Stanford University Home

Stanford News Archive

Stanford Report, January 12, 2000

Endangered tortoises face new threat on Galapagos Islands

When Charles Darwin arrived in the Galapagos Islands 165 years ago, he was astounded by the sight of thousands of giant tortoises, some with shells 7 feet in circumference. Darwin wrote:

As I was walking along I met two large tortoises, each of which must have weighed at least two hundred pounds: One was eating a piece of cactus, and as I approached, it stared at me and slowly walked away; the other gave a deep hiss, and drew in its head. . . . I frequently got on their backs but I found it very difficult to keep my balance.

At first, Darwin, like most human visitors, considered tortoises little more than food. But he was intrigued when the Spanish vice-governor of the Galapagos claimed he could identify which tortoise came from which island by the unique shape of its shell.

When Darwin returned to England, he realized that the vice-governor was correct: Nearly every major island in the Galapagos archipelago had its own variety of tortoise that had evolved over time. Thus, the giant tortoise became a cornerstone of Darwin's theory of evolution through natural selection.

Since Darwin's visit, the Galapagos giant tortoise (Geochelone elephantpus) has had a difficult time surviving in the wild. Hundreds of thousands of tortoises were slaughtered for food in the 1800s. Today the population is around 15,000, down from an estimated 250,000 in Darwin's day.

Three of the 14 subspecies of giant tortoise that originally inhabited the islands are extinct. Remarkably, Isabela Island alone boasts five distinct subspecies living on five separate volcanoes: Wolf, Darwin, Alcedo, Sierra Negra and Cerro Azul.

Volcanic eruptions have been part of the natural order in the Galapagos for millions of years, but biologists decided to evacuate tortoises during the 1995 Cerro Azul eruption because the volcano's unique tortoise population had become dangerously low.

In addition to its enormous size, the Galapagos giant tortoise also enjoys extraordinary longevity. One captive specimen is believed to be more than 165 years old, which means that it was alive when Charles Darwin journeyed to the Galapagos as a young man.

-- Mark Shwartz SR