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This had a huge impact on the males, which were forced to compete intensely with each other in a matter of weeks. The question has vexed biologists for three decades, and many have offered answers. Sperm competition drives the evolution of suicidal reproduction in mammals. A baby shrew suckles for days or weeks; a baby antechinus does so for four months. The females were better at raising their young if they gave birth just before the annual bonanza, and were well-fed enough to wean their joeys. But Fisher, who has been studying antechinuses for decades, favours a different idea. Fisher thinks that as the ancestors of antechinuses spread south through Australia and New Guinea, they encountered strong yearly fluctuations in their food supply. The females could only fit in one litter during the annual peak. Her team gathered data on the lives and environments of a wide variety of 52 insect-eating marsupials, from the fully semelparous antechinuses, to relatives where a small number of males survive past their first sexual liaisons, to species that breed repeatedly.

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