Sex is such a frenetic and stressful process for some male marsupials that it literally kills them -- and female promiscuity partly fuels this "suicidal" behaviour, according to a new Australian-led research.
Scientists had wondered for decades why some species of insect-eating marsupials dropped dead after mating, 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 of mammals that can die, are some species of small marsupials including the mouse-like antechinus and the phascogales, which is more like a possum. Die-off occurs in all males of the 12 Australian species of antechinus, three species of phascogale and the dasykaluta, a rodent-like cousin of the antechinus.
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 insect-eating marsupials in Australia, Papua New Guinea and South America -- not all of which self-destructed after sex.
The researchers found that among species with low male survival rates after mating, those with what is referred to as "suicidal reproduction" had shorter mating seasons and larger testes relative to body size, allowing them to fertilise many females.
"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.
The females also escalated sperm competition, not only by synchronising their annual mating period but by mating promiscuously.
"We conclude that precopulatory sexual selection by females favoured the evolution of suicidal reproduction in mammals," the paper added.
Fisher said across species, as the breeding season became shorter and shorter, there was a decline in male post-coital survival "until it reaches the pinnacle of extreme trade-off when you have to die".
Females had the advantage of timing their reproduction to be at the same time which encouraged mating competition, and ensured they got the best males.
"Rather than the males fighting, their sperm just compete," she 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.