The penis bone is considered one of the most diverse of all the bones. It vastly varies in size, shape and even presence in mammals – humans, for example, do not have one. But little is known about how and why the penis bone – or baculum – evolved. Its adaptive function has remained a mystery.

To investigate where it came from and what purpose it serves, anthropologists at University College London (UCL) looked through the ancestral record, examining penis bones across the ages. "Aside from documenting the presence and absence of the baculum across the mammalian orders, the evolutionary history of the baculum had not been studied until recently, leaving many questions unanswered," the team wrote in the Proceedings of the Royal Society B: Biological Sciences.

The scientists used an evolutionary comparative method to find out if increased intra-sexual selection (where animals compete) affects baculum evolution. Firstly, they reconstructed the evolutionary trajectory across the whole mammalian class. Their findings showed ancestral mammals did not have a baclum, but the ancestors of primates and carnivores did.

This let them to suggest the baculum must have first evolved after non-placental and placental mammals split 145 million years ago – but before the most recent common ancestor of primates and carnivores evolved 95 million years ago.

Following on, the team looked for a correlation between penis bone length and testes mass (previous research has shown where sperm competition is high, testes size increases relative to body mass). Their findings showed no clear link between the two.

penis bone
Comparing a small river otter baculum with the larger sea otter baculum. Travis/Flickr

Finally, the researchers looked at whether sexual selection pressures relating to mating systems and breeding patterns influenced the length of baculum length among primates. Findings showed they did – where there were competitive mating systems in place, the baculum was longer.

Specifically, polygamous mating systems and seasonal patterns resulted in longer bacula when compared to primates in other mating systems.

There was also a correlation between how long intercourse lasted and penis bone length: "Prolonged intromission predicts significantly longer bacula in extant primates and carnivores," the team wrote.

"The baculum physically supports and protects the male's penis and assists the transfer of semen towards a female's cervix. However, it also plays an important role in facilitating prolonged intromission, which itself may be a sexually selected behaviour, aimed at increasing reproductive success by delaying females from re-mating. Our results confirm that the prolonged intromission hypothesis remains robust."

Concluding, the team said: "These results suggest the baculum plays an important role in facilitating reproductive strategies in populations with high levels of postcopulatory sexual selection."

Further to this, the researchers say their findings might shed light on why humans lost their penis bone. Because we tend to be monogamous, competition is lower.

First author Matilda Brindle said: "Interestingly, humans have neither prolonged intromission durations, nor high levels of postcopulatory sexual competition. Given the results of our study, this may help to unravel the mystery of why the baculum was lost in the human lineage."