Can Tortoises Swim? How Giant Tortoises Cross Oceans

Can tortoises swim?

Not in the way turtles do. Tortoises are the land-living members of the turtle order, and their bodies are built for walking. Their legs are thick columns covered with hard scales, the toes are not webbed, and many species have a high, domed shell that is heavy and not streamlined. In water a tortoise can only paddle its legs slowly and clumsily, and it has little control over where it goes.

Sea turtles and most freshwater turtles are very different. They have flatter shells, and either flippers or webbed feet with long claws, and many can hold their breath for long periods. For more on how the two groups differ, see turtle vs tortoise.

Do tortoises float or sink?

Many tortoises float, at least for a while. A tortoise’s large lungs fill much of the space inside its shell, and the air they hold makes the animal buoyant. Giant tortoises in particular tend to float with the shell at the surface and can stretch their long necks upward to breathe.

Floating is not the same as being safe. Smaller tortoises and those with heavy shells may float low or tip over, and a tortoise that falls into a pond, canal or pool with steep sides often cannot climb out. It can tire, take in water and drown. Tortoises do not have the special adaptations that let aquatic turtles stay underwater for long.

Why tortoises still like shallow water

Although they avoid deep water, many tortoises spend time in shallow pools. Giant tortoises on Aldabra and in the Galápagos lie in muddy wallows and rain pools for hours on hot days to cool down, and they drink deeply when fresh water is available.

Tortoises in dry places get most of their moisture from food, but they will still drink from puddles after rain. In all of these cases they stay where their feet can touch the bottom.

How giant tortoises crossed oceans

Giant tortoises live, or once lived, on many isolated islands: the Galápagos, Aldabra and the Seychelles, Madagascar, and the Mascarene Islands of Mauritius, Réunion and Rodrigues. Most of these islands were never connected to a continent, so the tortoises must have arrived over water.

Scientists think they floated there. Giant tortoises have several traits that make them good accidental voyagers:

• Buoyancy: they float with the head held above the water.
• Endurance: their slow metabolism lets them survive for weeks, even months, without food or fresh water.
• Stored reserves: they carry fat and water reserves in their bodies.
• A breeding head start: a female can store sperm after mating, so a single pregnant female washed ashore could start a new population.

Genetic studies support this picture. The ancestors of the Galápagos giant tortoise floated about 1,000 km (600 mi) from South America on ocean currents, and their closest living relative is the small Chaco tortoise of the mainland. The ancestors of the Aldabra giant tortoise most likely came from Madagascar.

A real case: the Aldabra tortoise found in Tanzania

For a long time, ocean drifting by tortoises was a reasonable idea without direct proof. Then, in December 2004, a live Aldabra giant tortoise was found on a beach at Kimbiji, south of Dar es Salaam on the coast of Tanzania. It was roughly 740 km (460 mi) from Aldabra, the nearest population of the species.

The tortoise’s shell was covered with barnacles, marine animals that only attach to objects drifting at sea. From their size, the researchers who studied the case estimated that the tortoise had been at sea for several weeks. It was thin but alive, having survived the journey without food or fresh water.

Justin Gerlach and colleagues described the case in the Journal of Natural History in 2006 as the first substantiated case of a tortoise crossing an ocean. It showed that a giant tortoise could ride the currents of the western Indian Ocean from Aldabra to the African mainland, the same kind of trip its ancestors once made in the other direction. More about the tortoise’s home atoll is in Where do Aldabra giant tortoises live?

Myths and facts about tortoises and water

Myth: tortoises cannot go in water at all. Tortoises often soak, drink and wallow in shallow water. What they cannot do is swim well in deep water.

Myth: a tortoise will always sink like a stone. Many tortoises float for a time, and giant tortoises float well. But floating is passive; a tortoise in deep water cannot steer, and it may still drown if it cannot reach shore.

Fact: a long voyage is rare. Successful crossings to new islands are thought to have happened only a handful of times over millions of years. Most tortoises swept out to sea would die, and only a lucky few would reach land, let alone land with a mate or as a female carrying eggs.