The Galapagos giant tortoise is one of the most famous animals of the Islands; with the Archipelago itself being named after them (Galapágo is an old Spanish word for tortoise). Scientists believe the first tortoises arrived in Galapagos 2–3 million years ago by drifting 1,000 kilometres from the South American coast on vegetation rafts, where they underwent diversification into at least 14 species, differing in their morphology (particularly their shell shape) and distribution. Their population is currently estimated at 20,000 individuals.
Although there is a significant amount of variation in size and shape among Galapagos tortoises, two main morphological forms exist — the domed shell (similar to their ancestral form) and the saddle-backed shell. The domed tortoises tend to be much larger and do not have the upward thrust to the front of their shell – they live on the larger, higher islands with humid highlands where forage is abundant and easily available. Saddle-backed shells evolved on the arid islands in response to the lack of available food during a 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.
The Galapagos giant tortoise spends an average of 16 hours per day resting. The rest of their time is spent eating grasses, fruits and cactus pads. They can survive for up to a year without water or food. Breeding primarily occurs during the hot season (January to May), although mating may be seen at any time of year and lasting anywhere up to 2 hours. After mating, the female migrates to a nesting area, where she digs a hole with her back feet into which she lays 2 to 16 eggs, each the size of a tennis ball. The sun incubates the eggs, with young tortoises hatching after around 130 days.
In the 1800s Galapagos giant tortoise populations suffered drastic declines due to exploitation by whalers, buccaneers and fur seal hunters. An estimated 100,000 – 200,000 tortoises provided a source of fresh meat and oil, as they could be kept alive on a ship for several months without any food or water. Human introduced species has had further devastating effects on tortoise populations. Rats, pigs and ants predate upon tortoise eggs; feral dogs attack adult tortoises; cattle and horses trample nests, and goats compete with tortoises for food.
Following the establishment of the Galapagos National Park, giant tortoise eggs are now frequently collected from the wild and incubated at the Tortoise Breeding Centres. Keeping the hatchlings in captivity allows them to grow big enough to defend attacks from rats and dogs once they are released. The Galapagos Tortoise Movement Ecology Programme (GTMEP), co-funded by the Galapagos Conservation Trust (GCT), is helping to improve our understanding of tortoise migration so that more efficient management plans can be implemented for this iconic animal.
By Samuel Punnett