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Bio-bullets

A continent mysteriously empty of tortoises

Testudines failed to colonise dry land on the world’s largest island because of its combination of salt and fire.

professor caricatureProf. Mumblebard claims: “Australia is the only vegetated continent lacking land tortoises. Terrestrial testudines didn’t reach Australia because this continent is too isolated. Freshwater turtles have lived here since Gondwana times but couldn’t evolve into terrestrial forms owing to genetic constraints.” 

logoRobin and the Honey Badger respond: "Land tortoises and semi-terrestrial turtles are extremely effective at dispersing hundreds of kilometres across the sea. This was recently highlighted by the discovery of a healthy Aldabra giant tortoise on the East African shore – with barnacles on its legs and lower shell – that had drifted for at least 740 kilometres in the Indian Ocean. In view of the buoyancy of all genera of giant tortoises living on islands in both the Indian and the Pacific oceans, it’s reasonable to expect that the giant tortoise of Timor (the closest Indonesian island) would have reached Australia repeatedly by flotation. The strait separating Australia from this nearest landmass of Asia was only one-seventh of its current width during the driest times of the Pleistocene. Also native to Timor is a wood turtle, preadapted to foraging on land as well as in freshwater, which would have been capable of repeatedly floating to Australia. As for genetic constraints: the freshwater turtles which have always lived in Australia would need little modification to forage on dry land. Indeed, a species in the same family spends much of its time foraging out of water in the tropical forests of South America. The real reason for the lack of land tortoises in Australia is a combination of salinity and fire that occurs on no other continent or island. Tortoises survive wildfires mainly by aestivating underground, but aestivation is impracticable in saline environments. This is because aestivation involves desiccation and consequently a risk of excessive sodium in the blood."

Notice to readers: A more detailed account of why tortoises did not colonise Australia can be found in our blog-post, "Could the Australian tortoise trump the hare?"

 

Please join us here at the Bio-edge with your own comments. In the discussion below we encourage links to any evidence supporting either Prof. Mumblebard or Robin and the Honey Badger. Illustrations are welcome but please cite all sources or we may be forced under copyright to delete your comment.

 

A continent mysteriously empty of tortoises PART 1

A continent mysteriously empty of tortoises PART 2

Last modified on 16 July 2015

Comments   

0 #1 Zoophile 2014-02-10 14:12
The Aldabra giant tortoise, now called Aldabrachelys gigantea, doesn’t just float, it swims. See how well it uses its legs like flippers here:

arkive.org/.../video-06b.html
+1 #2 Biodiver 2014-02-10 17:10
Far out!! Check out these barnacles on the Aldabra giant tortoise’s legs:

0 #3 Taklamakan 2014-02-11 14:12
Here are some pics of the South American twist-necked turtle foraging terrestrially. It wouldn’t be much of an evolutionary leap for this turtle to turn into something you’d call a land tortoise…

pingleton.com/.../...
0 #4 Johns 2014-02-14 12:03
It’s a fair question why there are no land tortoises in Australia. Has nobody asked this question before? I did a google scholar but couldn’t find any literature on the subject.
0 #5 Johns 2014-02-18 10:26
This map really shows how close Timor was to the expanded Australian mainland at lowest sea levels in the Pleistocene. The darker grey land is continental shelf exposed at lowest sea levels. The distance from Timor across the Weber Line is only ca 80 km.


attributed to Maximilian Dörrbecker
0 #6 H. Farley 2014-02-18 10:35
It’s a bit of an irony that giant tortoises are famous for having got to remote islands like the Galapagos. Meanwhile Australia is the giant island but there’s nothing in Australia like a Galapagos tortoise.
0 #7 Taklamakan 2014-02-18 10:38
Wallace’s Line is supposed to separate the Asian biogeographic realm from the Australasian realm. Who knows how meaningful such a simplistic-look ing line is. Anyway there’s no denying that land tortoises crossed far beyond Wallaces Line into Australasia.


attributed to Alberto Salguero
0 #8 Gargantuan 2014-02-18 10:40
I’ve just discovered that there’s another giant turtle on the doorstep of Australia, and this one’s still alive! Who knew about the giant softshell turtle Pelochelys? Like Colossochelys, its range goes all the way from India to an island right next to Australia, in this case not Timor but southern New Guinea. In fact it must have reached Australia once because there used to be land connecting New Guinea with Australia. It’s funny that these two giant tortoises/turtl es, Colossochelys and Pelochelys, got so close to Australia but then didn’t go the ninth yard and colonise Australia too.
0 #9 Singleton 2014-02-19 07:45
i dont really see what the fuss is about. Lots of animals didnt reach Australia and tortoises are just another example. Whats the big mystery here?
0 #10 Johns 2014-02-19 07:48
Sorry if I’m getting a bit repetitive but I thought this map really shows how much vaster the continental shelf of Australia is than it seems at today’s higher sea levels. Look how narrow the dark blue line of deep sea is between Australia and Timor. Australia really doesn’t look very isolated when seen as a continental shelf. So I do think it’s odd that many Asian animals didn’t reach Australia or New Guinea.

