Release The Brood! Roly Poly Birthing Is Intense
Roly polies aren’t insects — they’re crustaceans, and they have more in common with kangaroos than you might think.
These land-dwelling isopods carry their eggs in a fluid-filled pouch on their underside, giving their young a tiny ocean to develop in until they’re ready to face the world.
In this episode of Deep Look, we take a close look at how roly poly moms protect their eggs, nurture their babies, and pull off one of nature’s most surprising examples of convergent evolution.
TRANSCRIPT
What does this roly-poly have in common with a kangaroo?
Here’s a hint …
It’s all about her parenting style.
Unlike mammals, most insects take a hands-off approach.
They find a good spot, lay their eggs and move on.
Good luck, little one!
But roly-polies, also called pill bugs, or doodle bugs, aren’t like most insects.
Actually, they aren’t insects at all.
They’re crustaceans, like shrimp and lobsters.
Roly-polies belong to a group of crustaceans called isopods, which originated in the sea.
About half of isopod species still live there.
But roly-polies’ ancestors ventured out onto dry land.
And they brought with them a maternal adaptation from their aquatic past.
After mating, a female roly-poly transfers her eggs into a fluid-filled pouch on her underside called a marsupium.
At first the marsupium just looks yellow.
But it doesn’t take long – just a few days – ‘til you can spot the eggs – about 40–60 of them!
Keeping them close is a good strategy.
If she were to lay her eggs on land, they’d dry out.
They don’t have the protective outer barrier of an insect egg to keep moisture in.
But inside their mom’s marsupium, the eggs have their own tiny ocean.
Roly-polies are clad in overlapping armored plates called pereonites.
If she gets spooked she can curl up in a perfect little ball to shield herself and her babies.
The marsupium is protected by more plates called oostegites.
But they’re thinner … almost clear, like a window into her nursery.
After 3 to 4 weeks, the babies, now called mancae, emerge from the eggs.
They look like miniature yellow versions of their mom.
But they aren’t ready to face the world yet.
They keep developing inside their mama’s pouch for another week or so.
As they mature, they start getting restless.
Sixty squirming babies in one crowded pouch.
A feeling only a mother could love.
The mancae eventually wriggle out from between the plates covering their mom’s pouch.
Then, after about a month of being little pouch potatoes, they’re finally out on their own.
They’ll go on to do the good work of detritivores, eating rotting wood and leaves … recycling the nutrients back into the soil.
And one day, the females will get to host their own pouch parties.
Marsupials and isopods tell a story of convergent evolution: two wildly different animals who come up with a similar solution.
But unlike a kangaroo, a roly-poly mom doesn’t invite her babies back in for a ride.
She gave them a nice, cozy upbringing.
Now it’s time for them to venture out and let the good times roll.
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Now, check out the six-rayed sea star, another very overprotective mom.
