Gopherus polyphemus / Gopher Tortoise

Gopherus polyphemus Interactions in the Smith Preserve: Many people consider Gopher polyphemus to be a keystone species in upland environments because it plays such a critical role in maintaining the structure of the ecological community. Tortoise burrows provide a place for other organisms to retreat from light, fire, dessication, temperature extremes, and predation. Additionally, the tortoise's fecal material accumulates at the deepest part of its burrow and supports a diverse community of microorganisms and insects.

In studies of gopher tortoise burrows, 60 vertebrates and over 300 invertebrates are known to use gopher tortoise burrows. Some include: newts, frogs, toads, American alligators, box turtles, green anoles, other lizards, skinks, snakes, bobwhites, owls, wrens, robins, sparrows, opossums, armadillos, domestic dogs, coyotes, foxes, raccoons, skunks, weasels, bobcats, squirrels, mice, rats, cottontails, land snails, crayfish, whip scorpions, pseudoscorpions, spiders, mites, ticks, centipedes, millipedes, crickets, grasshoppers, walkingsticks, cockroaches, true bugs, antlions, beetles, caterpillars, moths, crane flies, gnats, scavenger flies, robber flies, long-legged flies, dung flies, fleas, wasps, and ants. Some of these species, like Drymarchon couperi (Eastern Indigo Snake) are rare and threatened Florida species.

Other gopher tortoise interactions include 1) limiting plant populations by grazing, 2) being prey for carnivores, 3) and being hosts for parasitic organisms.

Gopherus polyphemus Biology topics:

 

 

I. Body Structure:

Gopherus polyphemus is a member of Order Testudines (Turtles) and Family Testudinidae (Land-Dwelling Turtles/ Tortoises). Like other members of this Order and Family, gopher tortoises have a highly complex shield covering the vital organs. The shield consists of a dorsal and ventral shell composed of modified ribs, pelvis, and other bones. As labeled in the photograph at right, the dorsal shell is the carapace, and the ventral shell, the plastron. The two shells are joined by an area called the bridge.

 

 

 

The carapace has an oblong shape and is widest near the posterior margin of the bridge. It also has the greatest height from the plastron in the pelvic region. As shown in the photograph at left, the carapace drops downward abruptly posterior to the pelvic region.

 

 

 

The individual plates or scales that make up the shell are called scutes. The carapace scutes are relatively smooth due to sand abrasion created when a tortoise enters and exits the burrow. Both the carapace and plastron scutes show concentric growth rings. The ten cm wide scute in the photograph at right was found near a tortoise carcass.

The longevity of gopher tortoises is thought to be about 60 years. The counting of growth rings on scutes can be used to tell the relative ages of tortoises living in the same locale, but counting the rings is not a definitive way to determine a particular tortoise's chronological age.

 

As shown below, adults have a dark-brown, tan, or grayish-black carapace, while hatchlings have a yellow to yellow-orange carapace with each scute bordered in brown. Photograph of adult: Courtesy of Sheri Arnold, Conservancy of Southwest Florida volunteer. Photograph of hatchling : Courtesy of Ian Bartoszek, Conservancy of Southwest Florida biologist.

 

The anterior end of the plastron has a gular projection. Gular scutes project below the chin. Males normally have longer gular projections than females, but gular projections sometimes break during aggressive and defensive encounters. The photograph at left shows the plastrons and gular scutes of a male (left) and a female (right.) Part of the female's head is visible over her gular scutes. Photograph at left: Courtesy of Ian Bartoszek, Conservancy of Southwest Florida biologist.

Full grown gopher tortoises reach 38.7 cm in carapace length. Newly emerged hatchlings are only five cm in carapace length.

Forelimbs are long, shovel-like, and covered in large scales. Hind limbs are short and elephantine. As seen in the photograph above, the skin of the head and limbs of a hatchling is bright yellow. The color darkens as it ages.

 

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II. Burrow Construction:

Gopher tortoises build their burrows in open, sunny locations where they use the burrow for nesting and thermoregulation. They choose sunny locations so they are close to their food source.

All gopher tortoises build burrows. Newly hatched tortoise burrows, like the one shown immediately left, are very shallow and many times just a few meters from the hatchling's birth place. Adult burrows like the one shown at far left average 4.5 m long and two m deep. The width of a burrow at a depth of 50 cm is approximately equal to the carapace length of the resident tortoise. The mouth of the burrow is eroded and enlarged. Most gopher tortoises dig several burrows during a year.

Photograph of the adult burrow above : Courtesy of Ian Bartoszek, Conservancy of Southwest Florida biologist.

An adult can dig a burrow several meters deep in about a day. The photograph at right shows an adult in the process of digging . Roll over the photograph to see the sand fly. Below, many tracks lead in and out of an active burrow.

 

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III. Diet:

Gopher tortoise diet includes over 300 species of plants. Their favorite foods are members of the following families: Poaceae (True Grasses), Asteraceae (Asters, Daisies, Sunflowers), Fabaceae (Legumes), Pinaceae (Pines), Chrysobalanaceae (Coco Plum, Gopher Apple), Cactaceae (Cactus), and Fagaceae (Beech and Oak Family). Members of all of these families exist within the Smith Preserve.

Tortoises enjoy eating foliage, flowers, and fruits. They are also known to eat mushrooms, scavenge carrion, and eat excrement, however in the Smith Preserve they have been observed eating only vegetation.

 

The Opuntia humifusa (Prickly Pear Cactus) pad at far left shows evidence of having been chewed by a gopher tortoise. The tortoise at left is feeding on Richardia grandifolia (Erect Richardia).

