The Bizarre Evolutionary Paths of the Penis

As June drifts into July in a picturesque bay in North Yorkshire something very strange happens. All male periwinkles of a particular species shed their penises. Like sky-lanterns released during Chinese festivities, they synchronously discard their organs, which float into the ocean beyond. The males remain as molluscan eunuchs, clinging to the sea wall. What’s happening?

The knee-jerk reaction might be to point the finger at parasites. Periwinkles are sea snails which are members of the class Gastropoda. Castration by parasites is rampant within the class. Opportunistic flatworms are the main antagonist. They often subvert the gastropods’ own hormonal system so that the ensuing chemical castration is self-induced. The flatworm smugly stays on board for the ride, gorging itself with nutrients that would otherwise have fuelled its host’s reproductive whims.

In the peculiar case of the periwinkle, we would be too hasty in blaming parasites. Something else is going on here. No parasite is calling the shots. The periwinkles are unmanning themselves. This might seem the very nadir of indignity but in evolutionary terms it makes a lot of sense. In much the same way that stags shed their antlers when the rut is finished, the periwinkle in all probability casts away its penis as a means of conserving energy. In the brief period leading up to the synchronous perishing of winkles, males reabsorb some of the usable nutrients locked up in the penis.

We tend not to think of the costs associated with maintaining a fully-fledged and functioning penis, but for the periwinkle it makes little sense to preserve the organ when the breeding season has ended. Especially when the amputee males can simply grow it back so that full maturity and former glory is restored just in time for Christmas.

Penis-shedding is a pretty extreme and rare phenomenon. Chromodoris reticulate is a hermaphroditic (with male and female reproductive organs) sea slug that amputates the top third of its penis after sex. The penis then grows back in a mere twenty-four hours, far faster than those of the lackadaisical periwinkles which can take as long as four months to regrow fully.

Human males by comparison seem boring. We certainly don’t shed our dangly-bits. Some of us are even rather fond of them. But certain morphological characteristics present in our ancestors certainly have been shed. Contrary to those of the periwinkle and C.reticulate though, these are not changes that occurred over night.

To see what features of the penis have been condemned to the similar relic fate of our once prehensile tail, we must first venture into our not-so-distant past. Seven million years should about do it. Considering life started 3.8 billion years ago; what we’ve just done barely constitutes a shuffle. Yet we’ve arrived at a time where the tip of our penis is not only adorned with spikes but the whole structure itself is supported by a bone: the baculum. This bone, which can reach truly monstrous sizes in certain mammals (the walrus’s baculum is 60cm long) is the most diverse of all bones in morphology. However it’s worth mentioning that whilst in chimpanzees the bone is shrunken relative to that of earlier primates, the bone has been lost completely in favour of a hydraulics-based system in only a few primates including tarsiers, spider monkeys and of course humans.

Let’s take a look at the spikes which are present in our closest living relative, the chimpanzee. They are hardly as foreboding in appearance as the veritable stalagmites found on the penises of beetles like the bean weevil. Nonetheless, the tip of the chimpanzee’s penis is studded with out-growths which are made of keratin – the protein component of hair and horns. In the bean weevil a sophisticated experiment (which involved lazering off the spines of certain males), showed that the spines punctured the females reproductive tract and allowed seminal fluid to seep into her blood-stream. Seminal fluid is a concoction of proteins which in insects can manipulate females’ behaviour for a males’ own benefit. The conflict between males and females striving for their own selfish goals is just as real as the war waged between a flatworm and its gastropod host.

So is the function of a chimpanzee’s penile spines similar to that of a bean weevil? Do they too use their penises like medieval maces first to damage the female, before injecting a potent mixture to control her behaviour? Probably not, though the exact function of the chimpanzee’s keratinised studs isn’t entirely known. They might play a stimulatory role. Or they might be an adaptation involved in scraping out residual sperm that is still lurking in a female’s reproductive tract from a prior mating that would otherwise compete to fertilise the female’s egg.

Conjecture aside, we are now in a position to ask: why do we humans have such smooth willies? A scintillating answer to this question came about four years ago from a group of Stanford researchers. They were studying the roles of a number of DNA chunks that have been “deleted” and are no longer present in humans but that are still present today in chimpanzees. DNA is like a molecular recipe book that encodes proteins which in turn determine everything from our shape to our behaviour. Interspersed between these coding recipes are non-coding regions, which are like advertisements in the recipe book. However, it’s an imperfect analogy for whilst adverts in a recipe book might not contribute to the finished product, non-coding regions certainly do. Their interactions can regulate the DNA recipe in complicated ways, drastically altering the finished product. It’s a deletion in a non-coding region which saw us lose our penile spines. It turns out this deletion did more than just render our nether-regions velvety, but explain that the evolution of our spineless penis was in conjunction with humans’ more monogamous system… and that it might be associated with our being able to maintain the sexual act longer than chimpanzees.

Whether it’s the peculiar penis-shedding of an inter-tidal snail, the amputating antics of a sea slug or the evolutionary history of our own member, the world of genitals is as daft as it is fascinating.

#Biology #Penis #GianlucaCerullo