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

Prof. 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.”

Robin 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.

Video link 1 at arkive.org: Footage of Aldabra giant tortoise swimming in ocean.

Figure 1. Aldabra giant tortoise with barnacle-covered legs (jeffmette.com).

Figure 2. South American twist-necked turtle foraging terrestrially (pingleton.com).