The Cave Dweller.
Here I was, exploring in the sandhills of South Carolina. The infrared video camera I had inserted into a long burrow had just given me a blurry, black-and- white image. As I stood staring at the monitor, I could barely notice the outline of a wide shell, worn smooth with age and many trips in and out of the close-fitting tunnel. But the staccato head-bob that greeted me was unmistakable. Yes! It was my first image of an ancient, secretive beast: the gopher tortoise. Scarcely could I contain my excitement!
I was amid a patchwork of land parcels razed for housing sites, clear-cut for timber production, and dissected by roads. But the presence of tortoises at this site had gone practically undetected by the outside world until the early 1990s. The previous generation of landowners had defended the property from intruders who might stumble upon their illicit moonshine operation. Inadvertently, they had protected the tortoises as well. Elsewhere, however, human demands have superseded the needs of the tortoise, threatening its existence. I hope that as more people become acquainted with these gentle creatures they will strive to find ways to share the habitat with its original occupants.
Tortoise or turtle?
A tortoise is a turtle, but not all turtles are tortoises. No, that's not a riddle. There are many types of turtles: Sea turtles live in the ocean, terrapins dwell in the salt marsh, other turtles make their home in freshwater habitats or on land. Many people think of a tortoise as any turtle that lives on land. In most cases, they'd be right. The box turtle, however, is more closely related to freshwater turtles than to tortoises. Tortoises, therefore, are not simply land-dwelling turtles; they also share a common ancestry.
There are only about 50 species of tortoises worldwide. Four of these dwell in the United States and Mexico, but the gopher variety alone has ventured east of the Mississippi River. Gopher tortoises occupy the coastal plain of Alabama, Georgia, and Florida, and they barely make it into Louisiana and South Carolina. Both the common name and the scientific name--Gopherus polyphemus--allude to the species' burrowing proclivity. Gopher is the vernacular term for gopher tortoise, as well as for several species of burrowing rodents, and polyphemus is undoubtedly a reference to the cave-dwelling mythical character by that name. By tortoise standards, the gopher tortoise is modest in size: up to 14 inches long. Yet it takes many years for one to reach full stature. It can live for 50 years or more, but it may not reach reproductive maturity until 15--20 years of age.
Contrary to common turtle lore, a gopher tortoise (or any other turtle) cannot crawl out of its shell and move into or regrow another, because the shell is fused to the backbone. A juvenile's somewhat flexible shell grows along with the rest of the body, gradually hardening as the tortoise gets older.
The upper shell, or carapace, can have a range of colors: gray, tan, and chocolate. By contrast, the lower shell, or plastron, is usually a lighter brown or pale, golden yellow. Both the plastron and carapace consist of a matrix of bony plates called scutes. It is often possible to estimate the age of a young tortoise by counting the rings (annuli) on a scute, much as you would count rings on the cross-section of a tree. In juveniles and adult females, the plastron is perfectly flat, but in mature males it develops a noticeable concavity toward its posterior. The male also has an elongated scute at the plastron's front end that is used to joust other males when vying for courtship rights with a female.
The gopher tortoise's columnar, elephantlike hind legs are well suited for overland travel, but their slow, almost jerky movements have been likened to the awkward lurches of a child's windup toy. The strong, flattened forelimbs (encrusted with hard epidermal scales in older animals) terminate in stubby, wide claws and are great for digging burrows. Even hatchlings with their spindly legs can dig on their own.
Home, sweet sandhill
Many people think a turtle's home is its shell, but that's a misconception because it ignores the need for proper habitat. Tortoises require sandy soils, open-canopy areas that receive plenty of sunlight, and tender, low-growing plants. A good sandhill habitat has a sparse upper canopy, dominated by longleaf or slash pine, and a subcanopy of mixed oaks and evergreen shrubs. Close inspection of the patchy, herbaceous layer reveals an astounding diversity of annuals and perennials--particularly asters, milkweeds, and legumes--on which the tortoise voraciously feeds. In areas where the soil surface has not been severely disturbed, clumps of knee-high wire grass provide the creature with forage material in early spring, before other plants emerge.
