Author's note: The following originally appeared as a guest post at A Primate of Modern Aspect and subsequently formed the basis for a technical comment published by Nature co-authored with John Hawks. This post is also notable in that it began my collaboration with artist Nathaniel Gold.

Chimp Pope

      "Chimp Pope" by Nathaniel Gold

There is very little known about the reign of Pope Benedict III except that clerics were generally satisfied with his testicles. Upon his coronation in 855 AD God's chosen messenger on Earth sat in a special chair resembling an ancient commode while the Holy See checked to make sure that the papacy was indeed infallible.

Two reliable clerics touched his testicles; witnesses who presented legal evidence of his maleness. . . At this the priest and the people responded, "Deo gratias" [Thanks be to God].

After all, you couldn't be too careful. The Bible was very clear that, "He whose testicles are crushed or whose male member is cut off shall not enter the assembly of the LORD." Healthy genitalia was a sign of spiritual purity and the Church made a point to check beneath the mantum of every Pope until up through the fifteenth century.

Of course, Christianity wasn't alone in this respect. The ancient Greeks saw the penis as a gauge to their proximity with the Gods, the Hindu god Shiva is worshipped primarily by paying homage to his penis, or linga, and the Sumerian god Enki was thought to have brought life to the Tigris Valley when he "lifted (his) penis [and] brought the bridal gift." The Pope seems to have merely been the latest in a long line of devout men who were dropping their pants for the Lord.

Now, scientists have gotten in on the act and have sought to understand human origins by studying our own little Bishop. From the standpoint of evolutionary biology this male obsession with their own genitalia makes perfect sense. Every animal alive today is able to stand and be counted because of a long line of ancestors who successfully reproduced. The natural world is a living erotic museum filled with variations in male genitalia, illustrating how natural selection has paid nearly as much attention to the male member as Catholic priests have.

But there's a sinister side to this obsession, by which of course I mean penis spines. Throughout the Order Primates, as well as in many other mammal species, males have developed small (and sometimes not so small) keratinized structures along the head and/or shaft of their penis that have been adapted to maximize reproductive success. According to the, rather appropriately named, primatologist Alan Dixson in his book Primate Sexuality, these spines can be simple, single-pointed structures like in macaques or complex ones with two or three points per spine like in the prosimians (lemurs and lorises). These different forms of penis spine therefore suggest different mating strategies that various species have adopted during their evolution.

Penile Morphologies in Primates

Examples of penile morphologies in primates. Note the obvious spines on I and K and the dots on N. Top row: Eulemur fulvus, Saimiri boliviensis, Macaca arctoides (stump-tailed macaque). Bottom row: Macaca fuscicularis (long-tailed macaque), Papio cynocephalus (baboon), Pan troglodytes (chimpanzee). Reproduced with permission from Dixson (2009).

However, a new study in the journal Nature has generated a great deal of titillation this week as Cory McLean and colleagues have revealed a sequence of DNA that promotes these penis spines, a sequence that humans appear to have lost. The genetic mechanism involved has already been explained extremely well by Ed Yong and John Hawks. However, the interpretation of what the loss of this DNA reveals about human evolution is perhaps the most provocative claim and has resulted in a flurry of media attention.

"Simplified penile morphology tends to be associated with monogamous reproductive strategies in primates," write the authors. According to their study, the loss of these spines would have resulted in a reduction in sexual sensation (because the spines are thought to be connected to nerve endings) and would therefore have allowed our ancestors to engage in more prolonged sexual activity that the authors associate with pairbonding and the evolution of social monogamy (citing Owen Lovejoy's Ardipithecus ramidus paper from 2009 as a model).

As Nature News wrote in their summary of these results:

It has long been believed that humans evolved smooth penises as a result of adopting a more monogamous reproductive strategy than their early human ancestors. Those ancestors may have used penile spines to remove the sperm of competitors when they mated with females. However, exactly how this change came about is not known.

