Galápagos tortoises with an explosive past carry the mark of a volcanic eruption in their genes.

Today, the biggest threat to the Galápagos tortoises that live on Alcedo Volcano is the herds of invasive goats that are stripping the slopes of their greenery. But 100,000 years ago, the tortoises faced something far more dramatic - a massive volcanic eruption that left its imprint in their DNA.

Alcedo is one of five major volcanoes on the island of Isabela, situated along the western edge of the Galápagos archipelago. Each of the volcanic peaks hosts its own population of giant tortoises, but Alcedo boasts the largest group, with between 3,000 and 5,000 individuals. Given these numbers, biologists from Yale University were surprised to discover that tortoises living on Alcedo's slopes have much lower genetic variation than those living on the four neighboring volcanoes.

The team has calculated that the genetic diversity that does exist among Alcedo's tortoises must have evolved from a single common ancestor that lived about the same time as the volcano's last major eruption - 100,000 years ago. The big event would have showered several feet of hot pumice over 3.4 square kilometers, wiping out most of the tortoise population. However, at least one pregnant female must have survived, giving birth to the descendants of today's tortoises. Although Alcedo endangered its reptilian inhabitants with the eruption, the volcano has also helped to protect its tortoises over time, since its great height kept whalers and buccaneers from exploiting the vulnerable animals.

Reptilian ancestors of the Galápagos tortoise probably floated to the islands on clumps of vegetation after rivers flooded on the mainland, dispersing the clumps into the sea. Photo: HonoluluZoo.org
Two giant tortoises graze on the slopes of Alcedo 100,000 years after the volcano's last eruption.
Photo: Jeff Waugh, Discover Galápagos
The endangered Galápagos tortoises (Geochelone elephantopus) can grow to over four feet in length and weight up to 500 lbs. Photo: H. Vannoy Davis, California Academy of Sciences.
Map by Colleen Sudekum