Valid Life Choices

Stiff Competition:
why one small marsupial is the only mammal to mate itself to death

If you think the competition is stiff in your dating pool, try being an antechinus (anti-KINE-us), a genus of tiny marsupial that literally mates itself to death.

They do WHAT now?

Every year in the early spring, a fresh batch of these small shrew-like critters spend the most important weeks of their lives in a perpetual mating frenzy, going at it for up to 14 hours a pop until the males literally collapse of disease and exhaustion.  The females don’t fare much better – in many cases, they only survive long enough to get their babies eating solids before their bodies give out.

The males don’t die from combat – these little males are extreme lovers, not fighters. They outcompete each other by having large testicles (the better to make sperm with, my dear) and going literally all night – the longer the sex lasts, the less likely another guy is to have his shot.
As fun as “mating yourself to death” might sound, this is not a pleasant way to go. Their hair falls out, they start to bleed internally, and become gangrenous.

As fun as “mating yourself to death” might sound, this is not a pleasant way to go. They throw so much of their energy into mating that their bodies fall apart.   Their immune systems shut down and they become riddled with infection.  They metabolize their own muscles. Their hair falls out, they start to bleed internally, and become gangrenous.  Then they drop dead – hopefully after they’re all out of sperm and have mated every female in the neighborhood.

How does this even WORK?

Dusky Antechinus by Catching the Eye
antechinus can’t believe
he let it get this bad

photo of dusky antechinus from Catching the Eye (CC BY 2.0)

At some point in their evolution, these species became locked on a runaway train of extreme “sperm competition” – whichever males put the most energy into making and sowing their seed had the most offspring, even if it killed them.

So the whole lives of these little insectivores became geared towards baby-making.  Hardly any of them survive past their first year.  By six months, the males reach full maturity, and devote the next five months to bulking up – they need to gain as much weight as possible before the females become fertile.  Once that happens, they won’t have time to eat.  They’ll be entirely dependent on stored energy for weeks as they run from female to female, engaging in marathon sex.

In fact, mating season drives them to such energy-saving extremes, they stop producing sperm about a month before the females even become receptive.  All the sperm they will ever make in their lifetime gets stored in the plumbing while the testicles themselves atrophy.  This isn’t the best location for safekeeping – they immediately start losing sperm in their urine.  This puts them in dire straights – they have irreversibly committed to the next breeding season, and the clock is ticking.

Their eggs (so to speak) are all in one basket, and spoiling rapidly. No wonder they’re so frantic to mate until they drop.

Luckily for them, fathering little antechinuses is not a “him or him” proposition – each litter can have young from more than one dad.  It doesn’t matter which male gets to a particular female first, as long as he gets to her eventually.  This is because the female hasn’t even ovulated yet when mating begins – instead, she stores all the sperm from the season in her ovaries, then ovulates after the breeding season is done.  It’s a bit of bet-hedging on her part.  If she has offspring with many different dads, her own genes are more likely to be passed on to her grandkids.

Why is this a thing?

$10 Words
Semelparity: breeding only once in your lifetime (often with deadly results)
Iteroparity: breeding multiple times in your lifetime
“Have babies or die trying” isn’t really an unusual phenomenon in the animal kingdom. It’s a sound strategy when you can produce a lot (think: dozens, hundreds, even thousands) of offspring in one go, and the odds of surviving to see more than one breeding opportunity are slim.

It’s almost unheard of in mammals, though – in fact, the handful of antechinus species are the only mammals we’ve ever found that practice what we call “sexual suicide.”

There are a few reasons mammals don’t.  For one, they aren’t capable of the quantity of offspring you want if you’ve only got one shot.  Antechinus have 14 young at most, and some species as few as four. The other problem is that mammalian offspring need care after birth to survive to adulthood.

Dying really throws a wrench in that plan.

It’s only in the last decade that we’ve really nailed down how Antechinus wound up this way.  It’s tempting to think of it as paternal altruism – that the fathers die to leave more resources for their offspring.  It was the accepted explanation for decades.  There are two problems with that: as it turns out, food is not actually scarce enough for this to be necessary, and the idea that individuals sacrifice “for the good of the species” is has very little support as a scientific theory.

The likely explanation for antechinus’ “sexual suicide” is far less poetic, and therefore less satisfying to our imaginations. The latest research strongly suggests that it all comes down to the predictability and seasonality of their food.

As these insectivorous marsupials moved from more tropical habitats to more temperate ones, the availability of bugs became more seasonal and predictable – prey numbers spike in early summer for about a month, and the females who wean their young just in time for the all-you-can-eat buffet have more successful offspring.

Swamp Antechinus with Pouch Young by Michael Sale
Who wouldn’t die
for these jelly beans?

photo of swamp antechinus by Michael Sale (CC BY-NC 2.0)
These little critters can only expect two breeding seasons at best, and marsupial mothers nurse their offspring for much longer than the average mammal (several months, instead of weeks or days).  Not only do nursing and weaning take a lot of calories, it also means they can only have one litter during their one breeding season.

That made these mamas understandably picky about giving that single litter its best possible chance.  And the best chance would come from weaning during the biggest bug bananaza.

So they focused on getting their timing just right.  In order to wean at the right time, they had to give birth at the right time; to give birth at the right time, they had to conceive at the right time.  The mating season shrank to the point where most of the mating occurred over a matter of days, and if that meant the males had to mate until they dropped dead, well…everyone involved was only getting a single shot at this anyway.

Now that’s commitment to a goal.

Learn More

References


Fisher, D., Dickman, C., Jones, M. and Blomberg, S. (2013). Sperm competition drives the evolution of suicidal reproduction in mammals. PNAS, [online] 110(44), 17910-17914. Available at: http://www.pnas.org/content/110/44/17910

Taggart, D., Shimmin, G., McCloud, P. and Temple-Smith, P. (1999). Timing of Mating, Sperm Dynamics, and Ovulation in a Wild Population of Agile Antechinus (Marsupialia: Dasyuridae)1. Biology of Reproduction, [online] 60(2), pp.283-289. Available at: https://academic.oup.com/biolreprod/article/60/2/283/2740961

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