The baculum, also known as the penile bone, is a bone that is found in the penis of many placental mammals. It is absent in humans, but present in many primates such as macaques, bonobos, chimpanzees, and gorillas. While many animals have a bone in their penis, the human species has lost this characteristic, likely due to monogamy. This bone, known as the baculum, is present in almost all animal species, including many primates. Its absence in humans is a unique characteristic, potentially caused by the shift towards monogamous relationships in our evolution. It is considered an important structure in the male reproductive system, but is not present in human penises, which differs from other mammal penises.