Sex is such a frenetic and stressful process for some male marsupials that it literally kills them, according to a new Australian-led research study, which also found that female promiscuity partly fuels this “suicidal” behavior.
Scientists had wondered for decades why some species of insect-eating marsupials dropped dead after mating. 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 and mammal ecologist Diana Fisher said yesterday. “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 afterward.”
Organisms that mate once and then die are common among plants and some fish, but rarer among mammals. Among the exceptions are some species 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.
Fisher, from the University of Queensland, said the male marsupials that die are so intent on mating that their high testosterone levels trigger a cascade effect of stress hormones, which cause 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 said.
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.
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 fertilize 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 synchronizing their annual mating period, but by mating promiscuously.
“We conclude that precopulatory sexual selection by females favored the evolution of suicidal reproduction in mammals,” the paper added.
Fisher said that across species, as the breeding season became shorter, there was a decline in male post-coital survival “until it reaches the pinnacle of extreme trade-off when you have to die.”
She said the 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,” Fisher said.