Galápagos Giant Tortoise
- CLASS: Reptilia (Reptiles)
- ORDER: Testudines
- FAMILY: Testudinidae
- GENUS: Chelonoidis
- SPECIES: 15
ABOUT
Galápagos tortoises are the giants of the tortoise world. Males can weigh more than 500 pounds (227 kilograms), and females average about 250 pounds (113 kilograms). They have thick, sturdy legs to hold up all that weight, but they still spend a lot of time lying down to conserve energy. There are two types of Galápagos tortoises: the largest, with big, round shells, called “domes,” and smaller kinds of tortoises with shells that curl up in front like a saddle and are called “saddlebacks.”
Although they are massive individuals, their shells are not solid. Instead, they are made up of honeycomb-like structures that enclose small air chambers. This makes it possible for a tortoise to carry the weight of their shell without much difficulty. The shell encompasses their ribs, so a tortoise cannot "walk out" of its shell like you may see in cartoons. Their lungs are located on the top of their body, under the top dome of their shell. Their shells also act as solar panels to generate heat and help regulate their metabolism. Galápagos tortoises, like all reptiles, are ectotherms—cold-blooded, dependent upon the ambient temperature and the sun for heat.
When threatened, a tortoise pulls itself into its shell with a hiss. The hissing sound is just the tortoise letting air out of its lungs. If a fight breaks out among males, tortoises face each other with ferocious glares, open their mouth, and stretch their head as high as they can. Whoever reaches the highest wins, even if he is much smaller overall than the other male! The loser pulls his head in with a noisy hiss, and the battle is over. To us, this looks like they’re just putting on a show; but it’s a serious matter to them, especially in the wild, where fights occur over mates and food.
HABITAT AND DIET
Naturalist Charles Darwin made his historic voyage on the HMS Beagle from 1831 to 1836. When the ship sailed around the Galápagos Islands, he and his shipmates marveled at the wildlife they found, from blue-footed boobies and marine iguanas to giant tortoises. One thing that caught his eye was that each of the 13 larger islands in the Galápagos Islands had a slightly different form of giant tortoise.
Each was uniquely suited for survival on that particular island’s environment. For example, domed shells were found on tortoises living in highlands with lush pastures. Saddleback shells gave tortoises on other islands more flexibility to reach sparse vegetation found higher off the ground, allowing them to stretch their neck to eat from bushes and cactus. On islands with sparse vegetation, tortoises have longer legs to climb a little and reach food growing up high; these tortoises tend to be smaller, possibly to help them travel through their harsher habitat.
Galápagos tortoises tend to lead peaceful, quiet lives that center around eating, relaxing in the sun, or wallowing in puddles. Because they are cold-blooded like other reptiles, they often soak in the sun to warm up. At night, they may rest partially submerged in mud, water, or brush to keep warm during cool evenings. Mud wallows can also keep a tortoise cool during the day.
There’s nothing like a nice, slow, ambling walk if you’re a tortoise. Their feet are turned inward, so each step moves a tortoise from one side to the other and never in a straight line. Tortoises rarely just stand up and walk, however; they prefer to put some weight on their front legs before pushing with their back feet and sliding forward.
Galápagos tortoises are herbivores that eat prickly pear cactus (a favorite) and cactus fruits, as well as flowers, leaves, and grasses. In fact, they can go without eating or drinking for up to a year, because they can store food and water so well. Tortoises don’t have teeth, so they use the bony outer edges of their mouth to bite off and mash food and then quickly swallow it. They don’t have a good sense of smell, so they use their eyes to find food. Anything red really gets their attention!
At the San Diego Zoo, Galápagos Giant tortoises eat Bermuda hay, cactus pads, and romaine lettuce.
FAMILY LIFE
When giant tortoises reach maturity, at about 20 to 25 years of age, they begin to prepare for reproduction. Their breeding season is generally between January and August. After mating, females may begin a journey of several miles to reach nesting areas. They look for dry, sandy ground and dig a hole about 12 inches (30 centimeters) deep. Here they lay hard-shelled eggs the size of tennis balls and cover them up with sand.
