The problem with a long penis (if you're a beetle)
Special muscles and soft tip help a male beetle's performance.
How do you quickly – and successfully – have sex when your penis is longer than your entire body?
Such is the lot of some beetles. But a study has found they get the job done by keeping the tip of their penis soft, allowing them to swiftly "shoot" their protracted members into a female's duct.
"Another benefit of having a long penis found in some other insects is that a male is able to efficiently 'scrape' the female's duct free of other males' sperm, which would compete with his own," says Yoko Matsumura, an entomologist at Germany's University of Kiel.
A female beetle has a very long reproductive duct, so only the longest penises can successfully make the distance and fertilise her eggs.
It also slows down mating, leaving her time to wriggle away.
Matsumura says this genital arms race has males evolving special muscles around their penis that catapult it with enough force to penetrate precisely and rapidly.
She and colleagues in Germany and Ukraine observed mating beetles and measured how far males were able to penetrate.
Their computer simulations showed that by being relatively stiff around the base, but progressively softer towards the tip, the penis hits its target more often than if it was rigid all the way along.
The work was presented at the annual meeting of the Society for Experimental Biology in Brighton, UK.