This is where it's important to know precisely what it is that we're talking about. Nature News referred to these structures in chimpanzees as "penis spikes" when the reality is more akin to goose bumps. Scicurious has posted a review of the only study that seems to have been done on these structures (published by W.C. Osman Hill in 1946) that found these "spines" to be only about 0.35mm wide, or the thickness of a human hair. Hardly a structure that would be useful for removing sperm.

Another problem is McLean et al.'s argument that loss of penis spines would result in reduced sensitivity and longer bouts of sexual activity. As Dixson points out in Primate Sexuality (p. 118), orangutans have more elaborate penis spines than chimpanzees do and yet their average duration of sexual activity is significantly longer than either chimpanzees or humans. Chimps engage in sexual activity for an average of 8.2 seconds while the average for humans (based on Kinsey's data) is less than 120 seconds. In contrast, orangutans range between a median of 840 seconds and a maximum of 2,760 seconds. Humans actually rank 14th in the duration of sexual activity (also falling behind macaques).

However, there's a more serious problem with the argument presented in this study. The source the authors cite in support of their argument for smooth penis monogamy is Dixson's Primate Sexuality, however he doesn't discuss what penis spines indicate about primate mating systems in that book. That's in his later book Sexual Selection and the Origins of Human Mating Systems, where his conclusions are somewhat different.

Penile morphologies

Four penile morphologies in single-partner or multi-partner primate mating systems (including humans). All categories show a significant difference except for penis spines. Reproduced with permission from Dixson (2009).

As Dixson's graph indicates, there are significant differences between a single-partner mating system (monogamy or polygyny) and a multi-partner mating system on three of the four categories: penile length, baculum length, and distal complexity. The only penis morphology type that isn't significant are penis spines. This doesn't necessarily invalidate McLean et al.'s argument for increased pairbonding, but it doesn't support it either. It shows that there is no correlation between penis spines and primate mating systems, the correlation that McLean et al. is arguing for. In contrast, Dixson concludes that Homo sapiens is a polygynous species. However, other factors suggest that a multi-male multi-female system is more accurate given the diversity of human sexuality.

There's also one final thing. Humans may not have lost their penis spines. Dating back to 1700 anatomists have identified what have now become known as pearly papules (also called Hirsuties coronae glandis). As reported by Denniston, Hodges, and Milos in 2009:

We have shown that, in the chimpanzee, these papules are a normal feature (spine-like) and are associated with nervous structures. It seemed to us that, in man, they may be a return to an earlier morphology.

To see a picture of these human "penis spines" click here [NSFW].

Five studies have been done involving nearly 2,000 patients in three separate countries, with an estimate that about 30% of all men develop these papules. In contrast, only four chimpanzees were studied in Hill's 1946 paper on penis spines so it's unknown how prevalent these structures are even within genus Pan. As Hill notes in his study:

The spines of the Chimpanzee are simpler structures than those of any of the other Primates, and the question arises as to whether they are degenerate remnants of a once powerful armature or a new product of evolution.

While the genetics of these pearly papules have yet to be studied, it doesn't seem that a strong case can be made yet for significant differences between our two species on this point. John Hawks goes on to note that the chimpanzee version can even be implanted into transgenic human foreskin fibroblasts:

That indicates that the overall genetic system to make penile spines is still there lurking in our genomes. If we could turn on the gene at the right time, replacing the function of the enhancer, we can still grow penile spines.

I'm confident that the bulk of this week's Nature paper will offer a host of tantalizing insights into the ways that humans and chimpanzees have travelled down different evolutionary paths since our common ancestry. At the same time, our evolutionary history has primed some members of our species to seek firm answers by looking to their respective, er, members. As we sit and try not to think about Pope Benedict's balls, we can muse on how potential revelations may indeed develop from these investigations. But it's also possible that we'll only be greeted with a cold hand and a cheap thrill before moving on to the next study.

ResearchBlogging.orgMcLean, C., Reno, P., Pollen, A., Bassan, A., Capellini, T., Guenther, C., Indjeian, V., Lim, X., Menke, D., Schaar, B., Wenger, A., Bejerano, G., & Kingsley, D. (2011). Human-specific loss of regulatory DNA and the evolution of human-specific traits Nature, 471 (7337), 216-219 DOI: 10.1038/nature09774