 

 

 

 

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IV. Excrement:

 

 

Since gopher tortoises are primarily herbivores, their dung contains fibers of plant material. As seen in this photograph, coarse-textured fibers are undigested.

 

 

 

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V. Reproduction:

For gopher tortoises, it is body size, not age that determines sexual maturity. Males probably mature at a smaller size than females. Size is influenced by diet and geographic location. In warmer climates which have a longer growing season, tortoises grow faster and become mature at a younger age than those living in more northern locations. In studies done in Central Florida, females were found to attain sexual maturity at nine to 16 years of age.

Photograph of tortoise : Courtesy of Ian Bartoszek, Conservancy of Southwest Florida biologist.

Mating and nesting activities take place primarily May to mid June. A male wanting to mate will position himself at the mouth of a female's burrow and display a head bobbing behavior. If the female is interested, she will exit her burrow. Then the male will walk in a circle around her, stopping along the way to perform the head bobbing display. If she approaches him, he will bob his head violently, bite her on her foreleg, head, anterior edge of the carapace, and gular projection. She will back in a semicircle; the male will follow. Eventually, the female will stop and extend her rear legs. She will rotate her body 180 degrees so her posterior end is near his head and the male will attempt to mount her.

Females often lay their single clutch of five to nine eggs in a flask-shaped nest cavity ten to 15 cm deep just outside her burrow. The brittle-shelled eggs are nearly spherical. The incubation period is about 80 days in southern Florida. Many of the eggs are killed by predators, but those eggs that survive hatch in August and September.

 

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VI. Predators and Parasites:

Predators:

Common predators of tortoise eggs include Solenopsis spp. (Fire Ants), Procyon lotor (Raccoons), Urocyon cineroargenteus (Gray Foxes), Mephitis mephitis (Striped Skunks), Didelphis virginianus (Opossums), and Dasypus novemcinctus (Armadillos).

According to David Alderton in his 1998 book, Turtles and Tortoises of the World, 90 percent of egg clutches may be destroyed before the end of the incubation period. Less than six percent of the eggs are expected to hatch with the hatchlings surviving one or more years.

The soft-shelled hatchlings and juveniles up to seven years of age are easy prey for birds and mammals as they sun themselves at the mouths of their burrows. Coyotes and domesticated and feral dogs and cats are also tortoise predators.

This photograph is of a juvenile (approximately six cm in carapace length) that was eaten. Only part of its carapace remains.


Parasites:

Tortoises of all ages are hosts of parasites that include mites, ticks, and bacteria. One type of bacteria, Mycoplasma agassizii causes URTD (Upper Respiratory Tract Disease.) This bacteria is easily spread from tortoise to tortoise. Once infected, a tortoise develops lesions in its nasal cavity, and has excessive nasal discharge and swollen and sunken eyes. In severe cases, it becomes lethargic, stops eating and dies.

 

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VII. Distribution And Predictions For The Species Future:

During the 1930s and 40s, gopher tortoise meat was a food source for people. At the time, hunting greatly affected tortoise populations. It was not until 1987 that Florida prohibited the hunting of gopher tortoises for food.

In recent times, another human activity is decimating tortoise populations: overdevelopment. As a result of overdevelopment, there has been a reduction of tortoise habitat; degradation of suitable habitat; and fragmentation of habitat, which isolates populations. A conservative estimate is that less than 20 percent of the xeric upland that existed in the 1960s remained in 2000.

In 2006 in Florida, Gopherus polyphemus was listed by the Florida Fish and Wildlife Commission as a threatened species. Threatened means "a species is likely to become endangered in the state within the foreseeable future if current trends continue." Some scientists feel the gopher tortoise should be listed as endangered now because it is one "whose numbers have already declined to such a critically low level or whose habitats have been so seriously reduced or degraded that without active assistance their survival in Florida is questionable.”

One approach to helping provide a healthy future for the tortoise is the purchase of land known to support their populations. The purchase of the eight-acre property, now the Christopher B. Smith Preserve, was made for this purpose. But with this purchase comes responsibility to keep the area in pristine condition for the tortoises.

At the time the eight-acre (19.76 hectares) property was purchased, a survey of the tortoise population was conducted and a fence surrounding the property was constructed to prevent the tortoises from escaping to the busy roadways in the adjacent urban community. Sixty to 70 tortoises were living on all parts of the property except the pond, which is uninhabitable.

The tortoise population was well above what has been determined by some researchers to be the optimal number for the size of the property. One research study (Eubanks, J.O., Hollister, J.W., Guyer, C., and Michener, W. K. 2002. "Reserve area requirements of gopher tortoises (Gopherus polyphemus)." Chelonian Conservation and Biology 4 (2): 464-471) maintained that for 50 adult tortoises, you need 25 hectares to 81 hectares. This was based on research tracking individuals and on calculations of burrow density. Other studies say 100 hectares is better because with less density there is less competition for food and space.

In spite of the high density of tortoises at the Smith Preserve, the tortoises seem healthy and unstressed. However because of the dense population, it is imperative that there be continuous maintenance of the habitat. Studies show that in the absence of fire for only a few decades, a good quality gopher tortoise habitat can become too overgrown to support the grasses and herbaceous vegetation needed as food. Without the possibility of fire in the Smith Preserve because of its proximity to the urban community, removal of plant overgrowth has to be done manually. Also, new plants have to be added as tortoises graze the existing vegetation.

 

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© Photographs and text by Susan Leach Snyder (Conservancy of Southwest Florida Volunteer), unless otherwise credited above.

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