The walrus penis bone, also known as an os penis or baculum, is one of the most popular objects at the Grant Museum. The human penis is haemodynamic, meaning an erection is achieved by blood pressure alone. In animals with an os penis, blood pressure still plays an important role, but the pressure functions to push a bone structure into the penis in order to achieve an erection. This has many benefits over an erection sustained by blood pressure alone, not least in keeping the glans open for sperm to pass through.

While the importance of shaft size and sperm competition has been discussed in my previous blog post, even the largest penis will offer no evolutionary advantage if sperm cannot escape: these much desired qualities will never be passed to offspring. This is not the only benefit. The os penis increases the potential duration of intercourse and also the frequency with which intercourse can take place. For example, a lioness can copulate 100 times per day, sometimes with only four minute intervals, but has only a 38% conception rate¹ – males need to keep up if they’re to achieve the best chance of paternity. It comes as a surprise to many people that the os penis exists at all, but in fact humans, woolley monkeys and spider monkeys are the only primates to lack this handy piece of anatomy.

With such benefits to structure, strength, endurance and recovery time, the next question must be: where is man’s penis bone? This is where the story becomes more intriguing…

Was it sacrificed for the sake of woman?

While the lack of a human penis bone may have interested evolutionary scientists for many years, it has recently attracted attention from professors of Theology as well. The debate was reignited in 2001 with the publication of a letter entitled Congenital human baculum deficiency: the generative bone of Genesis 2:21-23. Without wishing to delve into a religion vs. science debate, perhaps the most surprising aspect of this article is that it was published in the American Journal of Medical Genetics, with its central theory that God took Adam’s os penis to create Eve in the Garden of Eden, rather than his rib:

Ribs lack any intrinsic generative capacity. We think it is far more probable that it was Adam’s baculum that was removed in order to make Eve. That would explain why human males, of all the primates and most other mammals, did not have one.²

The authors then go on to cite possible mistranslations of the Hebrew word ‘tzela’, which can be translated as ‘rib’ or more generally as a supporting structure such as a baculum. More recently however, a study using the dissection and microscopic analysis of human, dog and rat penises revealed something rather unexpected…

Perhaps it was there all along!

With the comprehensive list of benefits discussed above, the root of this problem seems to be in discovering how on earth humans manage to function without an os penis. As a rule, larger penises in humans can give an evolutionary advantage, but without the toughness of structure provided by a bone, how do they avoid buckling under the pressure? Well, through both microscopic analyses and collagen tissue staining, the elusive evidence has finally been discovered. In the core of the human penis, there is a tunica (or lining) with many elastic fibres acting to keep the penis rigid in much the same way as an os penis. This has been referred to as the distal ligament, and is a structure so robust that even after some venous removal, erection can still be achieved without the need for a fully formed bone structure.⁴ In somewhat less good news for men, the authors of this study further hypothesise that if damaged, the distal ligament may take as long as a broken bone to heal.

So we are left with the final puzzle of why man lost his penis bone but retained a structure that serves a similar purpose. For the answer to this, we may need to look to women. Sexual selection works on the premise of ‘honest signals’. Put simply, females need to select high quality males to ensure the best genes for their offspring, and a penis that works predominantly through haemodynamics takes a lot of energy to produce. Therefore this is an honest signal of the most healthy males in the same way that the healthiest stags grow the largest antlers. In the end, for all the dissection, microscopic analyses and experiments with cadavers, Adam may have given up his penis bone for Eve after all.

Suzanne Harvey is a PhD student in Biological Anthropology, working on social interactions and communication in wild olive baboons. She is also a teaching assistant on the UCL Arts and Sciences BASc, a new interdisciplinary degree, and can be found on twitter @suzemonkey.

References:

^1. Rudnai, J. A. (1973). Reproductive biology of lions (Panthera leo massaica Neumann) in Nairobi National Park. East African Wildlife Journal II: 241 – 53.

