Why A Little Mammal Has So Much Sex That It Disintegrates
It’s August in Australia, and a small, mouse-like creature called an antechinus is busy killing himself through sex. He was a virgin until now, but for two to three weeks, this little lothario goes at it non-stop. He mates with as many females as he can, in violent, frenetic encounters that can each last up to 14 hours. He does little else.
A month ago, he irreversibly stopped making sperm, so he’s got all that he will ever have. This burst of speed-mating is his one chance to pass his genes on to the next generation, and he will die trying. He exhausts himself so thoroughly that his body starts to fall apart. His blood courses with testosterone and stress hormones. His fur falls off. He bleeds internally. His immune system fails to fight off incoming infections, and he becomes riddled with gangrene.
He’s a complete mess, but he’s still after sex. “By the end of the mating season, physically disintegrating males may run around frantically searching for last mating opportunities,” says Diana Fisher from the University of Queensland. “By that time, females are, not surprisingly, avoiding them.”
Soon, it’s all over. A few weeks shy of his first birthday, he is dead, along with every other male antechinus in the area.
The technical term for this is semelparity, from the Latin words for “to beget once”. For semelparous animals, from salmon to mayflies, sex is a once-in-a-lifetime affair, and usually a fatal one. This practice is common among many animal groups, but rare among mammals. You only see it in the 12 species of antechinuses and a few close relatives, all of which are small, insect-eating marsupials. (Although they look like rodents and are colloquially called marsupial mice, antechinuses are more closely related to kangaroos and koalas than to mice or rats.)
Why? Why do these marsupials practice suicidal reproduction, and why are they the only mammals that do so?
The question has vexed biologists for three decades, and many have offered answers. Some say that females don’t survive very well after breeding, so males are forced to hedge their bets by mating with as many as possible. Other suggest that it’s just a feature of the group, which have become locked into a weird breeding system through some unknown quirk of their evolutionary history. Yet others think the males are being altruistic, sacrificing themselves to leave more resources for the next generation.
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