Only one male in a naked mole rat colony has breeding rights, so there’s no need for zippy sperm (Image: Ron Austing/FLPA)

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.

“The authors have done a pretty good job of arguing that inbreeding is not driving the sperm degeneration, because the colonies they sourced sperm from were specifically outbred,” says Matthew Gage at the University of East Anglia in Norwich, UK. “It just goes to show how very important sperm competition is as an evolutionary force acting on sperm form and function.”

“The naked mole rat is actually a very good model for what happens in humans,” Maree says. Previous studies have found faster sperm in more promiscuous species. Humans are relatively monogamous, so sperm competition is fairly low and abnormalities are common. Typically, about 60 per cent of human sperm are motile, compared with 95 per cent in more promiscuous species.

Journal reference: BMC Evolutionary Biology, in press