Sexual cannibalism: why women sometimes eat their partners after sex

Spiders do it. Snakes do it. Luckily people don’t. Yes, we are talking about sexual cannibalism, the horrific, surprisingly common practice of eating your partner after procreation. The most famous example is praying mantis, where females often bite off their lovers’ heads after mating. The practice also occurs in spiders, and it’s what gave black widow spiders their common name — although sexual cannibalism in the species is rare.

Scientists have come up with a number of hypotheses as to why sexual cannibalism occurs, stating that males cook good meals, or that aggressive females get confused about what is prey and what is not. In many cases, there is a chilling, calculated logic behind the decision: Men are simply a good food source for expectant mothers. Males of some species even go so far as to offer themselves as a meal to females after they have finished mating.

But other smart men have come up with ways to avoid becoming a post-coital meal. Gifts of food are sufficient for some species, while others require quick silk lassoing. Anyway, it’s true what they say: it’s a partner-eat-partner world out there.

From father to food

Richard Dawkins’ 1976 book, The Selfish Gene, popularized the idea that the sole purpose of biological life forms is to pass their DNA from one generation to the next. From this perspective, the bodies we inhabit may be little more than vessels for the genetic information they contain; once those genes are passed on to the next generation, the physical body is of little use.

This certainly does not apply to every species (humans, for example), but it does confirm what biologists say semelparous animals – that die after mating. Many plant species die after reproduction, as do animals such as salmon and males antechinus. It also applies to male brown widow spiders, which offer themselves as live offerings to females. During mating, a male places his abdomen near the canine teeth of the expectant mother of his children. The female will consume her mate and use his body as food for her children.

Even more impressive: Male redback spiders perform what scientists describe as a somersault directly in the mouth of females after mating. The gymnastics feat is deadly, but also effective: men who sacrifice themselves have twice as many children.

Battle of the Sexes

In many cases, scientists believe that sexual cannibalism arose from a basic necessity of life. Expectant mothers need a lot of food to support their children, and the males provided a nearby source of protein. For example, a study in spiders found that females eating males… larger brood sizes than those who did not. Over time, women who ate their partner had more children and the behavior spread by evolutionary selection.

However, not every man is so selfless. In many species, the females behave in a way that is much more predatory. One theory holds that sexual cannibalism is a “maladaptive side effect” of female aggression. It is confirmed by studies showing that aggressive females eat males more often. Females that are more aggressive when hunting tend to get more food and are more likely to survive and have children. The unlucky males in this case may just get in the way.

Another theory for sexual cannibalism states that it results from women’s pickiness. For example, less impressive male wolf spiders were more likely to be eaten by the females, one study found.

It’s not just spiders who eat their partner after sex. Biologists have documented female anacondas strangling their partners after mating, probably to be used as food later. Like many species where sexual cannibalism occurs, female anacondas are much larger than males, making it easier for them to overpower their mates.

Isopods, a type of crustacean, also engage in sexual cannibalism, although in their case it seems to go both ways. Both males and females have been seen eat their friends after sex. Researchers aren’t exactly sure why male isopods sometimes eat females — after all, they just bothered to create offspring with them. It can only occur when females die shortly after mating, scientists say, or when males are dangerously deficient in food.

Sexual cannibalism, although rare, has also been observed in octopuses. In one case, a female blue octopus killed a small male after mating with her several times before dragging his body to her lair for dinner. The females often killing men by strangulationwrap a tentacle around the male’s mantle to cut off the flow of water to their gills, causing them to suffocate.

Men fight back

Faced with potentially deadly sexual encounters, the males of many species where this happens have sometimes devised ingenious methods of avoiding death.

Some take the road to appease their potentially hungry partner. Male maternity web spiders often come with wedding gifts — tasty insects — for their hungry friends. Those who do are cannibalized at much lower rates than men who show up empty-handed, researchers found.

Other males opt for the element of surprise. In a number of spider species, males have been observed to wait for a female to eat or molt an insect to mate. Such opportunistic mating strategies ensure that a female is sufficiently distracted, hopefully so much so that cannibalism is far from her mind. Likewise, a species of praying mantis will sometimes time their approach to the female with: sufficiently strong wind gustsprobably cover any sign of their advance.

Others opt for a more direct approach. Male maternity web spiders tie the legs of their future mates with silk before mating, something researchers have called a “bridal veil.” The strategy not only protects the males from predation, but also allows them to mate for longer, which increases the chance of offspring. A 2016 study confirmed that men who chose bondage significantly reduced their chances to be eaten by women. Take the plunge, indeed!

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