Spanish sailors discovered the Galapagos archipelago -- a chain of islands -- in 1535. The Spanish sailors named the islands "galapago," the Spanish name for tortoise. When Charles Darwin arrived in the Galapagos Islands in 1835, there were 15 recorded species of giant tortoises. However, throughout the 17th, 18th and 19th centuries, merchantmen, pirates and whalers hunted them as food and a source of oil, depleting the population. Today, only 11 types of giant tortoises still survive.
Sizes of the giant tortoises vary between the islands. The larger species of male tortoises can reach weights of 400 to 500 pounds. These tortoises are large enough to carry a human on their backs. In his famous book, "Voyage of the Beagle", Charles Darwin notes that on several occasions he got on a tortoise's back and rode it as it walked. The giant tortoises also reach an average of five feet in length.
The giant tortoise has two different types of shells: dome-shaped and saddle-backed. Tortoises with dome-shaped shells are larger and inhabit islands where vegetation is more plentiful. The dome shape limits the tortoise's ability to forage for vegetation situated higher from the ground, so vegetation must be plentiful within the tortoise's reach. On islands with less vegetation, the tortoises have saddle-back shells which allow them to extend their necks higher to reach higher vegetation.
The giant tortoise feeds on grass, leaves and cactus. The saddle-back tortoises in particular are physically well-suited, and their shell shape allows them to extend their necks further to reach the cactus fruit. Giant tortoises have a slow metabolism and large internal stores of water, allowing them to survive up to a year without eating or drinking. The slow metabolism of the giant tortoise in part explains the unusually long average life span, averaging 100 to 150 years.
