GIANT TORTOISES OF THE GALÁPAGOS

Live Galapagos Tortoise on exhibit

© AMNH / Denis Finnin

In the autumn of 1835, after four years at sea on the HMS Beagle, Charles Darwin arrived in the Galápagos‹an isolated chain of volcanic islands off the west coast of South America. The different forms of enormous tortoises that inhabited these islands intrigued Darwin and ultimately offered him clues for his theory of evolution by natural selection. Today, thanks in part to Darwin¹s work, these gentle giants enjoy celebrity status as symbols of the Galápagos Islands and their incredible diversity of life.

The Islands of the Tortoises

The Galápagos Islands are famed for the giant tortoises that live there; even the name Galápagos comes from a Spanish word describing the saddle shape of some tortoise shells. The islands were once home to more than 200,000 tortoises comprising at least a dozen separate populations. Most biologists consider these groups to be subspecies of Geochelone nigra. Today, about 10,000 giant tortoises live on the islands. A few subspecies are extinct and others are endangered.

Dome-Shelled

Galápagos tortoises have shells of two basic forms. Dome-shelled tortoises live on the larger, wetter islands, where they can graze on plentiful low-lying shrubs and grasses.

Saddle-backed

Saddle-backed tortoises tend to live on the drier islands. Their arched shells allow them to lift their heads up high to feed on cacti and other succulent plants that thrive in arid areas.

A Future for the Galápagos Tortoise?

Galápagos tortoises were once protected by hundreds of kilometers of open ocean. But after ships reached the islands in the 1700s, sailors started capturing these slow-moving animals for food and oil, often littering the shore with their shells. Outsiders also introduced foreign and invasive species to the islands, many of which harm tortoises: donkeys and cattle trample nests, pigs and dogs eat eggs, rats and fire ants prey on hatchlings, and goats compete with tortoises for food and destroy vegetation.

But help has arrived. Since the 1960s, the Charles Darwin Foundation has worked to protect and increase existing wild tortoise populations. The Tortoise Rearing Program hatches both wild and captive tortoise eggs and raises the hatchlings until they are big enough to fend for themselves‹a process called ³head-starting.² More than 2,500 tortoises have been successfully released into the wild.

Tortoises Back In Time

Modern DNA studies have shed light on the origin and ancestry of the Galápagos tortoise. Between six and twelve million years ago, a group of tortoises split into two populations, one of which would eventually evolve into the Galápagos tortoises and the other into the small Chaco tortoises of South America. Then, between two and three million years ago, a few members of the ancestral Galápagos population‹or at least one pregnant female‹floated from the mainland to the newly formed islands of Española and San Cristobal, from which they would later populate the other five major islands.