Sex and the single cannibal

This is a story about love gone horribly wrong. It’s a story where the girl always gets her man.

We are talking about males who end up murdered, or murdered and eaten, or, yes, even eaten alive. But this is the hard part: there are males who seem to want it to end like that, males for whom this is the essence of love gone wonderfully right.

A new study published in the journal Animal Behaviour starts out with a depressing little smorgasbord of heroic male self-sacrifice in the animal world.

“In honeybees, males have evolved genitalia that explode into the female, causing the male to die after a single mating. In some molluscs, crustaceans and fishes, males do not die when they mate, but they remain permanently attached to the female, rendering them unable to acquire additional matings.”

In the queenless ant species Dinoponera quadriceps, the female severs the end of the copulating male’s abdomen, killing him and leaving his genitalia linked to hers. And next – could we have a drum roll, please – the Australian redback spider, Latrodectus hasselti, turns self-sacrifice into a circus act. “Males actively sacrifice themselves during copulation by somersaulting their bodies onto the fangs of their mating partners in an apparent attempt to induce sexual cannibalism.” The crowd goes wild, but there is no encore.

The theory about male self-sacrifice is that, after a certain short-term awkwardness, the males benefit in the long run by increasing the number of their offspring and by reducing the likelihood that the female will mate again. They may also donate what the researchers call “somatic nutrients”, meaning their own bodies given up in the act of sexual cannibalism, and “these nutrients may be subsequently transferred to their own offspring, thereby increasing offspring quantity or quality.”

The new study tests these theories in dark fishing spiders, Dolomedes tenebrosus, from Nebraska. These spiders would be considered exotic to start with, for the behaviour that gives them their name. They lurk along the sides of streams and ponds, darting out across the surface to eat aquatic insects and even small fish. But let’s talk about sex.

In mating season, males outnumber females by three-to-one, meaning that they are highly expendable, even downright annoying. A typical female also weighs about 14 times more than a male.

During intercourse, the male inserts a pedipalp, one of the leg-like appendages on its front end, into the female. He inflates this copulatory organ and begins transferring semen, but then curls up and becomes unresponsive. His body “hangs from the female’s genital opening” lifelessly, for five or 10 minutes, although his heart may continue beating and sperm may continue to be transferred. What’s a girl to do?

She eats him.

The study, by researchers from the University of Nebraska-Lincoln and Macquarie University in Australia, concluded that sex means “obligate death” for these males. That is, they put so much into mating that the effort kills them, even if the female does nothing.

But surprisingly the male carcass does not then become a genital plug to reduce the likelihood that the female will be able to mate again, as happens in honeybees and some other species.

The researchers also found no evidence that cannibalising the male caused the female to avoid further matings. On the contrary. In one set of experiments, a second male was introduced into the female’s tank while she was still feeding on the first male. Half the time she had sex with him, too, followed by a light dinner. When a third male showed up, 68% of the females “cannibalised the third male prior to copulation”. The researchers describe this “precopulatory sexual cannibalism” as a “mechanism of mate choice,” a way to get rid of annoying “non-preferred males,” like a pretty girl handing out the wrong phone number at the bar. Only kind of worse.

Simona Kralj-Fišer, who studies spider mating behaviours at the Slovenian Academy of Sciences and Arts, praised the new study for “describing another strange sexual behaviour in spiders, when we think that we have already seen the most peculiar strategies.”

These strategies may sound like a really bad deal for the male. But the researchers haven’t yet taken the next step. DNA testing of the offspring may yet show that there really is something in this arrangement for the hapless dark fishing spider male, with male quality perhaps reflected in the number or robustness of their offspring. “Furthermore,” said Kralj-Fišer, “if 50% of females did not remate, what was the quality of the males where remating did not happen?”

Meanwhile, you know all those songs human males are always singing about broken hearts and other complications of our tangled love lives? Maybe we should just shut up, get the girl some dinner, and be thankful for what is, in the grand evolutionary sexual sweepstakes, a pretty sweet deal, after all.