Spotlight: Aldabra Tortoise
Imagine checking out at the grocery store, and as you politely socially distance from the customer in front of you, your eyes land upon a new tabloid. It’s a special edition, The Aldabra Enquirer! The shocking headlines include, “ Both Males AND Females promiscuous, expert says!”” Truth revealed — Esmeralda is actually a MALE!” “ I ran for my life when I saw them NOSING!” “ Heartless parenting, scientists declare” “ It must have weighed 600 pounds, and it was coming right for me!” and finally, “ Vacation in the Seychelles? Think again…what about the Aldabra CREEP? “
Like most tabloids, The Aldabra Enquirer includes a grain of truth in each headline, so read on to learn about these amazing, ancient giants!
Aldabra tortoises and their more famous cousins, the Galapagos Giant Tortoises, evolved from a common ancestor about 20 million years ago, and the Aldabras dispersed from Madagascar to the islands of the Seychelles, and also to Mauritius, Reunion, Rodrigues, and Zanzibar. Originally they shared these islands in the Indian Ocean with many other species of giant tortoises, but today only the Aldabras remain. Again like their cousins the Galapagos, they were prized by seafarers as the perfect source of food for long voyages, and were often stacked like firewood in the holds of ships. The short version is that their numbers decreased significantly, and they disappeared from everywhere except the Aldabra Atoll (and a few individuals have been relocated to zoos and to some of the neighboring islands and cays). The Aldabra Atoll is part of the Seychelles (it’s around 930 miles east of Africa, and it’s northeast of Madagascar), and is the world’s largest atoll, which is a ring-shaped coral reef. Today, the giant tortoises on Aldabra number from 100,000 to about 150,000.
There are various theories about why certain types of tortoises grow so large, but the one currently favored by the scientific community is the “founder effect,” when only a few individuals of a species arrive in an isolated place, like an atoll. If abnormal genetic traits, for example mutations like gigantism, enable them to better survive in this new environment, the small populations of survivors will inbreed. And because the genetic trait is advantageous, natural selection sees that these traits become fixed in this isolated population. That explains why the giant tortoises are endemic only to islands and atolls, never to mainlands.
But How Big is Giant?
The largest Aldabra tortoise documented weighed in at about 672 pounds! We know this must have been a male, because they are considerably larger than females. The male Aldabra’s carapace, which is the upper shell, can grow up to 4 feet long, and they generally weigh around 550 pounds. The females are considerably smaller, but relatively a little tubbier — 3 feet long on the carapace and 350 pounds. Aldabras grow pretty slowly, and when they’re 25 years old, they’re ready for breeding, though they’ve reached only about half of their full size.
Scientists speculate that these tortoises don’t grow continuously, and further that the growth rate slows as they get older, which means they’re not necessarily full-grown when they reach their 50 thbirthday, or “hatch day.” They are suspected of having very long life spans — perhaps up to 170 years — but because research scientists can’t compete with that impressive statistic, even those devoted to the species for their entire lifetimes can’t verify this. No members of the species have been in human care that long, either, so even in an environment that can be controlled, experts always say, “This individual may live to about 170.”
Not exactly role models for family life
Yes, promiscuous is an apt description! This shocking headline from our fictional Enquirer is accurate — but scientists prefer to describe Aldabras’ mating habits as polygynandrous. Both males and females have multiple partners. Time to now picture the relative sizes of the males and females — we might predict that the weight difference alone could make coupling dangerous for the females. However, they hold their own, and when they want to get rid of an aggressive male who is attempting to breed, they simply extend their front legs to make themselves as tall as possible; this has the effect of dropping the back of the shells to the ground, swiftly causing an unwanted male to tumble off and trudge away. Even without this neat trick, most mating attempts are unsuccessful, and though a female may lay from 9–25 eggs, only about half will be fertile.
