Scientists had wondered for decades why some species of insect-eating marsupials dropped dead after sex, with speculation including that they died from fighting or to leave more food for their offspring.
But research published in the US-based Proceedings of the National Academy of Sciences puts the "dying off" down to the animals' extreme efforts to ensure their sperm is successful in the short once-a-year window that females offer to mate.
"There's always a cost to reproducing -- it's an energy expensive thing that animals do," lead researcher, mammal ecologist Diana Fisher from the University of Queensland, explained Tuesday.
"But in this case they haven't spread out their effort over time, they do it all at once in a really short time. And they just die afterwards."
Organisms that mate once and then die are common among plants and some fish, but rarer among mammals.
Among the exceptions are some species of small marsupials including the mouse-like antechinus and the phascogales, which is more like a possum.
Fisher said the male marsupials that die are so intent on mating that their high testosterone levels trigger a cascade effect of stress hormones, which causes the animals' body tissue to break down and their immune systems to collapse.
"They mate for 12 or 14 hours at a time with lots of females, and they use up their muscle and their body tissues and they are using all of their energy to competitively mate, that's what they are doing. It's sexual selection," she told AFP.
"They just kill themselves mating in this extreme way."
The study, which included researchers from the University of Sydney and the University of Tasmania, compared 52 species of marsupials in Australia, Papua New Guinea and South America -- not all of which self-destructed after sex.
"We demonstrate that short mating seasons intensified reproductive competition between males, increasing male energy investment in copulations and reducing male post-mating survival," the paper said.
Fisher said, in the case of some of the marsupials, their life-and-death mating system seemed a shame.
"They have a nice temperament, they are very inquisitive little animals. They are quite interactive. It's a bit sad. But they don't know it's coming I suppose, it's just something that happens to them," she said
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