structural:scholar_edition
Gigantism

The discoverer of the Galápagos Islands, Fray Tomás de Berlanga, Bishop of Panama, wrote in 1535 of "such big tortoises that each could carry a man on top of himself." Naturalist Charles Darwin remarked after his trip three centuries later in 1835, "These animals grow to an immense size... several so large that it required six or eight men to lift them from the ground". The largest recorded individuals have reached weights of over 400 kg (880 lb) and lengths of 1.87 meters (6.1 ft). Size overlap is extensive with the Aldabra giant tortoise, however taken as a subspecies, the Galápagos tortoise seems to average slightly larger, with weights in excess of 185 kg (408 lb) being slightly more commonplace. Weights in the larger bodied subspecies range from 272 to 317 kg (600 to 699 lb) in mature males and from 136 to 181 kg (300 to 399 lb) in adult females. However, the size is variable across the islands and subspecies; those from Pinzón Island are relatively small with a maximum known weight of 76 kg (168 lb) and carapace length of approximately 61 cm (24 in) compared to 75 to 150 cm (30 to 59 in) range in tortoises from Santa Cruz Island. The tortoises' gigantism was probably a trait useful on continents that was fortuitously helpful for successful colonisation of these remote oceanic islands rather than an example of evolved insular gigantism. Large tortoises would have a greater chance of surviving the journey over water from the mainland as they can hold their heads a greater height above the water level and have a smaller surface area/volume ratio, which reduces osmotic water loss. Their significant water and fat reserves would allow the tortoises to survive long ocean crossings without food or fresh water, and to endure the drought-prone climate of the islands. A larger size allowed them to better tolerate extremes of temperature due to gigantothermy. Fossil giant tortoises from mainland South America have been described that support this hypothesis of gigantism that pre-existed the colonization of islands.