Giant Tortoises face off at meal time at the Charles Darwin Research Station. (Photo by Sue Cullumber)
The giant tortoises of Galapagos are among the most famous of the unique fauna of the Islands. While giant tortoises once thrived on most of the continents of the world, the Galapagos tortoises now represent one of the remaining two groups of giant tortoises in the entire world — the other group living on Aldabra Atoll in the Indian Ocean. The Galapagos Islands were named for their giant tortoises; the old Spanish word galapago meant saddle, a term early explorers used for the tortoises due to the shape of their shells.
The closest living relative of the Galapagos giant tortoise is the small Chaco tortoise from South America, although it is not a direct ancestor. Scientists believe the first tortoises arrived to Galapagos 2–3 million years ago by drifting 600 miles from the South American coast on vegetation rafts or on their own. They were already large animals before arriving in Galapagos. Colonizing the eastern-most islands of Española and San Cristóbal first, they then dispersed throughout the archipelago, eventually establishing at least 14 separate populations on ten of the largest Galapagos Islands.
Although there is a great amount of variation in size and shape among Galapagos tortoises, two main morphological forms exist — the domed carapace (similar to their ancestral form) and the saddle-backed carapace. The domed tortoises tend to be much larger in size and do not have the upward thrust to the front of their carapace; they live on the larger, higher islands with humid highlands where forage is generally abundant and easily available. Saddle-backed shells evolved on the arid islands in response to the lack of available food during drought. The front of the carapace angles upward, allowing the tortoise to extend its head higher to reach the higher vegetation, such as cactus pads.
Tortoise History in Galapagos
Of all of the native species of Galapagos, giant tortoises were the most devastated after the endemic rice rats, with the majority of rice rat species now extinct. One of the giant tortoise’s most amazing adaptations — its ability to survive without food or water for up to a year — was also the indirect cause of its demise. Once buccaneers, whalers and fur sealers discovered that they could have fresh meat for their long voyages by storing live giant tortoises in the holds of their ships, massive exploitation of the species began. Tortoises were also exploited for their oil, which was used to light the lamps of Quito. Two centuries of exploitation resulted in the loss of between 100,000 to 200,000 tortoises. Three species have been extinct for some time, and a fourth species lost its last member, Lonesome George, in June of 2012. It is estimated that 20,000–25,000 wild tortoises live on the islands today.
In addition to their direct exploitation by humans for both food and oil, giant tortoises faced other challenges due to the introduction of exotic species by human visitors. They suffered — and continue to suffer on some islands — predation on tortoise eggs and hatchlings by rats, pigs, and voracious ants, and competition for food and habitat with goats and other large mammals.
With the establishment of the Galapagos National Park and the Charles Darwin Foundation in 1959, a systematic review of the status of the tortoise populations began. Only 11 of the 14 originally named populations remained and most of these were endangered if not already on the brink of extinction. The only thing saving several of the populations was the longevity of tortoises, keeping some old adults alive until conservation efforts could save their species.
The taxonomy of giant tortoises has changed over the decades since they were first named. Today the different populations are considered separate species of the genus, Chelonoidis. There are currently 14 species, although recent genetic studies are beginning to show major differences among some populations on the same island. Giant tortoises were native to each of the big islands (Española, Fernandina, Floreana, Pinta, Pinzón, San Cristóbal, Santa Cruz, Santa Fe and Santiago) as well as the five major volcanoes on Isabela Island (Wolf, Darwin, Alcedo, Sierra Negra and Cerro Azul). Tortoises are now extinct on Fernandina (due to volcanism), Floreana, and Santa Fe (both due to exploitation). Pinta Island had a single known tortoise, Lonesome George, who lived up until June of 2012 at the Tortoise Center on Santa Cruz where he spent the last 40 years of his life. George’s remains are being preserved and will go on display permanently in Galapagos in 2014.
The Life of a Tortoise
Compared to humans, giant tortoises might deserve to be called “lazy,” spending an average of 16 hours a day resting. Their activity level is driven by ambient temperature and food availability. In the cool season, they are active at midday, sleeping in in the morning and hitting the sack early in the afternoon. In the hot season, their active period is early morning and late afternoon, while midday finds them resting and trying to keep cool under the shade of a bush or half-submerged in muddy wallows.
Galapagos tortoises are herbivorous, feeding primarily on cactus pads, grasses, and native fruit. They drink large quantities of water when available that they can store in their bladders for long periods of time.
Tortoises breed primarily during the hot season (January to May), though tortoises can be seen mating any month of the year. During the cool season (June to November), female tortoises migrate to nesting zones (generally in more arid areas) to lay their eggs. A female can lay from 1-4 nests over a nesting season (June to December). She digs the hole with her hind feet, then lets the eggs drop down into the nest, and finally covers it again with her hind feet. She never sees what she is doing. The number of eggs ranges from 2-7 for saddle-backed tortoises to sometimes more than 20-25 eggs for domed tortoises. The eggs incubate from 110 to 175 days (incubation periods depend on the month the nest was laid, with eggs laid early in the cool season requiring longer incubation periods than eggs laid at the end of the cool season when the majority of their incubation will occur at the start of the hot season). After hatching, the young hatchlings remain in the nest for a few weeks before emerging out a small hole adjacent to the nest cap.
Tortoises have several ways of communicating with each other. The only known vocalization in Galapagos tortoises is the sound that males make when mating — a bellowing, periodic “groan” that sounds similar to a loudly mooing cow. Female tortoises make no vocalizations at all. The main method of tortoise communication is behavioral. Like many other species, they have ways of conveying dominance and defending themselves. Competing tortoises will stand tall, face each other with mouths agape, and stretch their necks up as high as possible. The highest head nearly always “wins,” while the loser retreats submissively into the brush.
Fine Feathered Friends
It is quite common to see the tiny birds of Galapagos, such as Darwin’s finches and Vermillion Flycatchers, perched on top of the shells of their oversized giant tortoise companions. Some finch species have developed a mutualistic relationship with giant tortoises, feeding on the ticks that hide in the folds of the tortoise’s reptilian skin. In fact, these birds will dance around in front of the tortoise to indicate that they are ready to eat, and the tortoise then responds by standing tall and stretching out its neck to “expose the buffet.”