Temperature plays a role in whether a tortoise hatchling is male or female: if the nest temperature is low, more males hatch; if it is high, more females hatch. When young tortoises emerge from their shells, they must dig their way to the surface, which can take up to one month! It’s then up to them to survive on their own, as their mother does not stay to help them.
AT THE ZOO
The San Diego Zoo cares for one of the largest colonies of Galápagos tortoises in the world. We have had the honor of caring for giant tortoises since 1928, making them the oldest residents of the Zoo. That’s the year when Charles Townsend of the New York Zoological Society began his efforts to save giant tortoises from extinction by establishing conservation breeding colonies for them in North American zoos.
Many of these original tortoises are well over 100 years old today. The first Galápagos tortoise hatching at the San Diego Zoo occurred on October 21, 1958. Since then, we’ve hosted more than 90 successful hatchings at the Zoo.
CONSERVATION
Spanish explorers reached the Galápagos Islands in 1543. The wildlife on these islands had never experienced people, and there had been no large predators to hunt them. But people became a threat to tortoises. They hunted them for food for many years, and settlers also introduced new species to the islands—goats and cattle ate the plants tortoises needed, and pigs rooted for tortoise eggs and ate them.
When pirates, whaling ships, and merchantmen came through the Galápagos Islands between the 17th and 19th centuries, sailors loaded tortoises on their ships to use for food during the rest of their long voyage. Tortoise populations that once numbered in the tens of thousands were reduced to thousands, then hundreds, and even tens. What nature had taken millions of years to create, people had nearly destroyed in a few generations.
By 1959, when the Galápagos tortoise was in danger of becoming extinct, the Ecuadorian government stepped in and created the Galápagos National Park to protect tortoise habitat. Although visitors are allowed on the islands, these visits are strictly regulated. All groups must have a guide and stay on designated paths to protect wildlife.
In 1969, San Diego Zoo Wildlife Alliance became a partner with the Charles Darwin Research Station on the Galápagos Islands, funding a new tortoise-rearing facility and helping with propagation and reintroduction programs. Today, the greatest threats to tortoises come from introduced non-native species such as rats, dogs, and cats, which eat tortoise eggs and young tortoises. This is one reason why research station scientists offer hatchlings there a headstart by caring for them until they are big enough to survive on their own. They must still compete for food with non-native goats and cattle. At best, there are about 20,000 to 25,000 tortoises living today on the Galápagos Islands.
As Española giant tortoises are so rare (estimates were 12 females and 3 males in the 1970s), a male we cared for at the San Diego Zoo, named Diego, joined the Charles Darwin Research Station in 1976 as part of the conservation breeding program to help save his species from extinction. His contributions played a crucial role in revitalizing the population, becoming a father and then grandfather to perhaps one-third of the 3,000 or so Española tortoises alive today.
Fortunately, several species have recovered from near extinction and conservation efforts have generated hope for the future of giant tortoises. In 2012, renowned geneticist Oliver Ryder, Ph.D., was invited to an international workshop on the Galápagos Islands to plan the future of Galápagos tortoise conservation efforts. Dr. Ryder is the Kleberg Endowed Director of Conservation Genetics at San Diego Zoo Wildlife Alliance, as well as a creative problem solver with a deep capacity to understand the big conservation picture. The workshop utilized the expertise of people around the world in many different fields, including ecology, biology, horticulture, herpetology, physiology, genetics, and wildlife diseases, among others. Dr. Ryder was particularly pleased that the people who would be managing the areas and implementing the ideas, like park guards and local ecologists, were full participants.
Issues facing the delicate ecosystems of different islands, like invasive plant species (guava and blackberries), health and recruitment of wildlife populations, the elimination of nonnative species like goats and rats, and the containment of farm animals to protect native species, were addressed. “The San Diego Zoo has a long history of contributing to Galápagos tortoise conservation,” said Dr. Ryder. “This workshop was another great opportunity to provide thoughtful ideas and suggestions to the Galápagos authorities to guide conservation strategies into the future."
