by Stacy Hackner

This question was thrown at me at the end of a conversation about juvenile bone growth, and I was completely blindsided. I know my cat definitely has a bump in the place his navel should be, and I assumed all placental mammals have them.

Further research shows that indeed, all placental mammals start with a belly button (or navel, or umbilicus if you’re scientific). The navel is the remnant of the umbilical cord, which attaches a fetus to the mother’s placenta to deliver nutrients in utero. Thus animals that hatch from eggs don’t have them – this includes marsupials like kangaroos and wombats, which have not evolved a placental structure and instead incubate their young in a pouch. However, in most other mammals (and certain humans) they’re obscured by fur, and in some species they are a thin scar rather than a small bump, and fade over the course of the animal’s lifetime.

Umbilicus evident on a Grant Museum specimen of a fetal beluga whale.

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 Alicia Thornton

As someone whose background is in biological sciences, working in the Grant Museum of Zoology feels a little like coming home. Robert Edmond Grant collated the collection for the teaching of comparative anatomy and zoology, showing the differences and similarities between species. The collection is hugely diverse; from sponges and other marine invertebrates (in which Grant held a particular interest) to skeletons of primates, elephants, big cats and other mammals. The collection even has examples of some animals which are now extinct. Most notable are the quagga, a zebra-like creature from Southern Africa which was hunted to extinction in the wild around the time that Grant was teaching at UCL; and the thylacine or Tasmanian tiger. The thylacine was a marsupial native of Australia, also hunted to extinction, during the early 20^th century. The museum also has bones from a Dodo, which died out  as the result of a combination of factors, including hunting and predation by imported species introduced by European settlers.

For me, what the collection shows so well, through its diversity, is how every organism is adapted to the environment in which they live. Each species or subspecies has evolved to have a unique way of living and their biology gives a complete illustration of this.  For example, the shape of a jaw indicating the type of diet an animal has, or the dimensions of the limbs showing how an animal may swing through trees or stalk prey in grassland.  As I near the end of the first year of my PhD, I find that it is sometimes easy to get too engrossed in the details of my research and lose sight of what interested me about the topic in the first place. The Grant museum serves as a perfect reminder. My own research is focused on infectious diseases, and specifically human immunodeficiency virus (HIV). The way in which the virus has evolved and continues to evolve has been one of the biggest challenges for scientists and medics working in HIV treatment, care and research.

Like all viruses, HIV requires a living cell to reproduce. During infection, the virus enters the human cells and uses the machinery of the host cell to replicate, producing further infectious particles and releasing them to continue the infection cycle. In order to be successful and survive, the virus must find mechanisms by which it can evade the response of the host immune system that is designed to eliminate it. In fact, HIV is perfected suited to this; having the ability to infect cells which constitute a key component of the immune system as well as those which are out of the reach of the immune system.

Due to the nature of its replication, HIV evolves particularly fast and thus has the ability to survive changing environments.[1]  A huge range of drugs to treat HIV have been developed  since the beginning of the epidemic. These drugs have been a huge success, allowing people to live much healthier lives. Where they are readily available they have dramatically reduced the numbers of people who develop AIDS[2] and increased life expectancy of HIV positive individuals to almost that of HIV negative individuals[3].  Yet they never eliminate the virus completely and as new drugs are introduced, the virus rapidly evolves, giving rise to drug resistant strains and making treatment even more challenging.[4]

The 19^th International AIDS conference was held in Washington DC, USA in July 2012. This is the largest of the HIV conferences with over 20,000 delegates, taking place every two years, and is attended by a mix of medics, nurses, public health professionals, advocacy groups and policy makers. Finding a cure for HIV was a key theme of the conference and like all HIV conferences, a large volume of work presented was focused on the development of new drugs and drug combinations. Increasing the range of drugs available means that doctors are more able to combat the development of drug resistance and keep their patient’s viral replication supressed.

The extent of the HIV epidemic is the result of a complex combination of social and scientific factors. However, there is no doubt that the virus’ ability to continually change and adapt to the environment in which it survives is a one of the key reasons that the infection remains such a challenge to control.

[1] Rambault A, Posada D, Crandall KA & Holes EC.  The Causes and Consequences of HIV Evolution. Nature Reviews Genetics 2004; 5(1): 52-61.

[2] Mocroft A, Ledergerber B, Katlama C, Kirk O, Reiss P, d’Arminio Monforte A, Knysz B, Dietrich M, Phillips AN, Lundgren JD; EuroSIDA study group. Decline in the AIDS and death rates in the EuroSIDA study: an observational study. Lancet. 2003; 362(9377):22-9.

[3] Nakagawa F, Lodwick RK, Smith CJ, Smith R, Cambiano V, Lundgren JD, Delpech V, Phillips AN.  Projected life expectancy of people with HIV according to timing of diagnosis. AIDS. 2012; 26(3):335-43.

[4] UK Collaborative Group on HIV Drug Resistance; UK CHIC Study Group.  Long-term probability of detecting drug-resistant HIV in treatment-naive patients initiating combination antiretroviral therapy.  Clin Infect Dis. 2010; 50(9):1275-85.