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Caecilians are legless amphibians. Some of them are immune to cobra venom and in a certain sense, some of them eat their mothers from the inside-out and some eat them from the outside-in.

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Because you watch Bizarre Beasts,  Brilliant is offering you a  30 day free trial and 20% off an annual premium subscription  at There are three things you  need to know about caecilians. Not people from Sicily, I’m  talking about the animals.

One: caecilians are not worms or snakes or eels.  They are amphibians, like frogs and salamanders.

Two: many caecilians are immune  to the venom of certain snakes,   but they are not all immune in the same way. And, three: in a certain sense, some  of them eat their mothers from the inside-out and some eat them from the outside-in. [♪♪ INTRO ♪♪] If you want to support this channel,  and get an amazing pin every month, the Bizarre Beasts pin club will now be  open for subscriptions for the whole month! Sign up by March 20th and the first pin you  will get will be one of these weird little guys… Caecilians live in the tropical and subtropical  parts of the Americas, Africa, and Asia,   as well as on some islands, including Sri  Lanka, and the Philippines, and the Seychelles. There are around 180 species and  most of them live underground,   though one family found in  South America is aquatic.

They tend to be carnivorous  and they are not very picky… The terrestrial species eat things  like worms, insects, snails, and small vertebrates, while the aquatic  ones eat swimming insects, fish, and eels. They range in size from about  1.5 meters long   to around 10 centimeters long. And they are the group of amphibians  you are most likely to forget about,   even if you knew they existed in the first place.

Which, it’s okay if you didn’t,  that’s why we make the show. The thing that makes them pretty obviously different from other amphibians  is that they all have no legs. All other amphibians fall somewhere  on the have-legs spectrum, whether or not the legs are  useful, that’s another story.

Many frogs and toads basically  have super-legs, or, at least,   four limbs that they actually use to get around. Amphiumas, a family of aquatic salamanders,  have four hilariously tiny, mostly useless legs. And sirens, a different family of aquatic  salamanders, have just two tiny front legs.

But all caecilians have no legs and, like  amphiumas and sirens with their vestigial legs,   it’s probably because they adapted to wiggle through both water and dirt. It’s pretty common for both swimming and burrowing animals to reduce or lose their limbs over evolutionary time,   especially if they also become increasingly tube-shaped. And, for the burrowing caecilians,  this has brought them into conflict   with another classic legless, tube-shaped animal: snakes.

Caecilians seem like they would  be easy prey for the terrestrial   members of the family of snakes known as elapids. This family includes coral  snakes, mambas, and cobras,   and its members primarily have venom  that interferes with the nervous system. But, surprisingly, caecilians have a few tricks  up their sleeves to deal with these predators.

Now, caecilians are old. DNA estimates put the origin of their lineage  back something like 370 to 270 million years ago,   and the oldest fossil we’ve found of  them dates back 220 million years. They’ve been squirming around, spreading from  continent to continent for a very long time.

Elapids, on the other hand, are relatively young. The family only originated something  like 38 million years ago, but they have dispersed throughout the tropics and  subtropics of most continents since then. And when they first showed up in  places where caecilians lived,   it was probably a very bad time  to be one of these amphibians.

Elapids would have been an intense  selective pressure on caecilians. But some of them, obviously, survived. And it probably came down to a  certain amount of genetic luck, some individuals must have had natural mutations  that gave them some level of resistance to elapid venom.

Those survivors would have been  better able to reproduce and pass   those genes along to their offspring  than caecilians without those mutations. And we know the selective pressure was  intense because resistance to elapid venom evolved convergently at least 15 different times in caecilians. It also evolved in three different ways and some caecilian species have a combination of resistance mechanisms.

The mutations either block the neurotoxin  from reaching their nervous system receptors altogether,   change the shape of the  receptors so the toxins can’t attach to them,   or repel the toxins by switching the  electromagnetic charge of the receptors. Not bad for a terminally uncharismatic amphibian. If you want to know more about  this evolutionary arms race or   about elapid venom, head over to our  sister channel PBS Eons next week.

