One Shell of a Reptile
For the Galápagos tortoise, slow and steady wins the day (lots of them!)
BY Karyl Carmignani
Photography by Ken Bohn
Galápagos giant tortoises are among the largest terrestrial reptiles on the planet. Inhabiting specks of lava-formed islands off the coast of Ecuador—the remote archipelago that inspired Charles Darwin’s theory of natural selection—these reptiles are well adapted to their respective varied habitats. They avoid the fast lane, and their measured gait, healthy diet, and low metabolism help them to be one of the longest-lived animals in the world. Tortoises that hatched while Teddy Roosevelt was president are still lumbering along nicely today—and reproducing! Galápagos giant tortoises lead their gloriously slow lives methodically munching on greens, basking in the sun, and wallowing in puddles.
Among the 300 tortoise species, the Galápagos tortoise is a true giant. Their shells can reach six feet in length and four feet across, and adult tortoises can weigh more than a Shetland pony. Galápagos tortoises have a roster of near-magical skills, like the ability to go an entire year without food or water; a female’s capability of storing a male’s sperm for 2 years or longer, then laying her eggs when she’s good and ready; and a heart that beats a sluggish 6 to 10 times a minute. (Human hearts, in contrast, beat 60 to 80 times per minute.) Early whalers, pirates, and merchant sailors took advantage of the tortoises’ extended fasting ability, collecting them en masse for food aboard lengthy voyages at sea.
A tortoise’s shell may appear indestructible, but it’s not. It is made of two layers of sturdy keratin, sandwiching honeycomb structures that hold tiny air chambers, which makes the shell lighter to carry around and adds buoyancy to the animal at sea. The spine and ribs are fused to the top shell, called a carapace. The bottom shell is called a plastron. This encasing shield serves as a built-in panic room that the animal can retreat into at the first sign of danger…or when it doesn’t feel like socializing (it happens). Its front legs, which appear bowed when walking, enable it to effectively seal the opening with its raised bent elbows once its head is inside.
Like other island dwellers, all 11 Chelonoidis species are well adapted to their specific island habitat. For example, Galápagos tortoises with dome-shaped shells are found in highlands with lush pastures, where food is at eye level. Other species have saddleback shells, with room to stretch their necks up to reach sparse vegetation on bushes and cactuses found higher off the ground. This type of tortoise also has longer legs and a smaller body, enabling them to travel more easily through their harsher landscape. Both types exhibit a “finch response”: extending their crepey neck and sturdy limbs out of the shell to allow finches to rid them of parasites.
The Tortoise and the Egg
During breeding season, males spar for access to females; sexual maturity for both is 20 to 25 years of age. The saddleback tortoises defend cactuses (a resource for a mother-to-be, since they provide calcium) and perform neck-stretching battles with the competition. For dome tortoises, this is a slow-mo battle of the titans, with glaring and shoving. The loser retreats into his shell with a hiss, and the winner takes all. If the female is interested, the male, who is twice her size, mounts her with his forelegs propped up on the front of her shell. He may produce loud bellows, which carry for long distances. To lay their eggs, females dig nests in the sand over a foot deep, using their hind legs. It may take hours. Clutch size varies with the species—saddlebacks lay 2 to 7 eggs, domed lay 20 to 25 eggs. They may lay up to four clutches per breeding season. Eggs are about the size of a tennis ball, and after hatching, the youngsters stay in the nest for a few weeks before digging out. Each hatchling weighs about as much as a deck of cards. Their beguiling appearance cannot protect them from predators, and though they grow rapidly, it takes about a decade for their shell to harden into armor. Until then, they must outwit a gauntlet of non-native predators introduced by humans: pigs, dogs, rats, cats, and fire ants, as well as “normal” diseases, parasites, and volcanic island life risks. But if they survive their teenage years, they may live over 150 years!
At the Zoo
The San Diego Zoo is home to 13 Galápagos tortoises: 6 females and 7 males. Of those, two are of the saddleback variety. The oldest is Grandma, who is over 130 years old. The youngest is Jaws, a 580-pound fellow, a mere 52 years old. They all have different personalities, explained Jonny Carlson, senior keeper. “Chips loves to play and splash in the water,” he said. “Abbott is pretty lazy—I’ve seen him sit in a pool for two days. Augustus is the problem solver, often checking to see if we’ve left a gate open so he can access the females.” There have been 94 hatchlings at the Zoo since the first breeding group arrived in 1928. But we’ve not had any fertile eggs since 2001. “We have increased the time the sexes are apart from 12 months to 18 months, hoping that will increase the females’ receptivity,” he added. Keepers observe the tortoises for behavioral changes that might indicate they are gravid. For instance, Penelope is usually feisty and hangs with the others, but lately has been wandering off on her own. “If we suspect they have eggs, we let them stay in the tortoise barn, where there’s a soft sand pit four feet deep they can dig their nest in,” said Jonny.
In the mornings, the giant animals huddle around a tasty stack of hay, lettuce, kale, bok choy, and nutritious herbivore pellets. Some chew with their mouth agape, others concentrate on the duty at hand. Their exhales are audible, sounding like exotic woodwind instruments. “Everything about them is slow,” said Rick Watson, a longtime Galápagos tortoise volunteer. “The digestion of a meal will take about 12 days to complete, while humans digest a meal in 6 to 8 hours.” By and large, they are pretty healthy, rarely needing medical care. Walking, they drag their front feet a bit, which keeps their nails filed down. “We trim their beaks with a Dremel as needed,” said Jonny, since they don’t chew on tree bark here to keep that in check. They respond well to brightly colored foods like carrots and sweet potatoes, which are used for training rewards. Of the current Galápagos tortoise group at the Zoo, nine have lived here since 1928. That’s a mighty long time, no matter how fast your heart beats.