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 that 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."