I once saw two dogs get stuck together on a beach fucking, which is how I know that a long-lasting erection is not necessarily always a good thing. Mammals—like dogs, cats, rats, and racoons—have a little bone in their penis called the baculum. It's a smooth, slightly curved sliver of bone that looks like the sort of thing a beach bum would wear on a leather cord as a pendant around their neck.
The baculum is the reason that those two dogs got stuck fucking—facing away from each other, and whelping in pain—that day on the beach. Luckily, humans no longer have a baculum, which means you can beat a hasty retreat if your coital partner has you whelping in more pain than pleasure.
You can thank evolution for that, and new research explains why human boners are boneless, so to speak. Using a pioneering technology called Bayesian phylogenetics, a team at University College London tracked the evolution of the baculum through mammals and primates related to human beings through history.
"It's the technology used by Nate Silver to predict the last two elections," explains study co-author Matilda Brindle. "Only instead of predicting the future, we reconstructed the past. What phylogenetics does is look at how a trait, such as the baculum, is distributed across living species—sort of like a family tree. So if you know how different primates are related you can study things in light of their pattern of descent and make really robust predictions about how they evolved."
Using the technology, Brindle examined the baculum's evolution throughout human history by looking at our most recent common ancestors: chimpanzees and bonobos. She found that humans probably lost their baculum around six million years ago, when humans split off from chimps and bonobos in an evolutionary sense. I ask what purpose the baculum serves.
"Species with longer intromission periods [as in, the act of penetration] than three minutes tend to have longer baculums," Brindle responds. "That's correlated across the entire course of primate evolution." In essence, the longer the male has his penis inside the female, the more likely they are to have a baculum to prop up his hard-on. Consider the penis bone the evolutionary equivalent of an extra safe condom pumped full of anesthetizing gel.
Brindle explains the purpose of the baculum from an evolutionary perspective: "In males where there is a polygamous mating system, where multiple males mate with multiple females, or where the males only have a very short time to mate with the females, the animals tend to have a long baculum."
The baculum also helps males have the best chance of fertilizing the female. "If you prolong your intromission," she explains, "there's more of a chance for the sperm to meet the cervix before another male comes along and mates with the female."
Brindle believes that as humans evolved into monogamous or polygamous mating systems (where the males mate with multiple females but there is no competition from other males), the usefulness of a baculum diminished. "We don't have a strong level of male competition in humans, because our main mating strategies are monogamous or polygamous," she says, arguing that this means there is no evolutionary incentive for human males to stay harder for longer.
"The average time it takes for males to ejaculate during penetration is under two minutes," she says. "Obviously social and cultural factors may extend the length of copulation now… But if you take a male and ask him to fertilize a female, most can usually do that fairly quickly."
She pauses, then clarifies: "Well, they can try."