All male mammals have penises. Did you know that humans are one of the two primate species lacking a “baculum” or penis bone! Whales, rabbits, elephants, horses also lack a bone in their manhood.
Where’s the bone?
Some species have a bone to help the male maintain his erection long enough to penetrate the female and deliver the sperm. This bone, baculum, is in the male’s abdomen until he needs it. When he does his abdomen muscles contract pushing into into the penis, making an hard erection.
Speed is of the essence in nature, having to wait for the penis to fill with blood is too long for many species. The male lion, thanks to his baculum, can copulate 250 times in four days.
Where’s our bone?
As stated in IFLScience:
“Well, the reason is not entirely clear, but it’s believed to be down to our mating systems and strategies. In the 30th anniversary edition of his book, The Selfish Gene, evolutionary biologist Richard Dawkins suggested the lack of a human baculum is the result of “sexual selection” by female humans looking for healthy males.
That is, having a penis that relies on “hydraulics” to become erect (rather than a bone) means there will be some males with poor erectile function. If Dawkins is right, an innate female desire to guage a male’s suitability as a mate was responsible for “selecting” a penis that shows such suitability (or lack thereof).
Interestingly, our closest living relatives, including the chimpanzee, possess penis bones, but those bones are very small. It is possible that our primate relatives may too eventually lose their bacula.
Indeed, perhaps it’s more a question of why the other great apes still have bacula, rather than why we humans lack them. Complete loss of a baculum in humans seems to just continue a trend towards baculum size reduction which is found among the great apes.”