly the walrus. The walrus baculum, sometimes reaching two feet in length, is known in Alaska Native cultures as an oosik and was traditionally polished and carved into tools and artifacts. This large, dense, and often fossilized specimen can take on a smooth, off-white appearance that is superficially similar to actual ivory, which is also commonly harvested from walrus tusks.
Both oosik (baculum) and actual walrus ivory (tusk dentin) were historically collected, traded, and carved for similar decorative or utilitarian purposes. The high polish and dense nature of a dried or fossilized baculum could easily lead to an uninformed person mistakenly labeling it as ivory. Furthermore, the term “ivory” itself is sometimes used loosely to refer to any hard, white material derived from a large animal’s teeth or bones, further blurring the distinction.