What allows so many plant species to survive on sandy soils that are virtually devoid of moisture and nutrients? Most biologists believe the answer is linked to the historical importance of fire, which helps maintain the open canopy crucial for light-thirsty understory vegetation. A low-intensity fire also induces the production, release, or germination of the seeds of "fire- adapted" plants. Some plants, such as wire grass, cannot reproduce without such fires. The fire creates patches of bare soil suitable for germination sites, and nutrients released during the fire can nourish young seedlings.
The sandhills fauna can be just as diverse as the flora. Many resident animal species--such as the armadillo, pocket gopher, hognose snake, and burrowing owl- -circumvent the hot, dry conditions by adopting a fossorial lifestyle. Underground refuges reduce the risk of overheating, dessication, and predation, and may even protect the animals during periodic fires. The gopher tortoise is arguably the most well known fossorial creature of the sandhills.
Compared with the other three species of North American tortoises, the gopher tortoise spends proportionally more time in its burrow. The burrow passage, which can be from under three to over seven yards long, is usually just wide enough for the resident tortoise to turn around in, but it often ends in a large chamber several yards below ground surface. The large amount of soil excavated to construct the tunnel is deposited outside the burrow entrance, in a mound called an apron.
Gopher tortoises are ectotherms, which means they do not have an internal mechanism to regulate body temperature. Instead, they must modify their behavior. They avoid extremes of weather by retreating into the stable environment of their cavernous burrows, emerging on brief forays to graze on nearby vegetation. Conversely, on cool days they may perch atop the burrow apron to bask in the sun. Female gopher tortoises may also use the apron as a nesting site. On average, an individual tortoise will use between two and seven burrows per year, often the same ones year after year.
A gopher tortoise sometimes shares its burrow with another tortoise, but it usually occupies the burrow singly and may even defend it against others. I once observed two tortoises tussling briefly over burrow rights. One had approached the burrow, peered into the tunnel, and thumped its plastron on the apron. In response, the resident tortoise had come rushing up the tunnel, stopping just inside the entrance. As the intruder attempted to charge in, the resident dug its powerful legs into the walls and refused to budge. The rebuffed intruder eventually turned away in search of another home.
Despite occasional territorial disputes with other tortoises, the gopher tortoise is an accommodating host, sharing its home with other species--called commensals. Sixty vertebrate and at least 300 invertebrate species have been found in various tortoise burrows. Some guests use the burrow as a temporary refuge, but others--mostly insects and arthropods--have never been observed outside their benefactor's shelter. Ross Allen, the famous Florida naturalist, once found a diamondback rattlesnake, an opossum, a rabbit, a gopher frog, and a tortoise all living in the same burrow! By creating a home for many types of animals, the gopher tortoise increases the local faunal diversity, earning it the designation of a "keystone" species. Biologists warn that a decline in gopher tortoises could trigger the disappearance of other sandhill species.
As with many other species in danger of extinction, the gopher tortoise is most threatened by loss or degradation of habitat. Causes of habitat degradation include fire suppression and conversion of structurally complex, mixed-age longleaf pine forests (with a diverse understory) to slash or loblolly pine monocultures. The dry, sandy ridges favored by tortoises are also choice sites for roads, housing, and commercial developments. When several tortoises are likely to be affected, they may be relocated elsewhere. But relocation is a short-sighted, temporary solution, because it ignores the fate of burrow commensals (which are not relocated), increases the risk of spreading diseases, and fails to consider the complex interactions within tortoise populations.
The good news is that certain historical pressures on the animal have subsided. Gopher tortoises--called "Hoover's chickens" during the Depression-- are no longer eagerly sought for human consumption. Neither are they being collected from the wild for tortoise races. And most rattlesnake roundups--in which gasoline poured down burrows forced out the rattlesnakes but killed the slow-moving tortoises--have been discontinued. Conservation organizations, biologists, and local citizens are beginning to work together to acquire and protect some of the remaining habitats of the tortoise and to examine the intricate relationship between this creature and its community. Clearly, people are changing their attitudes toward the gopher tortoise and its unique habitat, but its long-term survival will require more than a piecemeal approach.
Tracey D. Tuberville has a graduate degree in conservation ecology and sustainable development from the University of Georgia. She is currently performing research on the gopher tortoise at the Savannah River Ecology Laboratory in Aiken, South Carolina.
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|Author:||Tuberville, Tracey D.|
|Publication:||World and I|
|Date:||May 1, 1999|
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