Asked by: Liz Jones, by email
My vote would go to the Australian brown antechinus, Antechinus stuartii, a rat-sized marsupial whose males indulge annually in a fortnight-long sexual orgy with as many females as possible. The fun for the males stops there, though, because they all die of exhaustion, leaving a population of females to give birth 27 days later. This mating strategy (called polyandry) saves the females picking out the ‘best’ males to mate with, as research on antechinus reproduction has shown that the sperm of such males can out-compete that of inferior males in the race to fertilise the egg.
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