It's an age-old question: why do we call them boners if the human penis doesn't actually have a bone? The baculum, as the dick bone is known, first evolved in mammals between 145 and 95 million years ago. Most mammals, like chimpanzees, bears, and dogs, still have a bone in their boners—but not humans. Why though? A new study suggests two main reasons: monogamy, and quick sex.
Published in the Proceedings of the Royal Society B, the study is titled "Postcopulatory sexual selection influences baculum evolution in primates and carnivores." Writing for the Conversation, study author Matilda Brindle explained more about dick bones than I ever expected to know.
First off, why do mammals have bacula (that's apparently the plural of penis bone) in the first place? The penis bone, Brindle explained, "helps prolong intromission, otherwise known as vaginal penetration." Rather than trying to stretch out the enjoyment of sex, Brindle wrote this prolonged intromission "is a way for a male to prevent a female from sneaking off and mating with anyone else before his sperm have had a chance to work their magic." She added, "This theory brings a whole new meaning to the term 'cock-blocking.'"
Over the course of evolution, having a penis bone at all is linked to longer sex—anything over three minutes. Furthermore, primate males with "longer intromission durations tend to have far longer bacula than males of species where intromission is short," Brindle wrote. So how'd that work out for dudes? Well, from penetration to ejaculation, the average human male lasts less than two minutes.
But that alone doesn't explain the fact that our members are missing a bone. After all, the bonobo, a type of chimpanzee, only goes for about 15 seconds with its 8mm member.
So what happened with humans? The researchers also discovered that mating strategies have a big effect on penis bones. According to Brindle, "males of species facing high levels of sexual competition for females have longer bacula than those facing lower levels of sexual competition." So promiscuous mammals have bigger penis bones. Unlike other mammals, humans are generally monogamous, so human dudes "have minimal sexual competition as females typically only mate with one male at a time," Brindle wrote.
There you have it: humans lost the bone in their boners because they were having monogamous quickies with their mates instead of getting around with a lot of mates for longer times.
