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Lizards are reptiles with four legs...usually. Reptiles without legs are snakes...a lot of the time. And amphisbaenians mostly don't have legs, except when they do...but they're definitely not snakes.

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Welcome to our first episode  of season 3 of Bizarre Beasts!

You know we always say that  profits from the pin club   and all our merch go to decreasing  maternal mortality in Sierra Leone, so we thought that it would be a good  idea to give you an update    on some of the things that you have supported so far: In April 2021, ground was broken on  the Maternal Center of Excellence,   and it’s on track to offer services in 2024, almost tripling the number of  maternal hospital beds in the region. And with all of your support, that project has  expanded in scope to include a dedicated   training center for healthcare workers.

If you want to help us out  and get some great pins, we will be keeping the pin club  open for an extra week this month.  We’ll be giving you all those  details at the end of the episode. There are millions of unique  species crawling, walking,   flying, and swimming around on our planet, most of which have yet to be formally  described by western science. And while every species may be unique,  not every species is original.

A lot of animals look alike  or live in very similar ways. One example of that: carcinization,   the repeated evolution of crab-like shapes  in crustaceans that are not really crabs. Another example is the scaled  reptiles, called squamates,   shrinking or losing their  legs over and over again.

Now, snakes might be the most obvious case, but there are also legless lizards, slowworms,  some skinks, and today’s bizarre beast, which may be the weirdest version of this. Meet the Mexican mole lizard: a pink, wrinkly,   burrowing creature that is not  a mole or a snake or a lizard, but a truly unique, if not entirely  original, combination of traits all its own. [ ♪ Intro ♪ ] The Mexican mole lizard is one of  around 200 species of a thing you probably have never heard of: amphisbaenians, a group of scaled, typically-legless  reptiles that are neither lizards nor snakes. Or, at least, we don’t think  of them as lizards or snakes.

Squamates are, taxonomically,  strangely complicated. You’ve got what we usually think of  as “lizards,” reptiles with four legs, things like geckos, western  fence lizards, chameleons. And you got snakes, which don’t have legs and seem  to be something totally different from lizards.

Like, snakes don’t have real  eyelids, while lizards generally do. And then you’ve got amphisbaenians, which  also don’t have legs, except for when they do. But, here's the weird thing:   when you put these three groups on an  evolutionary tree based on their DNA, you have groups of lizards,  like geckos and skinks,   that split off from a lizard ancestor  before snakes and amphisbaenians did.

And then you have groups of lizards, like iguanas,   that split off after snakes  and amphisbaenians did. So, from that perspective,   both snakes and amphisbaenians are just different  kinds of lizards that have lost their legs, But, because each group came  from its own common ancestor and the members of each group are more closely  related to each other than anything else, we call them “snakes” and “amphisbaenians.” Having separate names for them is  useful and we’re practical like that. Now, some research suggests that  amphisbaenians got their start in North America soon after the dinosaur-ending  Cretaceous–Paleogene extinction event.

And it looks like the ancestors of the Mexican mole lizard might’ve stuck around that continent, because today you can find  Mexican mole lizards buried   under the shrub land and desert  of the Baja peninsula of Mexico. It's that burrowing lifestyle that may explain why   the Mexican mole lizard is one of only four  amphisbaenians to have any legs at all, as well as one of very few in the group  to retain the skeletal scaffolding,    called a pectoral girdle, needed  for functional front legs. Its front legs are strong, with long,   shovel-shaped claws at the end  for digging, like a mini-mole, but Mexican mole lizards don't have back  legs for getting around like moles do.

Instead, underground, Mexican mole lizards  do what’s called ‘concertina locomotion,’ alternatively bending and straightening their  body to inch their way through the sand. Snakes do this too, usually in  tunnels or when climbing up trees. By bending part of their body  into a very tight s-shape, they anchor themselves in place  and avoid slipping backwards   as the other part of their  body slowly inches forward.

But for snakes, this takes a lot more  energy than other modes of locomotion,   like lateral undulation, which  is the classic snake slither. Mexican mole lizards are much more efficient  movers with concertina locomotion than snakes, and the reason seems to be those front legs. As the mole lizard pushes its head forward,   it can use its front legs to shovel and sweep dirt  out of the way, and to help stabilize its body, both of which mean more of its energy  can be used for forward propulsion.

And concertina locomotion isn’t the only  way that Mexican mole lizards get around. Their scales are arranged  in rings around their body, allowing them to crawl forward with the  accordion-like motion that earthworms use. Also kinda like earthworms,   Mexican mole lizards can lose parts  of their body and just keep on going.

They’ve been known to drop their own tail,   in classic lizard fashion,  possibly to distract predators. Unlike many lizards, though, this is a  one-time-trick, and the lost tail will not grow back. But what might be the coolest  thing about the Mexican mole lizard   is how it hears, without any external ears.

Which, by the way, is one of the features that  usually distinguishes snakes from lizards. Snakes don’t have holes in their head for  hearing, whereas lizards, even legless ones, do. Now, hearing is when an animal  senses vibrations caused by   alternating waves of low and high air pressure that are interpreted as sounds by the brain.

In reptiles with external ears, those  vibrations are picked up by the eardrum, which transmits them through the middle ear  bones, like the columella and extracolumella, to the cochlea, the part of the ear  where the sound-sensing hair cells live. But because amphisbaenians  don’t have external ears,   they’ve adapted to do things a little differently, think of it as evolution happening  to the parts that they do have. Most amphisbaenians have an unusually large  columella and an elongated extracolumella, but, unlike even other members of this group, the Mexican mole lizard doesn’t seem to rely on  the extracolumella to transmit sounds much at all.

Instead, it uses an inward fold of skin that  freely vibrates when exposed to sound waves, and transmits those sounds  directly to the cochlea. It’s kinda like an alternative eardrum. The “skin hearing” system is so effective that the   Mexican mole lizard has some of the most  sensitive hearing of any amphisbaenian, but it’s only about as sensitive  as that of the average lizard.

Not bad for not having external ears, though! With its pink, earthworm-y appearance,   lizard-like ability to drop its tail,  and mole-esque stubby front arms, the Mexican mole lizard is definitely unique. And while its individual adaptations  may seem somewhat familiar, seeing them all on a single animal is what  makes this amphisbaenian one Bizarre Beast.

The Bizarre Beasts pin club subscription  window will be open for an extra week,   from now through the end of July 10th. When you sign up, you’ll suddenly  have an excuse to tell your friends   all about amphisbaenians when you get  your very own in the middle of the month, and the pins after that around  the time each new video goes live. Check out this squiggly little guy!

He's really good! For more fun facts about the Mexican  mole lizard and other bizarre beasts,   please like and subscribe here, and follow us on Twitter @BizarreBeasts, and  on Instagram and Facebook @BizarreBeastsShow.   [ ♪   Outro ♪ ]