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Opossum Reproduction

In Part 1, we discussed the opossum’s ability to survive just about everything excluding the highway. And all that immune-to-venom, balk-at-botulism stuff was pretty wild, but it’s nothing compared to the world of marsupial sex. Crank up the BTO, ‘cus baby, you ain’t seen nothing yet.

When Opossums Rock It

Cute stuff first: A male opossum is called a Jack, a female is a Jill and babies of either sex are called Joeys, just like the kangaroo.

Wild stuff next: Jacks have bifurcated penises from hell. I feel like you might need a minute to let that sink in, so take your time. I’ll wait.

If you ever put a key into a doorknob or a plug into an outlet, you might imagine that Jill has a bifurcated vagina to match Jack’s penis. And you’d be right. She also has two ovaries, two oviducts, two uteri and two cervices. (And you just thought marsupials had a pouch.)

Opossum Sperm Pairing

When a Jack loves a Jill very much, they make beautiful, bifurcated love. Though they are prolific lovers, Jacks actually have incredibly low sperm counts – cows and sheep boast 500 million sperm per copulation, rabbits and rats between 150 and 50 million, whereas the lowly opossum makes do with less than 3 million spermatozoa per bifurcate-fest.

Luckily, Jack’s swimmers are remarkably effective, though they are few. Opossum sperm are fast, long-lived and – most interesting of all – cooperative.

Opossum sperm team up by sort of fusing their “heads” together, then beating their tails in unison. (“Reminiscent of watching a seagull in flight,” says our old buddy Krause.) Known as sperm pairing, this phenomenon is found only in American marsupials and affords Jack the boon of increased straight-line velocity. In other words, a boat with two paddles is more effective than a boat with one.

How effective? In the end, an average of 23 Joeys will be born, though some reports have noted more than 50.

The Magical Marsupial Birth Canal

You’re probably wondering why opossums haven’t taken over the earth if they’re capable of fertilizing 50 joeys at a pop. Well, the sad fact of marsupial reproduction is this – you can really only raise as many kids as you have nipples. But let’s back up a moment.

Opossum gestation is amazingly quick. In total, the little guys spend only 12 ½ days developing inside Jill’s double wombs. (Shortest gestation period of any mammal, woot woot!) But unlike antelopes, whales, humans and the rest of the placental mammals (or Eutherians), opossums give birth to tiny, only partially-formed monsters that will continue to develop in the safety of Jill’s pouch.

(Quick note: despite the distinction above, marsupials do indeed have placentas. They’re just short-lived and not as integral to the development of young. Just thought I’d save you the embarrassment of shooting off your mouth at a party about how marsupials don’t have placentas and getting pwned by a post-doc in front of all your friends. I’ve seen it happen.)

After less than a fortnight of pregnancy, the young opossums prepare for ejection. Unlike humans, Jill doesn’t hang onto a birth canal all year just for the hell of it. Instead, the marsupial birth canal (officially called the “median” or “pseudo-vaginal canal”) springs up out of nowhere like the Room of Requirement in Harry Potter. Essentially, the connective tissue down there just sort of splits around the young opossums, creating a shortcut from the inside to the outside. And once they’ve flown the coop, the median simply seals back up until the next pregnancy.

 

Pouch Potatoes

Little Joeys might not drop out of the womb ready to run like wildebeest, but they are equipped to make it to the pouch. In a word, claws. Since mother opossums don’t transport the babies from birth canal to nipple, it’s up to the Joeys – about the size of a honeybee, by the way – to find their way into the pouch by grabbing onto Jill’s hair and wriggling up in there. Krause describes the whole thing as an overhand stroke-like swimming motion that takes between 2-4 minutes.

Aside from temporary climbing claws – these daggers fall off once the Joeys reach a nipple and new, permanent ones start to form – baby opossums develop tiny, orienteering hairs in their inner ears just 24 hours before birth. The hairs allow them to sense gravity and crawl away from it. Jill simply sits on her haunches and lets the little guys cliff-hang their way up.

Once inside the pouch, Joeys grab onto a teat with their tiny mouths and hang on for dear life. This is quite literal, as a Joey without a teat is a dead Joey.

To prevent this, there’s some crazy nipple stuff we have to talk about. (Par for the course here at Bittel Me This.) When a Joey latches onto a nipple, its mouth opening starts to fuse together from both ends. At the same time, the tip of the mother’s teat starts to swell, creating a bulbous anchor that essentially locks the two together.

This intense suckle, called the fixation period, lasts around 60 days. (What would Freud have to say about that?) Krause on the importance of this uninterrupted connection, “If the young are removed (pulled off) from the teat during this period, the forming mouths are torn and the young are unable re-attach themselves back on the teats because the definitive lips have yet to form. The end result is that the young will die.”

It takes 10 weeks in the pouch before the Joeys start to resemble little opossums. At this point their eyes start to open and fur has formed. They are free to let go of the teat and vacation outside the pouch. In another week or so Jill might even get some time to herself, leaving the brood in a den while she goes hunting and gathering.

In total, it takes between 96-108 days after birth before they’re completely weaned. But in just another 3-5 months, these same opossums will be ready to rock it themselves – another reason opossums have mad staying power despite high death rates and short lifespans.

Opossum Awe

I don’t want to finish this post without a word on just how extraordinary the marsupial evolutionary strategy is and what a departure it marks from the rest of us mammals.

It’s not surprising that a gestation period of 12 ½ days yields half-baked cookies. What amazing, though, is the opossum’s ability to navigate from one warm, dark place to another with so few faculties at its disposal.

Krause notes that “approximately 91% of brain development occurs after the opossums’ migration and attachment to a teat.” While the little guys have a rudimentary sense of smell and gravity, “major body systems such as the digestive, urogenital, respiratory, and endocrine systems are only in the initial stages of development at the time of birth.” Similarly, Joeys are born “naked, blind, and deaf.”

And once they’re safely secured, Jill is a fully-functional walking nursery. Even after the babies are weaned, they can cling to her like a train in India. Throughout the whole ordeal, the ranging, nomadic lifestyle is upheld enabling the family to go wherever the food is.

Little wonder marsupials similar to the opossum existed alongside dinosaurs in the Jurassic. Think about that the next time you chase an opossum out of your garbage.

Warning: unrestricted cuteness

I’ve put you through a lot in this post: bifurcated penises, magical birth canals, claw-your-face-off pouch monsters. But all that nastiness produces some of the cutest little fur balls this side of Mammalia. Behold.

 

 

Opossum Opus: Part 3

In the final installment of the opossum saga, we’ll talk about the difference between “opossum” and “possum,” Pocahontas, and Alice Cooper. Just what you expected, right? 

Part 3: The Difference Between “Opossum” and “Possum”

Bittel Me Credits

Again, huge thanks to William J. and Winifred A. Krause and their e-book, “The Opossum: Its Amazing Story” – published by the Department of Pathology and Anatomical Sciences, School of Medicine, University of Missouri.

I also want to thank this amazing Tumblr for the Opossum Penis pic. And for all her other posts. Really cool stuff here, folks: arsanatomica.tumblr.com

Other image credits: 3 Lil Possums, TMNT Opossum and Sperm Pairing, Mama Opossum