Kangaroos have three vaginas

By Ed Yong | April 17, 2012 11:20 am

We interrupt your regularly scheduled news programming to bring you this wonderful piece of trivia about kangaroo genitals.

Regular readers will know of my love for Inside Nature’s Giants, the British documentary where anatomists cut up large animals to examine how their bodies work and evolved. It’s a truly incredible show, combining unbridled joy at the natural world, drama, and solid educational value.

So far, it has brought us the horrifying throat of a leatherback turtle, the mysterious bloodsweat of a hippo, and the exploding insides of a beached whale. But this week’s episode may have topped all of that with the triple vaginas of the female kangaroo. The diagram above (an annotated screengrab from the show) explains the complicated plumbing.

This set-up is shared by all marsupials – the group of mammals that raise their young in pouches. Koalas, wombats and Tasmanian devils all share the three-vagina structure. The side ones carry sperm to the two uteruses (and males marsupials often have two-pronged penises), while the middle vagina sends the joey down to the outside world.

Note that the ureters, which carry urine from the kidneys to the bladder, pass through the gaps between the three tubes. In placental mammals, like us, the ureters develop in a different way, and don’t go through the reproductive system. As we develop, the precursors to the reproductive tubes eventually fuse into a single vagina. In marsupials, this can’t happen.

The programme also suggested that this might explain why marsupial embryos are born at such a premature stage of development. A kangaroo’s joey is about the size of a jellybean when it leaves the vagina, and it must endure an arduous crawl into the pouch. It’s possible that with such a narrow tube to go down, it couldn’t get any bigger before its birth.

With its complicated reproductive set-up, a female kangaroo can be perpetually pregnant. While one joey is developing inside the pouch, another embryo is held in reserve in a uterus, waiting for its sibling to grow up and leave. Indeed, a mother kangaroo can nourish three separate youngsters at a time – an older joey that has left the pouch, a young one developing inside it, and an embryo still waiting to be born.

Comments (11)

  1. I can’t believe I have lived 32 years and only now learned this. Thank you, Ed, for sharing.

    Females, as the ones who most typically do internal gestation (at least, of the animals with internal gestation) hit up against gestation length as something that limits their total number of offspring. What is so cool about this system is that, as you’ve said, the female can have offspring at three different stages of development. Since these guys are all small, the energy it takes to do this is probably not too high, either, until the joey grows.

    Now I need to look up more about kangaroo mating systems. I wonder if multiple offspring in these situations would all be with the same dad, or different dads…

    /geekery

  2. Ooh yeah, lemme know what you find. There’s also some cool stuff about the mums being able to produce different types of milk to control the development of the different youngsters, but I didn’t have time to go through the literature on that today.

  3. Old Geezer

    Perpetually pregnant? Now if she could be made barefoot in the kitchen she could have been Rick Santorum’s running mate.

  4. Mindy

    I imagine this as the title of a children’s counting book, which would be THE AWESOMEST COUNTING BOOK EVER.

  5. Rel

    They can also pause their pregnancies if environmental conditions are less than optimal eg. droughts.

  6. Well, okay, I guess that’s pretty interesting.

    But you big science writers have consistently ignored the story of the monkey with four asses. Until you address the science behind that, I’m pretty much nonplussed. However, I’ll give you some points for the two-pronged penis.

    In all seriousness, though, this is truly fascinating. I’d really love to know more about the homeobox developmental program at work here. I recall PZ Myers speaking of HOXA13 as being essential to mammalian vagina development. I wonder if that’s the transcription factor which is lacking or fails to operate in the case of marsupials.

  7. That was the thing I had to run around telling EVERYONE about today. I can’t wait until I can finally see the episode of the show!!!

  8. More and more, males are getting the short end of the stick (so to speak) in these days of growing female ascendancy, and everybody here probably already knows this (he sighs soulfully), but male possums have forked penises.

    Yup. Possum ladyparts have two vaginas with one external opening, so lucky male possums get to have a more-or-less triad *every single time.*

    Almost as kinky, all the New World marsupials – like possums – have paired sperm that swim the female reproductive tract joined at the hip until the instant before fertilization. Seems kinda gay, but so far at least, nobody’s campaigning against it.

  9. metamanda

    This makes me feel like being a placental mammal is kind of a crappy deal.

  10. mfumbesi

    I learned something new Today. As I always do when I visit your blog. I also love that series, unfortunately for us (in the lowly 3rd world), there are no legal means to view it yet. Don’t ask me how I watch it.

  11. This isn’t true… the photograph is labeled wrong! Check out.. http://www.youtube.com/watch?v=D5Ag65dEJpY and @ 4:46 it describes the organs.

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