Giant tortoises are the epitome of Galapagos. Not only are they one of the best known and most charismatic species of the archipelago, they are also the reason the islands are so named, for galápago is an old Spanish word meaning tortoise. There are currently ten subspecies that live on the islands, but there were once at least 15. The only other extant (living) species of giant tortoise is Geochelone gigantea, the Aldabra giant tortoise native to the islands of the Aldabra Atoll, part of the Seychelles Archipelago in the Indian Ocean. Why, then, are the only giant tortoises on Earth found 9,500 miles away from each other on tiny volcanic islands?
It could be that the two species evolved into giants separately. In ecology, when two species independently evolve a similar characteristic it is termed convergent evolution. There are many examples in nature, an obvious one being the wing. Insects, mammals (bats) and birds have all evolved the ability to fly but have done so independently for they have not all evolved from a common flying ancestor.
It could be that they were both subject to a biological phenomenon known as island gigantism, whereby animals that reach isolated islands grow much bigger than their mainland relatives. The exact mechanism behind this occurrence is disputed but one possible cause is that, due to the absence of large carnivores on many islands, the constraints on size that some animals have due to predation is lessened. Numerous examples of island gigantism have existed around the world but many have since become extinct, often following human colonisation of the island. One example is the 1.5 m long giant iguana Lapitiguana that was found on Fiji until around 3,000 years ago, at which point Polynesian settlers reached the islands and probably caused their subsequent extinction.
Another reason, and one which is now widely acknowledged, is that the tortoises of Galapagos and Aldabra are simply the last remaining species of a race of giant tortoises that once roamed the Earth. Fossilised remains have shown that giant tortoises were once found in Australia, Madagascar, Malta, Fiji, the Mascarenes, India, Japan, Indonesia, Central America, southern USA and whole host of other countries. One publication states that at least 36 species of large (over 30 cm) and giant (over 1 m) tortoises have gone extinct in just the last 2.5 million years.
Some of these prehistoric giant tortoises were much larger than todays. Galapagos giant tortoises are the largest extant species, growing up to 1.8 m (6 ft) long and weighing up to 400 kg. The largest fossilised giant found to date belongs to Colossochelys atlas which once roamed throughout Asia and had an estimated length of 2.5 to 2.7 m (8.2 to 8.9 ft), height of 1.8 m (6 ft) and weight of 1,000 kg.
Whilst C.atlas will have had a close resemblance to modern day Galapagos giant tortoises, other extinct giants looked altogether more intimidating. Meiolania platyceps (pictured above), the second largest giant tortoise that has been discovered growing up to 2.5 m (8.2 ft) long, appears to have had a skull which sported two large horns and an armoured tail that had thorn-like protrusions coming off it. These menacing looking giants were found in Australia and the South Pacific and there is evidence that they still lived in the Vanuatu Archipelago only 3,000 years ago. Unfortunately, as with so many of the world’s megafaunal species, it looks like humans were the ultimate cause of their demise as fossil records of M.platyceps stop about 300 years after human settlement of the islands (for more details click here).
So it would seem that the Galapagos giant tortoise that we all know and love is actually the smaller, luckier and less sinister-looking cousin of some altogether more giant giants.
by Pete Haskell