Peculiar morphology The mammalian penis bone

A peculiar bony structure exists in the penis of many mammalian species, and this bone, often referred to as bac-

ulum or os penis, is probably one of the most puzzling and least understood bones of the mammalian skeleton. Present in a variety of orders including Insectivora, Chiroptera, Primates, Rodentia, and Carnivora, this bone does not occur in all Orders in the Mammalia, but also does not occur in all species within each Order. Within a species the bone also varies in size, with older individuals typically possessing longer penis bones. In the mammals, the largest penis bone in absolute and relative size occurs in the walrus (Odobenus rosmarus), where the baculum may reach up to 22 in (54 cm) in length. In contrast, the bone is mostly vestigial in rodents such as North American beavers (Castor canadensis), and very small in all felids (cats), which have spines on the penis.

There are obvious costs to possessing a penis bone as evidenced from accounts of penis bone fractures. Historical hypotheses suggested that the bone may provide additional support to the penis for copulation, may protect the urethra from collapsing and blocking sperm passage in species that copulate for long periods, or else may help stimulate females into ovulation. However, all hypotheses have weaknesses and the most current hypothesis explaining the evolution of the mammalian penis bone in carnivores suggest that the largest penis bone evolved in species with promiscuous mating systems as a way for females to assess male quality during copulation. However, explanations may not be exclusive and possibly other functions may exist in other taxa. Undoubtedly, the full significance of this bone into the evolution of mammals has not yet been fully understood and remains an enigmatic puzzle to solve for mammalian scientists.

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