Odds are not good for those 12 or so fertilized eggs, either, because like many tortoises and other reptiles, females lay the eggs and walk away. These rubbery, tennis-ball sized eggs are not even buried, because the nests are shallow indentations on the ground’s surface. Eggs therefore are especially vulnerable to predators (like giant crabs) as well as climatic events like flooding or rising temperatures. And those who do hatch are totally on their own. Fortunately, their herbivorous food sources, like grass, and small plants, are at ground level.
A Day in the Life
But let’s say a lucky hatchling does make it to adulthood, where her only predator would be a human being. What does she do with her day? Well, sleeping is quite popular — up to 18 hours a day. Then there’s grazing — as the Aldabras become larger, they’re able to eat tasty treats above ground level, often by knocking over small trees and shrubs — and this is actually beneficial to their ecosystem. So like elephants, the Aldabra Giant Tortoises are a keystone species, enabling the survival of many other creatures on the atoll who might depend on these food sources as well. These hefty giants also create pathways for their fellow atoll residents. Like all herbivores, Aldabras also provide the useful service of dispersing seeds through their feces, and there’s actually a species of land hermit crab, Coenobita rugosus, whose entire diet consists of Aldabra feces.
So let’s see — sleeping, grazing, landscaping , defecating…..then there’s stretching, a little walking (0.3 mph), resting, mating (or not), and nosing. Nosing? Nosing is actually kind of endearing, even though researchers have no idea of its purpose in the lives of these generally unsociable creatures. But anyway, one thing scientists are sure of about nosing is that it has absolutely nothing to do with mating. What happens is this: One Aldabra will lie down next to another, and then proceed to rub his or her nose on the companion’s head or neck, and continue doing so for several minutes. Then it’s time for more sleeping, probably, or maybe walking away, slowly of course. By the way, one of the names for a group of Aldabras is a “creep.”
Are they endangered?
The good news is that the Aldabra Atoll has been designated as a UNESCO World Heritage Site, and that means the tortoises are protected there. However, the IUCN has designated them as Vulnerable, just a step below endangered, and this is due to habitat loss as well as the introduction of introduced mammals which compete for land and resources, and also prey upon Aldabra eggs. And the greatest threat now seems to be rising sea levels around the atoll — which is projected to cause up to a 65% decrease in tortoise population on Aldabra Atoll in the next century. There are breeding programs in AZA zoos, but unfortunately, not many have yet been successful (similar to what happens in the wild). But maybe the continued opportunity to observe these unique creatures will result in new insights into breeding them — We hope so!
At the Reid Park Zoo
You can visit the creep at the Reid Park Zoo — it consists of two females, Georgie (maybe about 32 years old and about 155 pounds) and Dulcee ( maybe about 73 years old and 192 pounds), and the gigantic Herbie, who’s clearly the male, maybe going on 90, and weighing in at about 533! It’s a sure bet you’ll see them indulging in one of their nine activities — maybe even nosing. And if that whets your appetite, you’ll soon be able to see more marvelous reptiles in the Reid Park Zoo expansion — and each one will have its own unique adaptations and behaviors — but how can any of them compete with nosing?
NOT SO FAST — WHAT ABOUT ESMERALDA?????
Oh yes — our final shocking headline proves that if you don’t see the relative sizes of male and female Aldabra Tortoises, it’s awfully difficult to determine the gender of one of these giants. Esmeralda lives on Bird Island, a tiny coral cay in the Seychelles, and he weighs 670 pounds. He’s maybe 170 years old. And he got his name from a famous botanist and zoologist named Lyall Watson, who happened to be visiting Bird Island. A giant Aldabra Tortoise unconcernedly approached the zoologist, and he just seemed so mellow and happy that Mr. Watson, who only checks the gender of reptiles when he’s researching them, felt Esmeralda was the perfect name, and it stuck. No worries. To paraphrase William Shakespeare, “A male Aldabra Tortoise by any other name would still be as huge.”
Come to the Reid Park Zoo and take a gander at Herbie — you’ll see what we mean!