It’s ironic that the slang term “boner” is used for an erection when human males, unlike our closest genetic relatives, don’t have a penis bone.
The 'haves' and the 'have nots'
Gorillas and chimpanzees have a penis bone (bacula), but rabbits, horses, kangaroos, hyenas, and most of the largest mammals on earth, like whales and elephants, don’t.
Although most male mammals have baculae, these differ widely in shape and size according to the species. Mental Floss informs us that a gorilla’s penis bone is only a few millimetres long while a dog’s may be more than seven centimetres.
Read: Your penis
The baculum certainly helps to keep the penis stiff, but human males (and many other species) manage to get and stay erect without it.
How does it happen?
The way an an erection happens is as follows: Sexual arousal causes the brain to send chemical messages to the nerves in the penis, causing blood vessels to relax and allow blood to flow freely into the penis.
Two cylindrical chambers (corpora cavernosa) that act like sponges become engorged with blood, causing the penis to swell up and stiffen. This “swelling” blocks the veins that would normally allow blood to flow out of the penis.
When the sexual stimulus disappears, the arteries in the penis become narrower again, which lets the blood drain from the corpora cavernosa, allowing it to become flaccid.
As a penis bone doesn’t seem to be essential for successful penetration, intercourse and procreation, there are a number of theories why it is present in some species and not in others.
High levels of promiscuity
An article published by The Royal Society in December 2016 suggests that "the baculum plays an important role in facilitating reproductive strategies in populations with high levels of postcopulatory sexual selection" (consecutive acts of copulation).
The key concept to understanding the above is "promiscuity". The study found that there was a clear relationship between the length of the penis bone and the level of promiscuity of the species, i.e. the more promiscuous the species, the longer the bacula.
Species with clear mating seasons, as opposed to mating throughout the year, also have longer baculae. The third interesting fact is that the longer intromission (the time the penis spends in the vagina) lasts, the longer the penis bone tends to be.
In promiscuous species, the risk that a female is inseminated by another male before the sperm of the first male have managed to fertilise the female's eggs is high. Seasonal breeding patterns adds to the intensity because mating only happens during a restricted period of time. What this boils down to is that a bigger penis bone allows longer coitus, thereby reducing the likelihood of competing males' sperm impregnating the female.
How does this apply to humans?
Therefore, according to the above theory, the reason why human males don't have a baculum is because as a species we tend to be less promiscuous. Sticking to one partner, means that there's a notable absence of competing males eager to impregnate females with their sperm – which takes off much of the pressure.
Less reproductive competition definitely makes for a more relaxed and enjoyable sex life and allows finer emotions like love and affection to flourish. The only possible disadvantage, however, is the fact that there is no longer any biological need to prolong sexual intercourse . . .