Absence in Humans
Unlike other primates, humans lack an os penis or os clitoris; however, this bone is much reduced among the great apes: in many ape species it is a relatively insignificant 10–20 mm structure. There are reported cases of human penis ossification following trauma, and one reported case of a congenital os penis surgically removed from a 5 year old boy, who also had other developmental abnormalities, including a cleft scrotum. Clellan S. Ford and Frank A. Beach in Patterns of Sexual Behavior (1953), p. 30 say "Both gorillas and chimpanzees possess a penile bone. In the latter species the os penis is located in the lower part of the organ and measures approximately three-quarters of an inch in length." In humans, the rigidity of the erection is provided entirely through blood pressure in the corpora cavernosa.
It has been speculated that the loss of the bone in humans, when it is present in our nearest related species the chimpanzee, is because humans "evolved a mating system in which the male tended to accompany a particular female all the time to try to ensure paternity of her children" which allows for frequent matings of short duration. Observation suggests that primates with a baculum only infrequently encounter females, but engage in longer periods of copulation which the baculum makes possible, thereby maximizing their chances of fathering the female's offspring. For humans, it is almost impossible to tell when the female is fertile so frequent matings would be necessary to ensure paternity.
The evolutionary biologist Richard Dawkins also speculated in 2006 that the loss of the bone in humans, when it is present in our nearest related species the chimpanzee, is a result of sexual selection by females looking for honest signals of good health in prospective mates. The reliance of the human penis solely on hydraulic means to achieve a rigid state makes it particularly vulnerable to blood pressure variation. Poor erectile function betrays not only physical states such as diabetes and neurological disorders but mental states such as stress and depression.
Read more about this topic: Baculum
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