^2. Gilbert, S. F. & Zevit, Z. (2001). Congenital human baculum deficiency: the generative bone of Genesis 2:21-23. American Journal of Medical Genetics 101(3): 284-5.

^3. Hsu, G. L., Lin, C. W., Hsieh, C. H., Hsieh, J. T., Chen, S. C., Kuo, T. F., Ling, P. Y., Huang, H. M., Wang, C. J., & Tseng, G. F. (2005). Distal ligament in human glans: a comparative study of penile architecture.Journal of Andrology 26(5): 624-8.

^4. Hsieh, C. H., Liu, S. P., Hsu, G. L., Chen, H. S., Molodysky, E., Chen, Y. H., Yu, H. J. (2012). Advances in understanding of mammalian penile evolution, human penile anatomy and human erection physiology: clinical implications for physicians and surgeons. Medical Science Monitor 18(7): 118-25.

by Suzanne Harvey

Anatomy is destiny … The genitals themselves have not taken part in the development of the human body in the direction of beauty: they have remained animal, and thus love, too, has remained in essence just as animal as it ever was.

When Sigmund Freud wrote this in 1912, he may have been surprised to hear that some hundred years later, evolutionary theory would come to the same conclusions. Despite the frequently discussed individual variation in human penis size, the shaft of an average human penis is around twice the length and width of the shaft of an average chimpanzee penis. It is also useful to mention some more unusual facts: firstly, while chimpanzees have penises half the size of humans, they have testicles three times as large. Moreover, while silverbacks are formidable looking creatures, gorillas in fact have the smallest penis to body size ratio of any mammal. So, what causes these seemingly contradictory differences among the great apes, and how can evolutionary theory make sense of all creatures great and small?

Sperm Competition

As Freud’s quote suggests, the clue to the evolution of the penis is not just in their physical appearance but also in the social aspects of sex. In fact, generally speaking, the mating system of a species can be used to predict penis size. Chimpanzees live in large multi-male, multi-female groups, where females are able to mate with many males. Sperm can live for up to 4 days after ejaculation, and consequently when females mate with two males in close succession, sperm from two males can be in direct competition. The male who produces more sperm will have the best chance of fertilizing an egg. This evolutionary advantage of producing large amounts of sperm can explain the relatively large testicle size of chimpanzees. Correspondingly, the male gorilla’s huge stature is in fact the reason why he has such a small penis: when competition between males occurs through physical aggression, an alpha male may fight off rivals and control his own mating success without the need for sperm competition. Other physically smaller males have little access to females in the group.

Understanding the Human Penis

The mystery of the human penis is that ancestral hominids lived in similarly large and promiscuous social groups, but did not evolve the large testicles seemingly necessary to compete via sperm competition. One might be forgiven for thinking that larger penises evolved as a result of sexual selection; the theory that a preference for larger penises in females has led to greater reproductive success for males with larger penises, with these males passing on the trait to their offspring. However, the latest research shows that penis size may also be the result of sperm competition and natural selection.

The Semen Displacement Theory (Gallup and Burch, 2004) essentially explains the advantages of the size and shape of the human penis in terms of a device evolved to remove another male’s semen before fertilization.

As well as being larger and wider than other primate penises, the human penis has the unique shape of a shaft with a ridge leading to a wider tip, known as the coronal ridge. This is more pronounced than in other species. All of these elements are important in terms of semen displacement: the coronal ridge removes semen by ‘scooping it out’ as it passes over the tip, is trapped behind the ridge and pulled out during intercourse. Recent research shows that (using artificial genitalia) a penis with a coronal ridge will displace 91% of semen, while one without will displace only 35% (Gallup et al. 2003). Thrusting during sex creates a vacuum that aids this process, as the width of the shaft provides a plug in the vagina. In Gallup’s experiment, the same penis removed 90% of semen when fully inserted and only 39% when inserted three quarters of the way. Therefore, the length of the shaft simply improves reach and maximizes the amount of semen that can be removed.