0 #11 Gargantuan 2014-02-19 07:51
Wallace’s Line and the other boundaries didn’t keep freshwater turtles out of Australia because they belong to the Chelidae, a Gondwanan family not an Asian family. Some of the freshwater turtles of Australia have been there since the time of the dinosaurs.
0 #12 Gargantuan 2014-02-19 07:53
Colossochelys was wiped out as a whole genus at the end of the Pleistocene or thenabouts, but it was some tortoise! en.wikipedia.org/.../... Just thinking, this biggest-ever extinct land tortoise actually lived on Timor (the Asian island closest to Australia). Actually Colossochelys used to live all the way from India across Indonesia to Timor with a different species or subspecies on each of the islands. So the genus had this huge range and somehow it got to all those islands that are supposed to be more and more isolated the further you go from the Asian mainland. Why didn’t it make it the last step to Australia? Anyone got another theory?
0 #13 Gargantuan 2014-02-19 07:55
There were also big tortoise-like reptiles right in Australia in the Pleistocene, called meiolaniids. See en.wikipedia.org/.../Meiolania . Do you, ‘Robin and the Honey Badger’, know about them? Seems like they disappeared like the rest of the Australian megafauna that Tim Flannery wrote about in The Future Eaters.
0 #14 Biodiver 2014-02-20 07:55
Hey, I had no idea there are so many turtles/tortois es/terrapins in Australia and New Guinea! 32 species at last count!! Amazing, and that’s not counting the marine turtles. But they’re all aquatic. Not one land tortoise. Weird.
0 #15 Naturesprite 2014-02-20 08:06
I’m confused about where Asia ends and Australia begins, the whole Wallace Line thing. This map shows Wallace’s Line as if it’s a simple straight line drawn with a ruler.

(attributed to Mestska)

But other maps like

(attributed to Maximilian Dörrbecker)

and

(attributed to Altaileopard)

show quite a few lines separating Asia from Australia. Which one are we talking about for tortoises and freshwater turtles?
0 #16 Herpsrule 2014-02-20 08:12
It’s true that no turtle forages on land in Australia or New Guinea but that doesn’t mean that no species is adapted to drought in Australia. Chelodina (Chelodina) steindachneri lives in drainage lines but for much of the time (up to a year at a time) the drainage lines are dry because that part of Australia is semi-arid. Chelodina (Chelodina) longicollis can bury itself in forest and then remain in a torpid state underground for up to 480 days. It can also walk overland for up to 8 km and 40 days.
0 #17 Chimaera 2014-02-20 08:18
Something that a lot of people may not know about the Galapagos tortoises is that they reached the Galapagos by crossing not one but two oceans. First the ancestors of the South American tortoises crossed the still-narrow Atlantic Ocean to get to South America. Then once the South American tortoises had evolved into the genus Chelonoidis, they crossed part of the Pacific to get to the Galapagos. There were no land bridges involved. If you don’t believe me, read it for yourself in Le_etal_2006
iucn-tftsg.org/.../...
0 #18 Anaconda 2014-02-20 08:24
Heres some fun reading on chelonians:
environment.gov.au/.../...
0 #19 Gargantuan 2014-02-21 12:59
@Chimaera. Sea barriers don’t seem to be a problem for tortoises, but land barriers are. There are no tortoises in South America west of the Andes, at least in Chile. So they could cross the Atlantic and part of the Pacific but Chile is like too remote an ‘island’ as far as tortoises are concerned. Sometimes I think animal distributions are just a matter of luck ;)
+1 #20 Anaconda 2014-02-21 13:00
if tortoises are sensitive to the salinity level in soil, then how could they “float” across salty sea water over such vast distances and survive?!
0 #21 Taklamakan 2015-01-05 10:26
More evidence that the Timor straits were easy enough for reptiles to cross. The spotted tree monitor Varanus timorensis lives only on Timor and nearby islands, but belongs to an otherwise exclusively Australian and New Guinean subgenus, namely Odatria. So it seems that the ancestor of V. timorensis crossed from Australia (where a particularly closely related species, Varanus scalaris, is also arboreal) to Timor. If a monitor lizard, specialised for climbing rather than swimming, could reach Timor from Australia in the Pleistocene, surely land tortoises could make the same crossing in the opposite direction.

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