Perhaps by virtue of having adequately signaled my predilections over the past few years, or maybe due to my loved ones having so long endured fevered bouts of irritating, giggly prattle, two people sought to mark the occasion of my 30th birthday1 by giving me the same gift, a book called Nature's Nether Regions2 by Menno Schilthuizen. (I favor the latter explanation, being that these people were my girlfriend and my brother.)

natures-nether-regions-coverChock full of extremely fascinating and detailed examples of reproductive machinery—which comprise a breathtaking menagerie of complex, sometimes labyrinthine, members, attachments, and orifices—Schilthuizen's book attempts to sketch out how sexual selection gives rise to the vast array of genitalia seen in nature.3 As opposed to natural selection, the process by which traits are selected for based on suitability to an organism's environment, sexual selection works when traits increase mating success, not due to simple survival but because those traits are desired by the opposite sex. So while a crest of feathers on a bird, for example, might not confer a survival advantage, the crest may nonetheless look pretty sexy to a female, increasing the chances that the male will be able to mate and thus pass on sexy-crest genes to their offspring.

There are a number of different ways in which sexual selection can work, and Schilthuizen does a good job of illustrating these mechanisms, their limitations, and the controversies surrounding them with requisite humor but without the sort of offputting winkiness that often drenches articles about animal sex like bad cologne. On the contrary, from a scientifically inclined layperson's perspective, I think he does a fantastic job of balancing serious discussion of evolutionary concepts and florid illustrations of genitalia with the demands of a non-specialist audience. The scientific issues are simplified and made accessible but are dutifully explained, all of which makes for an enjoyable and engrossing read.

Much as with Holy Shit, I am tempted to copy reams of descriptions for you to read here. From sharks to humans to damselflies and spiders, the ways in which penises and vaginas have coevolved, sometimes in competition with one another, are endlessly fascinating. However, the mating habits of two species of insect, a cricket and a katydid, caught my eye as delightfully weird. For the purposes of understanding the following passage, you will need to be familiar with a couple of terms.

First, sperm competition refers to situations in which males devise strategies to give their sperm the upperhand within a female's reproductive tract. In some cases, males scoop out prior mates' sperm before depositing their own; in other cases, they may divert sperm into areas within the female that are less likely to bear that seed unto her eggs. These strategies exist because females evolve, by chance, genitalia that increase their sexual autonomy, or in other words, their ability to choose whose sperm fertilizes her eggs. Males, also by chance, "respond" when mutations result in changes to their genitalia or behavior that allow them, via force, subterfuge, or other means, to subvert the strategies of females.4  Females want the best quality sperm, essentially, while males want to make sure their own sperm wins out, because that is how they pass on their genes. This second concept, the "conflict" between reproductive goals, is known as sexually antagonistic selection. Cryptic female choice, on the other hand, refers to strategies employed by females to favor males whose genes are (involuntarily) deemed fit. Cryptic female choice consists of the ways in which females' genital tracts privilege some males' sperm over others, either by their very construction or by responses to purportedly stimulating features of male genitalia.5 Sperm dumping, when a female ejects sperm after coitus, is one such method of cryptic choice. And, last of all, a spermatheca is an organ in many female insects that holds the sperm until the female is ready to fertilize her eggs. Whereas human sperm dies in the female genital tract in two or three days, female insects can hold sperm for a much longer period of time.

Okay, now that we've got the background out of the way, you're ready for this:

Crickets also flush. Except that they don't use water but their own semen.6 The pretty green Japanese tree cricket Truljalia hibinonis has a huge (relative to its body length) member that it can push all the way up the female's spermatheca. When it then ejaculates, its sticky mass of sperm will force backward any sperm that·is already in there, pushing it out of the female's vagina and onto the penis shaft of the male—who then bends over and proceeds to nibble away at the freshly removed rival sperm as a postcoital snack! And in another cricket-like insect, the European katydid Metaplastes ornatus, sperm flushing is a part of sex in which male and female actually collaborate. The male inserts his genitalia, moves them in and out for a bit, and then withdraws. Since his genitalia are jagged and crenellated, pulling them out of the female brings forth a large portion of the female reproductive system, momentarily exposed inside out. What then happens is rather remarkable: the female doubles over and licks out the inside of her own genitalia, eating up any sperm packages of previous mates that still remain in there. This ritual is repeated several times before the male finally deposits his own sperm.

This last example may seem a little puzzling: why would a female join in on a male's attempts to empty her sperm stores? Again, we have to remember that a female has a vested interest in whatever the male does. A male that is able to persuade her to give up her stored sperm reserves probably has qualities that she'd do worse than to pass on to her sons. So, in some species, the whole sperm-scooping business has evolved to become incorporated into the mating ritual and, in a way, has become part and parcel of cryptic female choice—merging imperceptibly with sperm dumping.

Now, I think that's absolutely fantastic, and the rest of the book is replete with similarly captivating examples of sexual selection's effects on the genitals and sexual practices7 of all sorts of animals. There is also a companion website that I just found while writing this post, with what I hope are many more such examples and which I plan to peruse just as soon as I'm through the last seventy-odd pages of this wonderful genitaliary.