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The world used to be full of giant tortoises, and the ones that live on the Seychelles and Galápagos islands are all we have left. But how did these big reptiles get to the islands in the first place? And why is turtle anatomy so weird?

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Host: Hank Green (he/him)
Thumbnail photo: Dr. Dennis Hansen
There are a lot of things about  turtles that seem confusing to us.

Their shells probably are the most  obviously weird thing about them,   but the rest of their  anatomy is also very strange. Like, the way they breathe isn’t normal.

You might have heard that some of them  can breathe through their butts,   but the more immediate problem for me is that  their shell is made from their modified rib cage,   which usually has to expand and  contract for an animal to breathe. So, that seems like a problem. And when you dive deeper into  a particular group of turtles,   namely the giant tortoises,  things get even more bizarre. [ ♪♪ INTRO ♪♪ ] There are two different species  of giant tortoises alive today,   both of which include some number of subspecies, the number varies depending on who you ask.

The two species are the Galápagos  giant tortoise, which is found,   as you might expect, on the islands of the  Galápagos archipelago, off the coast of Ecuador. And the Aldabra giant tortoise,  which lives on some of the islands   of the Seychelles in the western Indian  Ocean. So nowhere near the Galápagos.

Males of both species can grow to be around 1.5 meters long and weigh as much as 250 to 260 kilograms, with the Galápagos tortoise  being the larger of the two, at least in the wild. And they’re all generally herbivorous,  snacking on the various grasses,   herbs, fruits, and flowers that  grow on their native islands. With the occasional meaty exception, like when researchers in the Seychelles filmed  an Aldabra tortoise eating a baby bird.

They can also live to be over 100 years old,   with one Aldabra tortoise named Jonathan  celebrating his 190th birthday in 2022. But, despite their similarities, these two kinds of tortoise are not each other’s closest living evolutionary relatives, which is confusing. The Galápagos giant tortoises are more closely related to smaller tortoises from the mainland of South America, 

while the Aldabra giant tortoises are more closely related  to smaller tortoises from Madagascar.

Which means that these two groups of  tortoises became giants independently! There’s even an evolutionary rule that seems to explain how these two species could do this.    It’s called  Foster’s rule or the island rule. Basically, it says that, on islands,   bigger animals tend to get smaller and  smaller animals tend to get bigger.

It’s the same effect that gave us pygmy mammoths on the Channel Islands  and  Komodo dragons in Indonesia. And giant tortoises seemed like the  poster children for the island rule,   especially after we learned that  their closest relatives aren’t giants. Here’s the thing, though: when  you’re studying evolution,   if you only look at the animals that are  alive today, you miss a lot of the picture.

Because, while the two species of  giant tortoises are the only ones around right now, they were not alone in the past. There used to be giant tortoises on every  continent, except for Australia and Antarctica. And it’s possible that they were already  giants, or, at least, large, by the time they got to the Seychelles and the Galápagos,  which they did by drifting on ocean currents.

They might not seem like it, but giant  tortoises are actually pretty buoyant   and they’re capable of floating  with their heads above water. They’ve even been seen swimming in the ocean. They can also survive without food  and water for weeks or months,   because they have large deposits  of fat and slow metabolisms.

So, rather than being evidence for  the island rule,    it looks like today’s giant tortoises are just the last survivors of a once-widespread distribution of giant tortoises. But this is not where the weirdness stops,   evolution has done strange  things to their anatomy, too. For example, in most vertebrates, the  shoulder-blades sit on the outside of the rib cage,  either on the back, like ours, or  on the sides, like in many quadrupedal animals.

But not the giant tortoises and the other turtles. Their shoulder blades have found  their way inside of their rib cage,   which is what forms their shells. See, turtles aren’t just skeletons  wearing their shells like an awkward coat.

Their shells are literally made of parts of their  skeleton, including their ribs and vertebrae. And that means they cannot breathe  the way other vertebrates do,   by expanding and contracting their rib cage. Instead, they have a unique set-up of  abdominal muscles that pumps their lungs.

They basically exhale by having one muscle  squish their organs against their lungs   to push out the air and inhale by using a  second muscle to pull that first muscle back. And while some species of turtles can  also kind of breathe a second way,   giant tortoises are not among the species  that can breathe through their butts. This is a thing more technically known as  ‘cloacal respiration’ which is something some freshwater turtle species can do to absorb oxygen when they’re stuck in places like frozen ponds.

Taking the long, evolutionary view  of giant tortoises   highlights both the strange things they share in common  with the rest of the turtles and also the unique things that make  them seem even weirder to us. From being the last survivors of an age of giants instead of the poster children for the island rule,  to being occasional predators of birds  and possible oceanic rafters, giant tortoises feel like rule-breakers. But when it really comes down to it, we  humans are the ones making the rules,   so why should we expect any bizarre beast  to know them, let alone follow them?

The Bizarre Beasts pin club subscription window  is open from now through the end of January 16th! This month, that gets you this amazing  giant tortoise pin, look at him, I love him! He's very happy!

And also Bizarre Beasts calendars are now 50% off! You can pick one up at So you can get your 2023 calendar.

It's got thirteen months of it, so you can carry it over into next January And, as always, profits from the pin  club and all of our merch go to support   our community’s efforts to decrease  maternal mortality in Sierra Leone. [ ♪♪ OUTRO ♪♪ ]