Prostitution is supposedly the oldest profession, which means that sex itself must be one of the oldest behaviors, a conclusion underscored by the recent discovery of two bugs fossilized as they coupled 165 million years ago. Simply being ancient doesn’t make a sex act interesting, but this one was noteworthy in part because behavior is generally so ephemeral, leaving none of the preserved remains we rely upon to tell us about how early forms of life ate, or ran, or saw the world.
The fossil also showed in breathtaking detail the positioning of the pair in flagrante delicto. While the finer points of the orientation of this particular duo may have been affected by the pressure of the sediment that caused their preservation, the finding underscores the unlikely seeming importance of sexual habits in evolution. Scientists care about the intricacies of how insects mate not out of a desire to create a bestial Kama Sutra, but for two reasons.
The first is that we can use such aspects of sex as mating position and the shape of genitalia to reconstruct evolutionary history and see how species are related to one another. Evolution is all about reproduction. What happens at the business end of an animal is essential to whether eggs are fertilized and genes passed on, and nowhere is the variation in sex organs more breathtaking than in insects. Illustrations of the private parts of insects look like something produced by the love child of Hieronymus Bosch and M. C. Escher, with a hefty measure of Rube Goldberg thrown in. How similar the sexual apparatus of two species looks is often a key to how recently they shared a common ancestor, or indeed to whether they are really two species at all.
The elaboration of insects’ genitalia also has a more diabolical side. We may see two damselflies forming a heart shape with their bodies as they couple, abdomens elaborately curled as they perch on a stem, but if you could look inside them, you would see what appears to be a minuscule Swiss Army knife with its attachments unfolded. That would be the male’s penis. A female damselfly may mate with more than one partner, but from each male’s perspective, the more of her eggs he can keep from rivals and fertilize himself, the better. The spines and scoops on his penis serve to remove the prior male’s sperm so he can replace it with his own. Nature may be red in tooth and claw, but she is utterly ruthless when it comes to genitalia.
But back to mating position, and to the second reason scientists care about bug sex. Sexual position dictates whether males or females have control over the outcome of mating, which in turn has a host of repercussions on the evolution of other aspects of behavior. The side-by-side position of many bugs, including the family of which the fossil duo is a member, is common, but so is a female-on-top version. In the crickets I study, for example, once a female has been wooed by the song of a male, she has to clamber onto his back and orient herself just so. Once she is securely situated, the male reaches up with his nether regions and proffers a tiny aliquot of sperm, neatly contained in a vessel of chitin. The end of the vessel has a long stem that must be threaded skillfully into the reproductive opening of the female, after which the sperm drains into her body for several minutes. The process is incredibly painstaking, and requires the full cooperation of the female.
Contrast this with mating in water striders, those leggy inhabitants of streams and ponds that skitter on the water’s surface. A water strider male simply leaps unceremoniously onto a passing female, sometimes clinging to her back with specialized hooks. The female usually attempts to dislodge him, sometimes successfully but often not; either way, the process is time-consuming and risks attracting the attention of predators.
People sometimes think animals, insects included, follow a kind of 1950s lifestyle, with subservient females and macho males, but the truth is much more unconventional. In some species of crickets and their relatives the katydids, for example, where females have the upper hand, so to speak, males offer females nutritious globs weighing up to 30 percent of their own body weight that they have manufactured from their body fluids. The female eats these during mating, and usually, the larger the nuptial gift, the more of the male’s sperm fertilizes her eggs. As you might imagine, however, a male does not produce such gifts easily, and, contrary to the stereotype of the indiscriminate lothario, tends to be rather picky about which female he chooses to be the recipient.
One last word about mating positions, this time in every New Yorker’s favorite insect, the cockroach. Cockroaches use an ancient pose, one that predates the missionary position by many millions of years. After emitting courtship pheromones and other seductive behavior, male and female back up to each other and join hind ends, each facing away. The procedure is oddly clumsy and almost touching, relying as it does on blind trust in the cooperation of one’s unseen partner. It would be easier if the two insects could glance over their shoulders to ensure they were backing up in the right direction, but since they lack necks, that is not possible. Perhaps a fossil a few hundred million years hence will show us an improvement. That is, if we humans are still around. The roaches are bound to be.
Marlene Zuk is an evolutionary biologist at the University of Minnesota and the author of Sex on Six Legs: Lessons on Life, Love and Language From the Insect World.