The truth about giant tortoises

Reputation: Giant tortoises live on islands. They can be aged by studying growth rings on their shells, which is how we know they are the longest lived vertebrate on record. Charles Darwin found they moved a whole lot faster than he'd imagined.

Reality: Giant tortoises are a recent evolutionary innovation and used to be everywhere, not just on islands. It's impossible to age them accurately unless you know when they hatched. They are actually pretty slow. Darwin was probably chasing them.

The largest tortoises in the world are to be found on the Seychelles in the Indian Ocean and in the Galápagos in the Pacific. But this truth has given rise to the false belief – often found in textbooks – that their large size is a product of island life. It almost certainly isn't.

This much is evident from a cursory inspection of the fossil record. A typical Galápagos tortoise has a carapace around 100 cm long. If this is the benchmark for "giant", it is clear that giant tortoises were not restricted to small islands. They were everywhere.

In the southern USA and Central America, for instance, there was a monster of a tortoise known as the southeastern giant tortoise (Hesperotestudo crassiscutata) that only went extinct around 12,000 years ago.

In Queensland, Australia there used to be a beast that goes by the name of Owen's giant horned ninja turtle (Ninjemys oweni). Before you ask, yes, it was named after the ninja turtles of teenage mutant fame.

Then there was the real mother of all giants, the Siwaliks giant tortoise (Megalochelys atlas), which was tramping around what is now the Punjab in India until a few million years ago. It was around twice the size of a Galápagos tortoise.

In addition, a 1999 genetic analysis of Galápagos tortoises suggests their ancestors were probably pretty large before they left mainland South America several million years ago.

In fact, it's been argued that being big was a necessary pre-adaption for the successful colonization of remote oceanic islands. If it finds itself in the water, a giant tortoise will bob along tolerably well, its long neck ensuring it doesn't take on too much water.

If there used to be so many giant tortoises, where are they all now? A recent study by the International Union for Conservation of Nature indicates that giant tortoises have suffered a far higher incidence of extinction than more modestly sized tortoises and turtles.

"These slow moving and non-threatening animals required minimal effort to find," wrote Anders Rhodin, director of the Chelonian Research Foundation and his colleagues.

Worse, giant tortoises can survive without food or water for long periods. That meant they could be stored – alive – to provide fresh meat many months down the line.

"Tortoises were, essentially, the earliest pre-industrial version of 'canned food'," they suggest. Early hominins opened their shells with stone-tool "can-openers".

It is only because humans came late to the Seychelles and the Galápagos that we can still marvel at these creatures today.

In these isolated spots, giant tortoises have become reptilian representatives, their extreme longevity casting them as a living link to a lost world. But how long do they live, really?

It is often said that you can age a tortoise by counting growth rings on its shell. Unfortunately, this is only reliable for the first year or two and is useless for aging an animal that is fully grown – which for giant tortoises is at the age of around 20.

The only reliable method of aging a tortoise is to record its year of birth. In a few instances where this has been done for giant tortoises, it is clear they can live for 150 years or more

Giant tortoises are not known for their speed. When in the Galápagos in 1835, Charles Darwin found that they moved faster than he'd imagined.

"One large one, I found by pacing, walked at the rate of 60 yards in 10 minutes, or 360 in the hour," he wrote in hisZoology Notes. "At this pace, the animal would go four miles [6.4 km] in the day & have a short time to rest."

More recently, researchers have been using tracking devices to record movements of Galápagos tortoises in more detail. It turns out they are not nearly so lively, most of the time making small movements around a relatively small patch.

"Our tortoises don't usually move more than absolute max of 2 km per day," says Stephen Blake, coordinator of theGalápagos Tortoise Movement Ecology Programme. "Darwin was probably chasing them."

They might be slow, but they are also probably smarter than most people imagine.

Research on the South American red-footed tortoise (a not-too-distant relative of the giant tortoises in the Galápagos) shows they use landmarks to create cognitive maps of their surroundings. They can also follow the gaze of another tortoise and learn from the behaviour of others. It seems likely that giant tortoises are capable of similar cognitive feats.

/By BBC/