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Just see if you can look at the picture on the right without wincing. But why have male beetles evolved such hellish genitals? What benefits do they gain by physically harming their partners?

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In this theory, the spines are important for winning these competitions, and the wounds they inflict are simply a nasty side-effect. Size, it seems, really does matter. The genitals of these different populations are very varied and to study them under a microscope, Hotzy and Arnqvist first had to fluff their subjects.

Under a microscope, they measured the length of the longest spines and the size of the entire spine-bearing area.

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They studied female beetles too, and all from a single population. Each one was allowed to mate once with a single male. A week later, Hotzy and Arnqvist dissected their sexual tracts to see how much scarring they had, using the distinctive black pigments on the scars to spot them.

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Two days later, the females were then paired with a second male, taken from one of the 13 worldwide populations. One week later, they counted all the eggs she laid. Every one of these must have been fathered by the second male, so their represents his ability to successfully oust the sterile sperm of the first partner.

Horrific beetle sex – why the most successful males have the spikiest penises

Either way, they have clearly evolved to allow the male seed beetle to out-ejaculate his rivals. This fierce competition for fertilisation rights is fairly common among insects and has led to the evolution of some truly horrific sexual strategies. Some species, like the dung flyalso have armoured genitals. Others, like the familiar fly Drosophila melanogasterhave toxic semen laden with chemicals that actually slash the lifespans of females, who can literally mate their way to death.

Their goal is to encourage the female to lay more eggs or reject further suitors, or even kill the sperm of other males.

And possibly the most cringeworthy sexual technique of all belongs to the male bed bug, whose penis is like a hypodermic needle. Doing so cuts the total length of mating by a third — this is one species where the females want the males to ejaculate sooner.

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