Reptiles: Galápagos Tortoise

Range: the Galápagos Islands, 600 miles west of Ecuador Habitat: open, grassy areas to rocky, volcanic outcrops

Adapting and surviving

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 animal life they found, from blue-footed boobies Sula nebouxii and marine iguanas Amblyrhynchus cristatus to the giant tortoises. One thing that caught his eye was that each of the 13 larger islands in the Galápagos Islands had a slightly different subspecies of the giant tortoise. Each was uniquely suited for survival in that particular island’s environment. For example, domed shells are found on tortoises that live among highlands with lush pastures. Saddle-back shells give the tortoise more flexibility to reach sparse vegetation found higher off the ground, stretching their necks to eat from bushes and cactus. In arid areas, the tortoises also have longer legs to reach food growing up high, and the ability to climb a little. They are also smaller and need less food.

How about those shells!

Although they are massive animals weighing several hundred pounds (kilograms), their shells are not solid. Instead, they are made up of honeycomb structures that enclose small air chambers. This makes it possible for the tortoises to carry the weight of the shell. The shell is also attached to their ribs, so a tortoise cannot "walk out" of its shell, like you may see in cartoons. The tortoise’s lungs are located on the top of its body, under the top dome of the shell, which is why the tortoise can be in trouble if it is turned over—the weight of its body can crush the lungs.

Courting tortoises

When giant tortoises reach maturity, they begin to think about mating. In the wild, their breeding season is generally between January and August. After mating, the females begin a journey of several miles (kilometers) 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, then 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 will hatch; if it is high, more females will hatch. When the young tortoises emerge from their shells, they must dig their way to the surface, which can take up to one month! Then it’s up to them to survive on their own.

What a life!

Giant tortoises tend to lead a peaceful, lazy life that centers on grazing, relaxing in the sun, or wallowing in water puddles. Because they are cold-blooded like other reptiles, they like to soak up the sun to warm up. At night, they sleep partially submerged in mud, water, or brush to keep warm during cool nights. They are herbivores that eat prickly pear cactus (a favorite) and fruits, as well as flowers, water ferns, leaves, and grasses. At the San Diego Zoo, the tortoises are offered kale, collard, mustard and dandelion greens, bok choy, and seasonal fruits and vegetables. Galápagos tortoises can retain a great amount of water, which allows them to survive the long dry season on the islands. In fact, these tortoises can go without eating or drinking for up to a year because they can store food and water so well.

Trouble in paradise

When pirates, whaling ships, and merchantmen came through the islands from the 17th to the 19th centuries, sailors killed the tortoises for food. Nearly 200,000 were lost this way. By 1959, when the giant tortoises were 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, it is strictly regulated. All groups must have a guide and are asked to stay on the paths so the vegetation isn’t trampled on and the animals are not disturbed.

Today, the greatest threats to the tortoises come from introduced nonnative species to the islands, such as rats, dogs, and cats, which eat tortoise eggs and young tortoises. They also must compete for food with goats and cattle, which causes food shortages. At best, there are about 10,000 to 15,000 tortoises living today on the islands.

Helping these giants survive

In 1969 the San Diego Zoo became a partner with the Charles Darwin Research Station on the islands, funding a new tortoise-rearing facility and helping with captive propagation and release programs. The Zoo also has one of the largest captive colonies of Galápagos tortoises in the world and has had these giants in the collection since 1928. That’s the year when Charles Townsend of the New York Zoological Society began his efforts to save the tortoises from extinction by collecting them and setting up colonies in zoos.

The original tortoises sent to the San Diego Zoo—yes, they are still with us!—are well over 100 years old today, although we do not know how long a tortoise will continue to produce offspring. In addition, because Hood Island tortoises Geochelone nigra hoodensis are so rare (estimates were 12 females and 3 males in the 1970s), the San Diego Zoo’s male, number 21, was sent to the Charles Darwin Research Station in 1976 to be part of the breeding program there. So far he has fathered hundreds of tortoises!