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Genitals and Other Previously Unappreciated Accomplishments of Evolution



Rob Hardy


Sex is one of the things we humans find really worth doing, and we will spend money, or throw away marriages, or hazard careers in its pursuit. For all its delights, complicated emotional implications, and astonishing anatomical and physiological details, though, human sex is pretty simple, compared to the examples given in Nature's Nether Regions: What the Sex Lives of Bugs, Birds, and Beasts Tell Us about Evolution, Biodiversity, and Ourselves (Viking) by evolutionary biologist Menno Schilthuizen. The author describes his own work with snails that not only have their shells spiraled either clockwise or counterclockwise, but have their penises spiraled in the same direction. That is, however, only a few pages within this diverting book, which intends to popularize genitalia research. We know what Ulysses calls the "energetic piston and cylinder movement" involved in sex for us, but it turns out that that is just our branch of the evolutionary tree. Evolution has been at work on our genitals and those of every other animal, and it has produced some of its finest and most bizarre creations in the very organs that bring forth progeny. 




It is only in the past few decades that evolutionary biologists have been looking at genitalia. Darwin, like other Victorian gentlemen, averted curiosity from the region, concentrating on the less naughty "secondary sex characteristics," like plumage, antlers, and prongs on insects' heads. Genitals, Darwin thought, were simply functional, accomplishing the job of inserting or accepting sperm for fertilization. Only in 1979 did a researcher show that the tiny penis of the damselfly not only did the job of insemination, it was shaped to scoop out any sperm from predecessors who had already enjoyed the damsel's favor. It was the first hint that genitals might play a role in sexual selection, as well as in sperm delivery. Since that initial discovery, researchers like the author have been finding that the sizes, shapes, and activities of animal genitalia come in vast varieties. 




Why should there be so many different forms of genitalia? In beetles, for instance, one beetle may look almost exactly like another, and the only way to tell one species from another is to put specimens under a microscope and look at the penis and vagina, which can have wild variation of tabs and slots and knobs and hooks and daggers. Genitals seem to be more of a playground for evolutionary tinkering than, say, feet. It used to be thought that this was because of a "lock and key" principle. A vagina would not admit a penis that did not fit, because to do so would be to allow a sort of bestiality between beasts, with no progeny produced. It was an intuitively appealing idea, but didn't make sense upon examination. After all, why should Mr. and Ms Beetle expend the effort to send all the olfactory, auditory, and tactile signals of proposed hubba-hubba only to find just before the moment of bliss that they key didn't fit the lock? 




It is now known that the variety of genitalia has purposes far more refined than just keeping the wrong species from hooking up. Darwin would have loved hearing about how sexual selection really works at the level of genital anatomy and physiology. Males, like the damselfly, may have penises that are shaped to expel the sperm of predecessors. One beetle has gone overboard with a bristle-brush of a penis that is so efficient at expelling the semen of others that it carries some of that semen away, and then the predecessor semen may actually be used when the beetle mates with another female. One of the interesting concepts here is humping; we humans do it all the time, but animals have all sorts of ways of ejecting and injecting liquids into other animals. Indeed there are animals like the spider Harpactea sadistica, with male genitals that literally pierce the belly of the female to inject sperm. But coition does involve genitals moving rhythmically for sperm transfer in around three-quarters of creatures, and in some insects, even if there is no thrusting, there is throbbing of the penis. Schilthuizen says that such movements do more than just make sperm release possible; they let the male use the penis not just as a delivery nozzle but as an "internal courtship device," whereby the knobs and grooves of his equipment best stimulate the female and influence her choice to accept him as suitor and his sperm as the ones she wants to use. There are species of craneflies that have grooves on the male hardware that, when drawn in and out of the female hardware, produce vibrations. Yes, they have their own vibrators. It is unreasonable to think that craneflies have the same refined sense of pleasure that we humans do, but it is not unimaginable that in a smaller way, the neuroreceptors in the cranefly produce a sensation in the female that she can indeed feel and might influence her to proceed with reproduction. 




Female genitalia and behavior indeed also play a role. Darwin knew how females partook of sexual selection, but they do so, too, at the tiniest anatomical levels. Some female nematodes will take on the sperm of some suitors, and then will dump it with hopes for some better guy to come along. Storing sperm from previous males is done in many species. Some crickets eventually decide that the best way to use sperm from previous encounters is to dine on it, which not only provides nutrition but also clears a space for the sperm from the next male. Female goblin spiders hoard the sperm of the different males they have taken on, storing it separately and then deciding long afterwards which sperm they want to use as sire to their next eggs. One of the campaigns Schilthuizen champions in this book is the need for increased attention to anatomy of female genitalia, which has still not been studied in most species as closely as has that of the males. The reason may simply be the convex nature of one and the concave nature of the other, with one being "out there" to study. Deeper reasons may have to do with societal regard for what is proper for study, with females once again getting short shrift. Another campaign: Schilthuizen says it is easy for the media to make fun of his area of research: "Studying the finer details of the private parts of some inconsequential animal can effortlessly be cast in terms of frivolity beyond words." There are, he insists, practical reasons (like in livestock reproduction) to understand exactly how genitalia function, but that's secondary. Knowing these biological details is fun, and increasing knowledge is simply good in itself. 




Schilthuizen has just the right sense of humor in introducing us to "the animal world's multipage sex-aid catalog." It isn't just anatomy; there are lots of strange behaviors, and plugs inserted by males to keep their sperm from being dumped, and hijacked hormones, and much more. The odd behaviors and anatomies are all designed to play their role in getting genes into the next generation, and we can be sure (at least in lower creatures) that it is all invigoratingly amoral. One researcher exclaimed with glee over the mating behavior of his subject sea slug, "Everything the church forbids is present in this species!" Schilthuizen has a few direct lessons for our own species as well, though he is not sure he agrees with the research that shows that human penises, like those of so many insects, have a shape to drive out previously-inserted semen from someone else. We have evolved to have sex but there are plenty of ways we have our sexual fun, and anyone who uses the internet can see how strange some of those ways of humans are. It is refreshing to see just how vanilla our activities are compared to the ducks, snails, and spiders here. This is a grand book for page after page of the "Oh, wow!" experience.



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