When sex kills

praying mantis 

Mention reproductive conflict and many people will nod knowingly. It is not unusual to find differences in the sexual needs of men and women. Conflict may arise over frequency of intercourse and length of foreplay, not to mention the differing proclivities of partners. Yet that’s nothing compared to the female praying mantis who celebrates successful mating by eating the head of her partner.


It may seem nonsensical that what represents sexual fulfillment for one partner leads to the death of the other. However, sexual fulfillment exists only in connection with reproductive success. Whatever behavior leads to greater reproductive success will be favored, regardless of the unfortunate impact on the sex that is victimized.


This is highlighted in an article in this month’s issue of Current Biology, Sperm Competition Favors Harmful Males in Seed Beetles. Co-author Goran Arnqvist, discussing the phenomenon of reproductive conflict says:

One especially tricky case involves species where the males have mating organs that are supplied with hooks, barbs, and flukes that cause internal injuries in females during mating. This is extremely common among insects, but it also occurs in many other animal groups. 

According to the press release announcing publication of the article:

Goran relates that the males’ mating organ is rather similar to a medieval spiked club and that it causes severe wounds in females during mating. But since it is never a good idea for a male merely to injure a female, the researchers have assumed that these structures serve another purpose and that the injury is an unfortunate side effect.

“Females’ injuries as such do not benefit the male she mated with. It has been suggested rather that the injuries are a side effect of other benefits the males reap from the barbs. Now, for the first time, we are able to show that this is the case,” says Göran Arnqvist.

Longer barbs cause more injuries to the female, but they also increase the likelihood of male reproductive success. When females mate with two males, the male with the longer barbs is more likely to fertilize the eggs. As in other cases of sexual conflict, the interests of the male and female are different, and it is reproductive success that ultimately determines whose needs are more important.


In the past, Goran and colleagues have proposed that a coevolutionary arms race takes place between males and females. Since reproductive success is maximized by an injurious mating organ, it is favored despite injuries to the female. In reponse, female seed beetles evolve reproductive ducts with more connective tissue, minimizing the risk of injury even as the male reproductive organ evolves to maximize it.


That makes a certain amount of sense, but, obviously, there are some injuries that cannot be overcome regardless of evolution. The male praying mantis cannot survive long without its head and no amount of evolutionary adaptation can change that. Therefore, there must be a very strong boost to reproductive success than arises from the female eating the male’s head during mating, and indeed there is.


The male praying mantis can complete mating after being decapitated. In fact, biting off the male’s head actually appears to improve sexual performance. Decapitating a  male praying mantis causes it to reflexively assume the mating position.


Second, the male’s head is an excellent source of protein and nutrients. Ingesting her partner’s head allows the female praying mantis to produce higher quality eggs, and thereby increases the reproductive success of the species.


While reproductive conflict may be responsible for unhappiness among human beings, it appears to have significant advantages among animals. The next time any man complains about not “getting enough” he should pause to consider that it could be far worse. Instead of getting his head bitten off figuratively, his partner could make biting off his head the literal consequence of sexual satisfaction.