On Monday night a newspaper article caught my eye – the reporter had picked up on a study suggesting that, if you’re a female praying mantis, eating your partner during sex can actually be quite beneficial…
The idea that female praying mantises eat their sexual partners during the act has been around for a long time – I remember hearing it when I was a child (and that’s a long time ago!). Steven Jay Gould wrote an excellent essay on the subject (“Only his wings remained”) – and points out that although the idea of sexual cannibalism has a certain horrid fascination, it’s a relatively rare event.
For male mantises, the whole enterprise of mating is fraught with danger. Approach her from the front & you’re likely to be history; you’ll be greeted as prey and not partner. So male mantises approach (with caution) from behind, leap on the female’s back, hold on tight – and leave rather quickly once it’s all over. Even then, if he’s not in quite the right position, the female will simply turn her head and bite his off. And then consume him, bit by bit, from the neck down. (There’s a wonderful article on sexual cannibalism here.)
Now, at first sight, this seems rather counter-productive – why kill and eat your mate right in the middle (or perhaps at the start) of mating? Certainly, the female is getting a nice fresh nutritious meal that would go a long way towards meeting the energy & nutrient requirements of forming and laying eggs. But surely that’s not much good if her partner’s now deceased? And there’s nothing in it for him, is there?
Gould points out that decapitated males are actually better in bed than their intact brethren: they perform harder and for longer, thus potentially transferring more of their sperm to the female. This is because much of the mechanical movement of mating is controlled by a nerve plexus at the end of the male’s abdomen, and this plexus is in turn controlled by the cerebral ganglia (the ‘brain’) in the insect’s head. Removing the head is followed by immediate, repeated, and prolonged mating behaviour.
And there’s certainly evidence from other species that duration of copulation is linked to the amount of sperm transferred – and many examples of males bearing gifts for their partner and through this obtaining a longer sex act. In hanging flies, for example, duration of mating is directly linked to the size of the ‘nuptial gift’, typically another insect: males who offer bigger gifts get to mate for longer and transfer more sperm to the female (and are less likely to be consumed themselves). In some species of hanging flies, the male first wraps his gift in silk – it takes a while to unwrap & in the meantime he’s begun copulating.
So the gruesome circum-coital behaviour of some female mantises may not be as pointless as it seems. But don’t get me wrong – the male isn’t a consenting parther in all this. Instead, he actively tries to avoid ending up on the dinner table – just like males in all the other species where the female is larger, much more aggressive than the male, and very very hungry.
(PS hanging flies are called this because they use a couple of legs to hang off leaves and stems.)
Steven Jay Gould (1985) Only his wings remained. pp 40 – 55 in ‘The Flamingo’s Smile: reflections in natural history”, pub. Penguin.