Thursday, 15 December 2016

Why Humans Lost Their Penis Bone 

Assorted mammalian bacula
Why humans lost their penis bone | Science | AAAS

Despite the assumptions behind certain slang terms, human males don't really have a penis bone, or baculum, unlike some of our closest mammalian relatives such as chimpanzees and bonobos. The reason why was not really clear until now although evolutionary explanations have been suggested.

The penis bone first put in appearance in mammalian evolution between 145 and 95 million years ago, so was present in the last common ancestor of apps and carnivores, so at some point, and in the case of humans after we diverged from the chimpanzee/bonobo line, the penis bone was lost.

It has been suggested that, partly as a consequence of our upright gait which puts the male genitalia on display, female sex selection may have caused a larger penis to evolve. Riding on the back of this may have been an incidental selection for good health, in that poor health tends to cause loss of erectile function. Losing the penis bone not only allows the larger penis to shrink back to a more manageable size when not in use but also allows erectile function and fitness to be on display. Now researchers may have identified another reason, at least for why the bone was not retained.

According to Matilda Brindle and Christopher Opie of the University College Department of Anthropology, London, UK, there is strong correlation in primates between the presence of the baculum and the amount of time the penis remains in the vagina (intromission time) during intercourse, and this in turn depends on the mating strategy.

The extreme morphological variability of the baculum across mammals is thought to be the result of sexual selection (particularly, high levels of postcopulatory selection). However, the evolutionary trajectory of the mammalian baculum is little studied and evidence for the adaptive function of the baculum has so far been elusive. Here, we use Markov chain Monte Carlo methods implemented in a Bayesian phylogenetic framework to reconstruct baculum evolution across the mammalian class and investigate the rate of baculum length evolution within the primate order. We then test the effects of testes mass (postcopulatory sexual selection), polygamy, seasonal breeding and intromission duration on the baculum in primates and carnivores. The ancestral mammal did not have a baculum, but both ancestral primates and carnivores did. No relationship was found between testes mass and baculum length in either primates or carnivores. Intromission duration correlated with baculum presence over the course of primate evolution, and prolonged intromission predicts significantly longer bacula in extant primates and carnivores. Both polygamous and seasonal breeding systems predict significantly longer bacula in primates. These results suggest the baculum plays an important role in facilitating reproductive strategies in populations with high levels of postcopulatory sexual selection.

Matilda Brindle, Christopher Opie
Postcopulatory sexual selection influences baculum evolution in primates and carnivores
Proceedings of The Royal Society B DOI

© 2016 The Authors.
Reprinted under the terms of the Creative Commons Attribution 4.0 International License (CC BY 4.0)

Our findings suggest that the baculum plays an important role in supporting male reproductive strategies in species where males face high levels of postcopulatory sexual competition. Prolonging intromission helps a male to guard a female from mating with any competitors, increasing his chances of passing on his genetic material.

Matilda Brindle, UCL Anthropologist. Co-author.
Quoted in Phy.Org
In species such as the chimpanzee and the bonobo, mating is polygamous with multiple males mating with multiple partners whereas in humans mating tends to be monogamous. Polygamous mating means there is intense selection pressure on males to minimise access to the female by other males and longer intromission times the less likelihood there is that she will mate again with another male soon afterwards, so increasing the likelihood that his sperm will fertilise her egg.

After the human lineage split from chimpanzees and bonobos and our mating system shifted towards monogamy, probably after 2mya, the evolutionary pressures retaining the baculum likely disappeared. This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans.

Dr Kit Opie, UCL Anthropology. Co-author.
Quoted in Phy.Org
A baculum facilitates this by keeping the penis erect and the urethra open. In effect, there is postcopulatory sex selection because the female is less likely to select an alternative mate if her current mate keeps her occupied for longer. This also gives his sperms a head start on their journey to the egg.

So it seems probable that human males lost their baculum after the split from the other apes and after the mating system changed to become more monogamous or at least polygynous so there was reduced male postcopulatory competition. This may have been as recent as 2 million years ago so it will be interesting to see if traces of the baculum are ever found in Australopithecines such as Australopithecus afarensis or Au. sediba. This in turn would give us a clue as to their mating strategies and so to their tribal/family structure.

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