Galápagos Tortoise

Galápagos tortoises can go for long periods without eating or drinking, with sources ranging from months to up to a year without food. They manage by metabolizing stored fat and by resting about 16 hours a day, mostly in sun or shade, with long periods of inactivity during harsh conditions on arid islands. They are the primary herbivores of the islands and feed selectively on grasses, leaves, flowers, shrubs, fruits, and prickly pear cactus (Opuntia). They also disperse seeds, carrying them 100 m to more than 4 km (330 ft to 2.5 mi) from the parent plant. Walking speed is about 0.26 km/h (0.16 mph). Hatchlings weigh only 70 to 90 g (2.5 to 3.2 oz), while the largest adult males reach about 250 to 300 kg (550 to 660 lb).

Galápagos tortoises have two main shell shapes, with some intermediate forms. Domed tortoises live in cooler regions, while saddleback tortoises live in dry, coastal areas. The saddleback’s shell flares upward at the front, which lets it reach tall cacti. Saddlebacks also perform longer neck extensions, possibly because competition is stronger on arid islands. Saddlebacks lay 2 to 7 eggs, while domed tortoises lay about 20 to 25 or more. Body length reaches up to 1.2 to 1.8 m (4 to 6 ft), and males can be two to three times larger than females. Raising the head also helps tortoises watch for approaching threats. Galápagos finches hop in front of them to prompt them to rise up on their legs and extend their necks, so the birds can remove ticks from the skin folds.

Nest temperature decides the sex of Galápagos tortoise hatchlings. Embryos incubated above a pivotal temperature during a critical window develop as females, while cooler embryos develop as males. On Santa Cruz, nests at low and middle elevations were much warmer, and about 90 percent of juveniles there were female, while about 80 percent of juveniles from the high nesting zone were male. A rise of roughly 50 to 100 m (164 to 328 ft) in elevation lowered nest temperatures by about 1.6 °C (2.9 °F). Females dig nests for several hours, and incubation takes about 80 to 175 days, longer in cool months. Nesting runs June to December, hatching December to April. Researchers warn that predicted warming of 1 to 4 °C (1.8 to 7.2 °F) could change nesting conditions and sex ratios.

Some Galápagos tortoises migrate along elevation gradients, following food. On Santa Cruz, GPS loggers on 17 adult tortoises over two years showed that only adults migrate, and larger individuals are more likely to. Younger tortoises stay in the lowlands year-round. Dominant males can travel up to 10 km (6 mi) into highlands about 400 m (1,300 ft) above sea level, where dry-season fog keeps vegetation growing. Sources differ on timing: one says tortoises move uphill in July to October early in the wet season, and another says around June at the start of the dry season. Repeated use creates trails from highlands to lowlands and to water sources. In a four-species study, three species averaged 45 to 100 m (148 to 328 ft) per day, up to 3,000 m (9,843 ft) per month.

Galápagos tortoises are active by day, rest about 16 hours, and are mostly solitary, but they gather around food, shade, and water. They wade communally in water or muddy pools for hours, which helps regulate temperature and reduces parasites and mosquito bites. They also take dust baths, possibly to cool down and control parasites. In warm hours they extend the head and limbs to shed heat, often in open areas, and in cool hours they withdraw them to retain heat, often near hillsides, vegetation, or other tortoises. One study reported nighttime groups, possibly for warmth. Finches and vermilion flycatchers remove ticks and seeds from their skin folds, and albatrosses use open areas created by tortoises as runways for takeoff and landing. Cold tortoises walk slowly in the morning, supported by three legs.

Male Galápagos tortoises establish rank partly through ritual neck stretching. Males stretch their necks and compare how high they reach, and a local saying holds that he who can stretch his neck highest wins. Males are usually dominant over females, and male-male disputes are more common than male-female ones. The ritual signals rank without physical contact, but not every dispute stays peaceful: agonistic behavior also includes chasing, pushing, mouth gaping, and biting, usually to the head and rarely causing wounds. Submissive tortoises lower the head, close the mouth, withdraw into the shell, or move away. Few vocalizations are known. Hissing is used in defense, and males grunt and bellow during mating. Some also bob their heads while relaxing, for reasons that are unknown. Dominance hierarchies are poorly studied, especially in the wild.