Or right now, it depends on when you watch this. Unfortunately for caecilians,   snakes aren’t the only thing trying to  eat them: their own babies are, too, at least, in some species. And yes, this is going to get kinda gross.

They either, number one: lay eggs that hatch  into aquatic larvae, like many other amphibians; two: lay eggs that hatch into  miniature versions of adult caecilians,  which is called direct development; or three: they give birth to live young. And caecilians are surprisingly good mothers. They do things like guarding  their nests, for example.

Some of them also take care  of their newly hatched babies. In some of the species with direct development,  the mother even feeds the babies her own skin. And look, I don’t mean that she  sheds it and then they eat it.

I mean, they literally tear and peel it off  of her body with their baby caecilian teeth. This behavior actually has a name,  it’s called maternal dermatophagy   and the mother’s skin cells prepare for  it by becoming especially rich in lipids,   which have a lot of calories for their weight. But that’s not the only wild maternal  feeding behavior found in caecilians.

In the species that give birth, the mothers  feed their offspring before they are born. Which doesn’t sound like that big of a deal,   because like all mothers have to provide  nutrition to their developing young, but caecilians have taken  it to a very weird place. In the mother’s oviduct, the tube where  embryonic development takes place,   the offspring start out by hanging out  in their egg membranes, feeding on yolk.

But when the yolk is gone, the fetal caecilians  leave the egg membrane in search of other food... which they get by scraping the walls of their  mother’s oviduct with their specialized baby teeth    and consuming the lipid-rich secretions  and tissue lining the tube for nutrients. Sounds, you know, unpleasant! But I don’t know, I’ve  never had that happen to me!

Caecilians, for all that they have  been around for something like 300 million years,  are actually pretty poorly  understood compared to other amphibians. But now you know at least three things about  them, even if you might wish you didn’t. And that’s the beauty of Bizarre

Beasts: sometimes the things that make animals weird to us are awesome   and sometimes they are  gross and sometimes they are both. Don’t forget, sign up for the pin club to celebrate these amazing animals by March 20th if you want a caecilian pin! You can do that at Thank you!

You know caecilians are amphibians,  like salamanders and frogs. You might also know that many  amphibians are poisonous, they secrete toxins to keep  predators from biting them.  And until 2020, we did not think  that any amphibians were venomous, possessing a toxic bite of their own. But it turns out that caecilians  might actually be venomous!

They have glands in their mouths  that produce saliva containing enzymes found in the venoms of  snakes, scorpions, and wasps. But, rather than injecting the  venom through fangs, like snakes do,   their delivery system is basically ‘coat  the teeth in saliva, and then bite.’ The researchers who published this finding  originally are still following up on their results,    because the enzymes they  found aren’t exclusive to venom,   but it’s pretty cool that there might  be a venomous amphibian out there! Also, in case you were wondering, the name  ‘caecilian’ means ‘blind one’ and it’s pretty fitting,   as they tend to have eyes that are  either tiny or completely covered by skin.

Instead of seeing, they pick up sensory  data from their environment using tiny tentacles located between their nostrils and eyes. The origin and spread of weird adaptations, like caecilians convergently  evolving resistance to venom, is governed by mathematical probability. And with Brilliant’s new Introduction  to Probability course,    you can build a foundation in probability to better  understand the likelihood of events,   and learn how to answer real data  questions using probability and simulation.

And that’s just one of the many courses  you can explore with Brilliant,   the online learning platform with thousands of interactive  lessons in science, computer science, and math. Brilliant lets you learn anywhere at any time  and if you aren’t sure what course to take,   Brilliant has a quiz you can take when you sign   up to be matched with content that  fits your skill level and interest. You can try it for free for 30 days  at or by clicking the link in the description down below.

And that link also gives you 20% off an  annual premium Brilliant subscription. Thanks to Brilliant for supporting  this episode of Bizarre Beasts! [♪♪ Outro ♪♪]