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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.

But Fisher, who has been studying antechinuses for decades, favours a different idea. 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.

It’s their diet that matters. These animals feed on insects, and some experience a glut of food once a year but very little at other times. This seasonality increases the further you get from the equator. The species with the most seasonal menus also had shorter breeding seasons, and their males were more likely to die after mating.

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 were better at raising their young if they gave birth just before the annual bonanza, and were well-fed enough to wean their joeys. Their mating seasons shortened and synchronised, collapsing into a tight window of time.

That probably wouldn’t have happened if they were placental mammals like shrews or mice, which could have produced several litters during the peak of food. But they were marsupials: their babies are born at an incredible early stage and rely on their mothers’ milk for a long time. A baby shrew suckles for days or weeks; a baby antechinus does so for four months. The females could only fit in one litter during the annual peak.

This had a huge impact on the males, which were forced to compete intensely with each other in a matter of weeks.  They didn’t fight. Rather than using claws or teeth, they competed with sperm. The more they had, the more females they impregnated, and the more likely they were to displace the sperm of earlier suitors. Indeed, Fisher found a clear relationship between suicidal reproduction and testes size. The biggest testes of all, relative to body size, belong to species whose males die en masse, followed by those where a minority survive to mate again, and then by those with several breeding seasons.

The males that put the greatest efforts into sperm competition fathered the most young. It didn’t matter if they burned themselves out in the process, if they metabolised their own muscles to fuel their marathon bouts. These animals are short-lived anyway, so putting all their energy into one frenzied, fatal mating season was the best strategy for them. Living fast and dying young was adaptive.

This idea was first proposed in 1979 but Fisher’s data, although mostly correlative, provides fresh support for it. She certainly finds it more plausible than the idea that the males are selflessly sacrificing themselves for the next generation. After all, the males usually live outside the females’ home ranges, so are unlikely to compete with their own young for resources.

“Antechinus mating habits have appeared in many documentaries, and the explanation of males selflessly sacrifing themselves to increase food supply for young is the one given in all the ones I have seen,” says Fisher. “I hope that documentaries and textbooks now start to give an evidence-based explanation of sexual selection.”

Reference: Fisher, Dickman, Jones & Blomberg. 2013. Sperm competition drives the evolution of suicidal

reproduction in mammals. PNAS http://dx.doi.org/10.1073/pnas.1310691110

31 thoughts on “Why A Little Mammal Has So Much Sex That It Disintegrates

  1. Can’t believe I’ve not heard of this before. You continue to be a treasure trove of information from where I pick up nuggets of information to drop into the next holiday family dinner and stop the conversation dead. Not that that is my primary motivation for reading your work—I appreciate the science behind all this as well, but I certainly liked seguing from carving the turkey to talking about explosive duck penises to the horror of a couple of my more repressed in-laws.

    [Providing conversation-stopping conversation topics is my raison d’etre – Ed]

  2. When I saw the statement, “It’s their diet that matters,”
    I expected to read about nutrition, not a lifespan (and the according reproductive cycle) that coincides with the annual cycle of feast and famine.
    The title leads to a discussion about a mammal’s fatal sexual encounters, and the article wavers between observations of the marsupial mice and the male or female of the species.
    This article is about the male’s fatal sexual encounters.
    The (“short lived”) females obviously live through gestating and borning and suckling and weaning. Do they survive for subsequent sexual seasons? Or are the female’s sexual encounters also ‘fatal,’ only slower and more complex?
    The article mentions that by the end of the mating season, “females are, not surprisingly, avoiding [the physically disintegrating males].” So do the females desire ‘suicidal’ sex or do they get overpowered by the virile males, and only escape the dying males? Do any females attempt to avoid sex from the get go? Do they all?
    It’s quite a conundrum, to consider the marsupial mice mating season along with immediate (male) or eventual (female) death, and then to determine the procreative activities as fatal (death is inescapable for male or female) or suicidal (the male death is immediate and therefore intentional).

    Now I’ll have to go look for the article about explosive duck penises.
    For reading while my next roast duck is in the oven.

  3. My all time fave little Aussie. I remember first being introduced to Antechinus by my wildlife lecturer, Peter Brown, at Deakin Univeristy. Peter had an absolutely classic photo of a totally shagged out male slumped over the back of a female who was still going about her business while he determinedly clung on for more action. Had the lecture theatre in fits of laughter as the blokes squirmed uncomfortably when they learned this incessant randiness was terminal hee hee.

  4. Semelparity. I wish I’d known there was a word for it when, as a teenager, I was so often accused of doing it.


  5. One of the more interesting aspects of marsupial behavior that is so often overlooked is their propensity to defy the understood tendencies of placental mammals. The egg laying and marsupial mammals may look like us, or mice in this case, but the ‘wiring’ differences between us are as gaping as those between bats and birds.

  6. So is it a string of different males or the same jaded female that persists in living behind my frig? I thought we had a relationship but now I’m feeling used and grubby.

  7. The scientists took three decades to find the reason for male Antechinus dying for sex, yet they are on assumptions. We are living in scientific world were science is reading minds through machines. why don’t they read their mind through such? why don’t the the doctors read their minds or the science is fake with their inventions of machines? and the whole mind reading, lying detecting stories are a lie?

  8. There are a lot of people who think that the human race should be that way! What this article proves without saying it is that nature is diverse in its evolutionary methods. Sometimes when we think about our children we conclude that even though we will one day kick the bucket, through our children we will live on, and if they remain prolific down the line we will be around for a couple of thousand years. Perhaps this species has the same idea along with the idea that “the more the merrier”

  9. This article was a fascinating read. The title alone grabbed me. I mean, whose curiosity would not be peaked by the concept of “suicidal reproduction.” The male antechinus species were having so much sex to the point of death. Female antechinus had a very hard time surviving the breeding of the young, so was it possible that their male counterparts sacrificed themselves to secure the future of the next generation? There were many arguments presented in this article as to why this form of mating came into practice. While reading this, I couldn’t help but compare the male antechinus to our species. I guess the male antechinus took the biblical verse be fruitful and multiply to a whole new level!

  10. I suppose there is some humor in this but I was wondering what would happen if you isolated a male antechinus but fed him the same diet, would he die all the same or would his lifespan be expanded?

  11. I think, if you were to isolate him, he’d die of thirst/starvation pining away for it… or just go bug-nuts -er- literally heheh

  12. Karen, at the beginning of the post, it says, “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.” Is it possible there may be a correlation and maybe more than a singular explanation?

    Don’t burn your duck and fuque it up. 😉

  13. What happens if these satyromaniac antechinuses (antechini?) are kept away from the females at the time or, say, for the first yr. Do they continue to live? Then do they continue to produce sperm in time for the next breeding season?

  14. Why “a month ago, he irreversibly stopped making sperm, so he’s got all that he will ever have”? And the title of the reference study killed me! “Fisher, Dickman, Jones & Blomberg. 2013. Sperm competition drives the evolution of suicidal reproduction in mammals.” Dickman is one of the researchers! 😉

  15. What this poor animal goes through afterwards is awful. Basically just falls apart little at a time. What a way to die!

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