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 ribcage,
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.
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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 ribcage,
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 ribcage,
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 ribcage.
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 ComplexlyCalendars.com.
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.
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