It's fair to say that we belong to a species obsessed with sex. We are among the only species to have sex for fun, not just for reproduction. For some other species, though, sex is far from fun. In fact, as two recent review papers show, it's a war zone, involving things like penis fencing and love darts.
In 1897, the Italian zoologist Constantino Ribaga discovered a strange organ in female bedbugs, halfway up the abdomen. He suggested that they used it to produce sound, like cicadas. But something wasn’t right: in the bundle of cells underneath this organ, he found large quantities of sperm. How did they get there? At the time, puzzled scientists concluded that males must flood females with sperm, and the female digested the excess—as a “nuptial gift”—using this organ. But this idea was tenuous at best.
It wasn’t until 1913 that males were observed stabbing females through this organ with a horrifying syringe-like penis, then copulating with the wound. Sperm then swim directly to the ovaries through the body cavity. This process has been termed “traumatic insemination."
In the first of the two papers, appearing in Biological Reviews, Rolanda Lange and colleagues at Tübingen in Germany and Sheffield in the UK show that similar behavior occurs across invertebrates.
In snails, which are hermaphrodites, amorous advances involve “traumatic secretion transfer,” blasting potential mates at close range with “love darts” covered in psychoactive mucus. Understandably, neither party is keen to play the female role, which involves being shot. In sea slugs, attempting to avoid this fate results in “penis fencing”—each party attempting to penis-stab the other. An inflicted wound inoculates the recipient with sperm.
To stab or not to stab
Why would a male want to impale the mother of his future children? In a paper in Annual Review of Entomology, Nik Tatarnic and colleagues from Sydney, Australia and Sheffield, UK focus on answering this question using examples from arthropods. They explain that stabbing is, in evolutionary terms, a game-changing tactic for males.
To sire offspring obviously requires mating, but this is only a prelude. Much more crucial is fertilization, and females understandably want to control when, where, and by whom their eggs are fertilized.
In many cases, females are highly successful at this—by, for example, using their reproductive tract as a powerful tool to screen out all but preferred males. Females often simply eject unattractive males’ sperm or filter it out chemically and sometimes can close their tract entirely. Female control is especially widespread in insects, where females store sperm in a sac, sometimes for years, opening it to fertilize eggs at their leisure.
Each male that mates with a female would prefer his own sperm to fertilize the offspring. To achieve this, he must both overcome the female’s defenses and beat her other mates—two never-ending arms races.
Males can beat rivals to the first step—that is, mating—by impressing females through courtship. This may also win favor at the fertilization phase. But males are more sneaky than that, with outlandish adaptations to ensure their sperm win the race, like plugging females up or scooping out rival sperm. Our own human organ may indeed have a dual function as a “sperm scoop.”
Another solution is to try to directly overcome challenges posed by the females' reproductive tract. Fruit fly males spike their sperm, drugging females into releasing more eggs, even though this shortens females' lives. Wood mice females produce mucus that only strong-swimming sperm can cross. Ever resourceful, males’ sperm team up into long “snakes” that, together, altruistically push one lucky sperm through the mucus to success.
Stabbing for the win
Penis-stabbing, though, shifts the goalposts entirely. By injecting sperm directly into females' body fluid, male bedbugs bypass the female’s adaptations that control access to her eggs. The female is injured in the process, reducing her number of offspring. But in the cold language of statistics, this may be worth it for the male—a healthy female is no good if she doesn’t bear his offspring. Males can both prevent females from excluding their sperm and also from preferring other males.
This has been such a successful strategy for males that it has evolved repeatedly across the animal kingdom. Male Myzostoma worms secrete corrosive enzymes from their penis, dissolving a hole in the female’s skin into which they ejaculate. Male giant squid inject sperm packets into females' arms—although occasionally they end up inseminating themselves, shooting themselves in the foot. In Harpactea sadistica spiders, the male bites the female, then stabs her with needle-like genitals, ejaculating into the wound (as hinted at by the Latin name).
In some groups, like bat bugs, males spear females at random. In many others, though, females have evolved damage-limiting adaptations such as the bedbug “spermalege” (the organ discovered by Ribaga), offering an easy entry point to discourage indiscriminate stabbing. Some species have entire “paragenital systems” guiding sperm to their ovaries, while the former reproductive apparatus shrivels from disuse.
Males will often jump on and penis-stab anything that comes their way, even females of other species, sometimes killing them in the process—a phenomenon that has driven some species to evolve apart. Male bedbugs regularly jump other males by mistake, which is such a problem that males in one species have evolved their own damage-control spermaleges.
In an arms race, though, neither side can win—each can only gain a temporary advantage—and females are fighting back. Astonishingly, some female bedbugs have evolved modified spermaleges that mimic those of males to reduce harassment. Others have evolved ways of digesting the injected sperm and using it to repair their wounds, minimizing the damage.
The course of true love never did run smooth. For males and females locked in an arms race, though, it could be said to run in circles—vicious ones at that. And all to inject a little bit of yourself into the next generation.
This article was originally published on The Conversation.