So yes, when it comes to penises, size – and shape – matters when it comes to natural selection!

Suzanne Harvey is a PhD student in Biological Anthropology, working on social interactions and communication in wild olive baboons. She is also a teaching assistant on the UCL Arts and Sciences BASc, a new interdisciplinary degree, and can be found on twitter @suzemonkey.

References:

Freud, S. (1912). On the Universal Tendency to Debasement in the Sphere of Love. Oxford Literary Review 30: 109-146 DOI 10.3366/E0305149808000199, ISSN 0305-1498

Gallup, G. G. & Burch, R. L. (2004). Semen Displacement as a Sperm Competition Strategy in Humans. Evolutionary Psychology 2: 12-23

Gallup, G. G., Burch, R. L., Zappieri, M. L., Parvez, R. A., Stockwell, M. L. & Davis, J. A. (2003). The human penis as a semen displacement device. Evolution and Human Behavior 24: 277–289

by Sarah Chaney

Katie’s recent post on the ethics of displaying human remains in museums, along with the recent Grant Museum exhibition on the topic, raised some important questions about collection and display. Unsurprisingly, this is a frequent topic of concern in medical museums – particularly in hospital museums, teaching collections tend to focus around anatomy and pathology, a large part of which consists of specimens of human remains. But what exactly constitutes “human remains”? This is, on occasion, a surprisingly difficult question to answer.

Many medical collections were created in the eighteenth and nineteenth centuries, as anatomy schools grew and increasingly began to offer practical training for medical students, alongside lectures. One such fascinating collection can be found at Benjamin Franklin House, in Craven Street near Charing Cross. An ordinary Georgian townhouse, which was home to American Founding Father Benjamin Franklin for nearly sixteen years, the building also housed a private anatomy school. The school was run by the landlady’s son-in-law, William Hewson; who had previously worked with the well known anatomist William Hunter. Hewson sadly died young, of septicaemia contracted during a dissection, but the remains of his school were uncovered during the restoration of the house in recent years, in a pit where the back yard would have been.

It is not unusual for bones to be unearthed when foundations are laid for new buildings in London, something explored not so long ago in an exhibition at the Wellcome Collection, London’s Dead. These skeletal remains are what first springs to mind when we think about human tissue. It is skeletons that we often expect to see in museums; the absence of flesh suggests age and the process of decay. How long, though, do we continue to regard these skeletons as the remains of our loved ones? Cemeteries, for example, tend to offer burial plots for periods of only around fifty or sixty years. By this time, the body will have decomposed, and the land can legally be re-used. In Sheffield, part of the former city cemetery has been turned into a park. In London, memorial grounds are built over all the time. Once people have faded from memory, it seems that their remains do too.

But what other human remains do we find in medical collections? Many of the specimens are dissections of various sizes and complexity. It seems obvious which of these are tissue and which not, but even here the recent Human Tissue Act has struggled to define humanity. We might assume that any part of our physical form constitutes human tissue but, legally speaking, this is not the case. Body parts that regularly grow and are removed, for example, are something of a grey area; for example, locks of hair, often kept as mementos of a loved one, can legally be kept or displayed by any museum. But what about blood? The status here is uncertain. On the other hand, a tumour which has been surgically removed is considered human tissue, despite the fact that the person operated on may well have considered it to be alien to their own person.

Stained Brain Specimens in the UCL Pathology Collection. Courtesy of Bethlam Heritage.

One of the things that intrigues me most, however, is the place of foreign bodies in medical collections. Foreign bodies are objects that have been swallowed, inhaled or otherwise inserted into the human body. Often, particularly when these items end up in the bladder, the body creates deposits around these objects, protecting organs from sharp edges or corrosive material. When removed, the foreign body may be invisible within layers of mineral coating. These objects are faintly mysterious: created by the human body, they are nonetheless not considered to be human at all. They lie beyond the regulations on human tissue, but could not have come into existence in the first place without having had a relationship with that tissue.