Fatal distraction

A praying mantis, apparently unawre of its likely fate. Photo: Getty Images

Sex has been the downfall of many species – and that includes German turtles and unwary praying mantises. This month, scientists
have been speculating over what killed a brace of copulating turtles mid-act. Essentially, the turtles’ attention seems to have been diverted, with fatal effects – an all too-common phenomenon in the natural world.

The turtles were found fossilised in a German quarry. Some 47 million years ago, the quarry was a volcanic lake-bed. It was found that six pairs of turtles had been fossilised while mating, so the smart money is in poison gas entering the water as the killer blow.

There is always a risk attached to taking your mind off the business of staying alive, which is part of the reason that the existence of sexual reproduction remains something of a mystery to biologists.

Asexual reproduction – that is to say, cloning – doesn’t shuffle genes and help a species cope with parasites or a changing environment. But it does ensure that an organism’s entire genome makes it through to the next generation. And it is nowhere near as dangerous as sex.

Seeking a sexual partner, whether by physical display or by singing your lungs out, risks drawing unwanted attention from predators. And the act of having sex certainly makes you vulnerable to them. In (somewhat bizarre) laboratory experiments, biologists have shown that the fish that eat small crustaceans known as copepods find it easier to snack on mating pairs. It’s no wonder that copepods usually don’t mate
if they know that there are predators around. Water striders, too, suffer for sex; the female bugs are twice as likely to be eaten by a predator during copulation.

Not that males always have it easy. The male praying mantis, for instance, mates by jumping on to the female’s back, gripping hard and jumping off again when the job is done. If, however, he slips and comes within reach of the female’s mandibles, she will bite his head off.

It’s no skin off her nose – the nerves that drive the muscle movements of copulation are in a mantis’s abdomen, so a headless mate is just as good a sexual partner. And she gets a tasty, nutritious snack to boot.

Male mantises seem to have a nose for this kind of tendency; the hungrier a female is, the fewer mates she will attract. Male orb web spiders have no such sense: females almost always eat the male after copulation.

Evolution has compensated the males for this inconvenience by endowing them with a detachable penis, which acts as a plug to stop others from mating with her and ensures that their death is not in vain.

Horny dilemma

Going even further, evolution has given the wasp spider two detachable penises. Yet a study by researchers at the University
of Hamburg showed that this just delays the inevitable – and puts the male on the horns of a dilemma. Try to escape in order to fertilise the eggs of a second mate, or go for maximum fertilisation during the first encounter? Either way, you don’t make it past lunch.

Male wasp spiders make their decision by weight and sexual history. If they encounter a heavy and never-impregnated female (it seems wrong to label any spider a “virgin”), they go for the double with that partner. In any other scenario, they attempt to get away to a second encounter, invariably ensuring that the second partner is heavier than the first. That is because a high maternal weight increases the survival chances of any offspring.

Even in the face of sex, or death – sometimes both – some creatures are still capable of making good decisions.

Michael Brooks’s “The Secret Anarchy of Science” is published by Profile Books (£12.99)

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