baculum

The baculum, also known as the os penis, is a bone found in the penis of most placental mammals, some marsupials, and some reptiles. It is absent in humans, horses, and cetaceans (whales and dolphins). The baculum varies greatly in size, shape, and complexity across different species, often reflecting mating strategies and sexual dimorphism. Its primary function is to facilitate and maintain penile erection, particularly during prolonged copulation. The bone's structure can provide support and rigidity, enabling penetration even when the penis is not fully engorged with blood. Studies of bacula have been used in taxonomic research and in understanding evolutionary relationships between species. In some species, the baculum's size and shape are correlated with the female reproductive tract, possibly influencing sperm competition or ensuring successful insemination. The development and presence of the baculum are hormonally regulated. While generally considered a simple skeletal element, its intricate morphology in some animals has led to detailed anatomical and functional investigations. The evolutionary loss or reduction of the baculum in certain mammalian lineages, including primates, is a subject of ongoing scientific inquiry.