Question: why do praying mantis females bite the heads of of the males before ending sex?

Sam Tazzyman answered on 22 Jun 2011:

In fact in some species the male mantis cannot ejaculate unless he has been decapitated! So the short term answer is “because otherwise there would be no reproduction”.

Of course there is a more in-depth answer too. In many insect species, the males have to bring the females some killed item of prey before the female will mate with the male. The female then eats the prey while the male mates with her. The prey item is called a “nuptial gift”. In the case of preying mantids, the nuptial gift is the head of the male! Trying to think of how on earth it could be evolutionarily good to have your own head bitten off is a strange thing. But the reason is as follows: Praying mantids are solitary, and quite rare. It’s very unlikely that a male mantid will encounter more than one female mantid in his lifetime. So if he mated with the female, and didn’t get his head bitten off, and she didn’t eat him, he would be very unlikely to go on and meet another female and mate again. So his genes would only be passed on once whether he survives or not. Therefore from an evolutionary perspective it makes no difference whether he survives the mating or whether he doesn’t. However, if the female eats the male then she gets some more food. This means she is more likely to survive, and so the offspring of the mating are more likely to survive. And so genes that cause a male to allow himself to be eaten by the female will spread in the population than genes that make a male run away!