Galapagos Tortoises Visit NYC

Galapagos giant tortoise, Geochelone elephantopus.


Early this morning, I visited the two Galapagos tortoises, Geochelone elephantopus, who currently reside on the third floor of the American Museum of Natural History. Even though the sign outside their large glass-enclosed area claims they are female, their handlers now suspect that they are both male. Apparently unperturbed by their mistaken gender, these teen-aged giants act as unofficial greeters to the crowds that are flocking to the newly opened Darwin exhibit at the museum.

"They're brothers. That's Frank and this is Charlie," Beth said, pointing to each tortoise in turn. The closest tortoise, Charlie, looked serenely through the glass at us, a piece of hay hanging out of his mouth.

Even though they are brothers, Beth explained, they are distinct and their caregivers can easily distinguish them by the shape of their scutes, the thick scales comprised of keratin that cover their bony shells. Keratin is a protein that is also found in hair, nails and hooves. But there are other physical differences between these brothers, too.

"For one, Frank is heavier, and Charlie is taller," Beth explained. I leaned down and squinted my eyes, trying to compare their respective shell heights from their level and found myself wishing for a tape measure.





Frank and Charlie's ancestors are terrestrial reptiles found only on the Galapagos Islands. The Galapagos lie in the Pacific Ocean, west of Ecuador (click map above for larger image in its own window). The Galapagos are really a group of more than 15 islands and islets, and each one has a different habitat and is occupied by a distinct combination of plants and animals, including giant tortoises. Historically, approximately 250,000 tortoises inhabited these islands, but unfortunately, their numbers plummeted because sailors ate them and routinely abandoned their domestic animals, such as dogs, pigs, goats, cats and rats, on these islands. Because these introduced species either ate tortoise eggs, preyed on young tortoises or competed with the adult tortoises for limited plant resources, only 10-15,000 individuals are alive today.

After a century of scientific study, it is not known for certain if each island's tortoise population can be classified as a true species, or if they might instead be subspecies or local variants of one species. However, recent DNA evidence suggests that at least a few of these island populations are legitimate species.

Giant tortoises have long been the focus of great scientific and historical significance because they were one of the many species that fascinated Charles Darwin on his voyages.

"I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted," Darwin wrote about these tortoises in his popular 1839 book, The Voyage of the Beagle.

After a conversation with the Vice-Governor of the islands, who claimed that each tortoise population was characterized by marked physical differences, Darwin discovered that he too, could identify a tortoise's home island, particularly by carefully noting the tortoise's overall size and the shape of its carapace (the bony shell). Just as selective breeding by humans caused the wolf to be modified into hundreds of distinct breeds of dogs, each breed with its own talents, these tortoises were likewise modified by the demands of their new environment. Over the millennia, this selective pressure shaped the original tortoises into distinct populations inhabiting each island. Because these changes are influenced by nature instead of humans, this essential process of evolution is known as natural selection. Consequently, because these animals illustrated the correlation between geographic isolation and morphological divergence, they became instrumental to the formation of Darwin's concept of evolution through natural selection.

In Darwin's day, there were 15 distinct populations of Galapagos tortoises. Representatives from 14 populations were formally described by the scientists of that time and 11 of these populations are still live today, although some are endangered.

These populations fall into two "morphotypes" based on the shape of their carapace, which is one of the tortoise's physical adaptations to the habitat found on their particular island home. It was noticed that generally, tortoises living on larger and wetter islands are very large, with domed carapaces and stubby, thick legs. These are the "dome-backed" group, of which Frank and Charlie are representatives.

The other morphotype, the so-called "saddleback" tortoises, are found on smaller and drier islands in the Galapagos archipelago. They are physically smaller than their dome-shelled cousins, with longer and thinner legs, and their carapaces flare out above their necks and legs. It is thought that these physical modifications provide the saddlebacked animals with greater mobility necessary to reach the succulent pads of the Opuntia cactus (interestingly, this cactus, which is a major source of water on the dry islands, evolved a tree-like form in response to the demands of hungry and thirsty tortoises). Because the saddle-backed tortoises reminded the early Spanish explorers of a type of riding saddle called the "galapago", this group inspired the name for these islands.

But who were the ancestors of all these tortoises and how did they get to the remote Galapagos islands? New DNA data reveal that the giant Galapagos tortoises are close relatives to the much smaller chaco tortoises, Geochelone chilensis, that are native to South America. It is thought that the Galapagos islands were colonized 2-3 million years ago by either a pregnant female tortoise or by several individuals that rafted from the mainland to the newly formed volcanic island of Espanola or San Cristobal. From this tenuous beginning, the resulting offspring of these tortoises then colonized the other islands in the Galapagos archipelago.

Unlike their mainland cousins, these island tortoises are huge animals. Male Galapagos tortoises from some island populations can attain a carapace length of 130 centimeters (approximately 4 feet) and can weigh up to 270 kilograms (600 pounds). Males are much larger than the females, who never exceed 300 pounds. Galapagos tortoises reach sexual maturity at approximately 40 years of age and can live to be 150 years old. So as these tortoises go, Frank and Charlie are mere whippersnappers: they are roughly the size of a footrest and weigh approximately 80 pounds each.

"They just celebrated their 13th birthday this past August," said Beth to a crowd of their admirers who were watching the two boulder-shaped animals move sedately around their enclosure. She noted that it's a good thing that these tortoises are relatively small because the larger and stronger adults tended to walk through closed doors when they so desired.

"If we had a full-grown male on display, he could crash through the wall of the display," Beth explained. I peered around the large reptile hall for a moment, imagining what it might be like to be greeted by a 600-pound giant tortoise out for an early morning stroll across the gleaming floors.

"Do they bite?" I asked after a moment, watching the piece of hay disappear into Charlie's mouth.

"No. Well, not really," Beth paused. "If they do bite, they just grab a little bit of fabric [of your clothes]."

As you might have already surmised, Galapagos tortoises are herbivores. Depending upon the island where they originated, they eat a diet consisting of those particular species of prickly pear cactus and fruits, bromeliads, water ferns, leaves, shrubs and grasses that are native to their island homes. Beth noted that they have good color vision, and show distinct preferences for food items that are red, green or yellow in color.

"This is probably because foods with these colors had the highest nutrition content on their islands," she said.

The tortoises also have tremendous water storage capacities, and have been known to survive as long as one year without water. This led to their popularity as a menu item from the 1700s onward for hungry sailors craving fresh meat and oil during their long voyages.

Because Galapagos tortoises are protected species, Frank and Charlie were domestically bred animals: they hatched in Oklahoma and then were purchased seven years ago by Reptileland in Pennsylvania where they usually live when not visiting museums in New York City. Beth was not certain which island was home to Frank and Charlie's ancestors because one parent's ancestry cannot be verified.

"They're probably mutts," she said.

Regardless of their ancestry, Frank and Charlie are fascinating animals with a remarkable story to tell about the origin of all life on earth.


Acknowledgements


GrrlScientist sincerely thanks Beth for answering her many questions.


More Information


AMNH tortoise cam. Image refreshed every 30 seconds.

The Endangered Galapagos Tortoise, Discover Galapagos. Provides information about each tortoise population.

Galapagos Tortoise, Honolulu Zoo (includes video of nesting tortoises).

Interactive Galapagos map.


tags: evolution, biogeography, Darwin, Galapagos, tortoise, NYCLife

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