Why Lady Mantises Eat Their Mates

If one out of every four of your sexually active friends got eaten, you’d probably start to consider a sex-free lifestyle. Unless, of course, you’re a male praying mantis, in which case you’d go for it anyway. The male bugs’ apparent eagerness to go to their mid-coital deaths has puzzled scientists for some time. One 2016 study may have an explanation: being eaten could actually increase a male’s chance of passing on his genes. The study was published in the Proceedings of the Royal Society B. 

Mantis sex is … intense. About 25 percent of encounters include sexual cannibalism, in which the female mantis literally bites her mate’s head off. But a little thing like that is not going to deter the missing male, who will go on humping like nothing has happened, even as the female continues to nibble him into nothingness. This can take hours.

True romance: a female mantis eating her mate's genitals. Image Credit: Oliver Koemmerling via Wikimedia Commons // CC BY-SA 3.0

He is, in essence, offering his body as a present. This sexual gift-giving is not uncommon in insects—the males in one cricket species even make gummy snacks for their mates—but most of these gifts are not also the gift-givers. From an evolutionary perspective, it makes a lot of sense: females can share the nutrient boost with their fertilized eggs, which gives the kids a better chance of surviving. Was that what was going on in mantises? To find out, researchers set up a little mantis love hotel. 

The first step was to provide a romantic meal. The scientists treated small crickets with radioactive amino acids, then fed those crickets to a group of male mantises. Then each male was paired with a female and allowed to mate. Half of the mantis pairs were separated before the female could get her chow on. The other half were left alone to play out their grisly romance.

In eating her partner, a female also ate the contents of his stomach. So by tracking those radioactive cricket particles, the scientists were able to determine how females’ bodies were using their most recent meal.

As it turns out, even uneaten males were giving their partners a little something. On average, male mantises who survived passed on 25 percent of their radioactive amino acids via their ejaculate. But, as expected, a dead dad could do even more for his kids; gobbled-up males gave their mates a full 90 percent of their nutritious amino acids, and their mates passed the goodness on to their young.

Being devoured also improved a male’s genetic success by increasing the number of eggs laid by his partner. Females in the coitus interruptus pairings—those who didn’t get to eat—laid about 37 eggs apiece. Those who finished their dinner dates, on the other hand, produced around 88. 

We’ve still got plenty to learn about these beautiful creatures and their slightly terrifying sex lives. But these findings do, at least, make sex-positive male mantises seem slightly less foolhardy.