Abandon the rat race at your peril. Naked mole rat colonies contain just one sexually active male – and the lack of competition has left its sperm shrivelled and sluggish.
Liana Maree of the University of the Western Cape in Bellville, South Africa, and colleagues took sperm from captive naked mole rats and subjected them to a battery of tests. Only 7 per cent of the sperm actually moved, and they swam at around 35 micrometres per second – possibly the slowest sperm of any mammal.
"The reason they look so ugly and swim so slowly is there is no sperm competition," says Maree. Naked mole rats live in colonies dominated by the queen, who chooses one male at a time to mate with. She suppresses the reproductive instincts of every other male in the colony. Because the chosen male has exclusive mating rights, he can afford to produce sperm that will dawdle on their way to the egg.