The Human Penis is a Spineless Wonder

Evolution of human penis form may be tied to pair-bonding and monogamy

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Have you ever given thought to what a chimpanzee’s penis has in common with a popular variety of condom? I would guess not often, but there is a connection of sorts. In a manner reminiscent of the classic ‘French Tickler’ condom, with its tip covered with rubberized protuberances, many male mammals - including the chimpanzee, our nearest relative among the apes - also have numerous small spines covering the outer surface of the head of the penis [see footnote1]. These spines are made from a fibrous protein known as keratin, the same material which gives us fingernails, hair, and the rest of our skin. The interesting story here is that, while our nearest living evolutionary neighbours (and many other mammals) possess penile spines, human males do not. This begs the obvious question for human males: where oh where did our little spines go?

Cory McLean and his colleagues at Stanford University in California were interested in just such anomalies of anatomy and in particular their underlying genetic (DNA) basis. Such research has been made easier in the past few decades given that we have successfully produced full genetic sequences for humans and many other animals (and plants), as well as our nearest evolutionary ancestors, the Neanderthals. McLean and his colleagues were interested in seeing how DNA differences might affect anatomical development. They were particularly interested in the impact of genetic sequences - strings of DNA - that were present in one species but missing in the other, known as deletions in genetic parlance. They focused their research on DNA sequences that are known to be highly conserved [see footnote 2] and which tend to be stable across species. In comparing the chimpanzee and human genomes, they found 510 such regions that were present in chimpanzee DNA but were missing in humans. Interestingly, 88% of these missing genetic regions were also absent from the Neanderthal genome, indicating that they were lost at a time in human evolution prior to the appearance of this ancestor several hundred thousand years ago. How long these regions have been lost we do not know as - so far - the Neanderthal genome is the oldest we have been able to fully reconstruct.

Of course, this is just the first step in such an analysis - the big question is what impact would the deletion of normally conserved DNA have on gene expression and trait development? They found that the missing DNA sequences in humans enriched (occurred more frequently) in two areas: near genes connected to steroid hormone receptor signaling, and near genes involved in neural function. In terms of penis spines, it is the first of these areas that interests us. This is because the research showed one such region flanked a piece of genetic code responsible for androgen receptor production. Such receptors determine the tissue-level response to circulating sex hormones, known as androgens (for example, testosterone in males among others). What McLean and co. were able to show was that this particular deletion, in modifying androgen receptor function, ultimately impacted gene expression responsible for the production of both facial vibrissae (whiskers, present in many mammals including all primates except, of course, humans) and genital tubercles (penis spines, also missing in human males) [see footnote 3].

So why would natural selection deprive human males (and those among our ancestors at least as far back as Neanderthals) of an anatomical feature widely conserved among primates and other mammals? The authors argue that one possible reason resulting in this outcome might be tied to the development of monogamous mating structures, pair-bonding and increased parental care at some point in our evolutionary past. They suggest that the spineless human penis is one of several features that reflect such a mating pattern, including smaller canine teeth and testicle size in males, and concealed ovulation [see footnote 4] and enlarged breasts in females.

To understand how this might work regarding penis spines we need to consider the purpose of these spines in the first place. Unlike our French tickler analogy above, in which the 'spinous ticklers' of the condom are meant to simulate the female and facilitate orgasm, penile spines in males are meant to work in an opposite fashion. They effectively hyper-stimulate males so that they achieve orgasm and ejaculate relatively quickly. When we think about it, this outcome would have considerable evolutionary value in highly male-male competitive mammalian social groups by allowing a male engaging with a receptive female to complete the process of insemination in short order, as it were. This would be particularly handy if the male in question was of subordinate rank in that group and thus wanting to (a) impregnate a female and (b) not be pummeled by the dominant male. So back to humans - what then is the benefit of a spine-free penis? In a monogamous pair-bonded species conflict (that is, pummeling) between males over reproductive access to females is greatly reduced: one male, one female equals one pair. As McLean and co-workers suggest, loss of penile spines would result in extending duration of copulation which, arguably, could in turn enhance and maintain the bonding of male and female. That said, while this is a reasonable working hypothesis, it is, in fact, untestable - as so many such hypotheses are concerning our deep evolutionary past.

[Footnote 1] Properly known as the glans, this bit of male anatomy is the developmental equivalent of the female clitoris.

[Footnote 2] A highly conserved genetic region is one which has shown little change across species over time, indicating that it has been maintained by natural selection to 'conserve' its important biological function.

[Footnote 3] The importance of androgens for development of whiskers has been demonstrated in, for example, castrated mice, which produce much shorted whiskers. Also, mice given more testosterone in development grow longer whiskers. Similarly, castrated primates do not grown penile spines.

[Footnote 4] Most primate females 'advertise' their reproductive status though features such as genital swelling; baboons are a classic example. At some point in our evolution human females ceased to advertise - effectively concealing when they were most likely to conceive.

Where to Find the Science:

McLean, CY et al. 2011. Human-specific loss of regulatory DNA and the evolution of human-specific traits. Nature 471: 216-219. March 10 https://doi:10.1038/nature09774 

Updates and Related Pieces:

Reno, PL et al. 2013. Penile spine/vibrissa enhancer sequence is missing in modern and extinct humans but is retained in multiple primates with penile spines and sensory vibrissae. PLOS One December https://doi.org/10.1371/journal.pone.0084258  

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