Gopherus polyphemus

The gopher tortoise is a relatively large (carapace length often 15-28 cm, but up to 38 cm) terrestrial turtle with a domed carapace, short elephantine hindlimbs, shovellike forelimbs, a gular projection from the anterior plastron, and a short tail. The anterior surface of the flattened forelimb is covered with 7-8 rows of large scales. Often the surface of the carapace is quite smooth in adults, reflecting the abrasion it receives as an individual enters or exits its burrow. The carapace is keelless and oblong, with the greatest width just anterior to the well-developed bridge (connecting the carapace to the plastron), and the greatest height in the sacral region. The carapace drops off abruptly to the rear of the highest region (Ernst and Barbour 1972). The carapace of an adult varies from dark- brown to grayish-black. In Florida, individuals from coastal areas are generally darker than those from central populations. The gular scutes of the robust, hingeless plastron project below the chin. Males often have longer gular projections than do females. However, because both sexes use their projections during agonistic encounters, the gular projections are often broken and may not be an accurate diagnostic feature of the sex of an individual (Mushinsky et al. 1994). Most gopher tortoises have well defined "growth rings" on the scutes of the yellowish plastron. Use of the growth rings to age individuals must be done with caution, as there is much variation in the number of "false" growth rings throughout the range of this taxon.

Female gopher tortoises become sexually mature at a carapace length of about 23-24 cm. Males are somewhat smaller at maturity and do not obtain the large body size of females. The best indicator of the sex of an adult gopher tortoise is the depth of the plastral concavity (Mushinsky et al. 1994). Mature males have a shallow depression in the posterior, central portion of the plastron to facilitate mounting a female for copulation. Large females may have a shallow plastral concavity (2-4 mm) compared to the deeper concavity found on mature males (5-8 mm). Males often have larger integumentary glands under the chin than do females (Ernst and Barbour 1989), but the size of these integumentary glands varies seasonally. Based upon numerous anatomical measurements, McRae et al. (1981a) developed a discriminant function that accurately identified the sex of adult individuals. Using a stepwise multiple regression on numerous morphological measurements, Burke et al. (1994) developed a non-invasive sex identification technique for determining the sex of hatchling and juvenile gopher tortoises.

Hatchlings emerge from their eggs at a carapace length of generally about 3-5 cm. Coloration of the vertebral and costal scutes of the carapace of hatchlings is yellowish to yellowish-orange, and each scute is bordered by brownish coloration (Allen and Neill 1953). The skin on the head and limbs is likewise brightly colored yellow to yellowish-orange. The bright coloration of hatchlings darkens during the first year or two of life. The gular scutes of young tortoises do not project forward as in the adult tortoises, and the claws of young tortoises are long and sharp (Allen and Neill 1953). Hatchlings dig their own burrows, often just a few meters away from the nest from which they emerged. Hatchlings and juveniles, up to an age of 5-7 years, have relatively soft shells and are highly vulnerable to predation (Wilson 1991).

Eggs are white, nearly spherical, and brittle-shelled. For photographs of eggs see Allen and Neil (1951) and Pope (1939). Iverson (1980) reported an average maximum egg diameter of 42-43 mm and an average wet mass of 40.9 g (also see Arata 1958, Landers et al. 1980).

This is the only terrestrial tortoise east of the Mississippi River. The only other terrestrial turtle within the geographic range of the gopher tortoise is the eastern box turtle. Box turtles are smaller than gopher tortoises, have a high domed carapace, and a hinged plastron. Box turtles can close their shells completely so that neither the head nor any appendage is visible when retracted completely. Gopher tortoises cannot withdraw completely into their shells. A startled gopher tortoise will withdraw its limbs into the shell, but the limbs are visible when retracted.

Gopher tortoise differs from the desert, Texas, and bolson tortoises (none of which occur in the range of the gopher tortoise) in having relatively smaller hind feet. In the desert, Texas, and bolson tortoises, the distance from the base of the first claw to the base of the fourth claw on the forefoot is approximately equal to the same measurement on the hind foot; in the gopher tortoise, the measurement is smaller on the hind foot (Ernst and Barbour 1989).

Commonly occupies habitats with a well-drained sandy substrate, ample herbaceous vegetation for food, and sunlit areas for nesting (Hallinan 1923, Landers 1980, Landers et al. 1980, Diemer 1989). These habitat types include sandhill (pine-turkey oak), sand pine scrub, xeric hammock, pine flatwoods, dry prairie, coastal grasslands and dunes, and mixed hardwood-pine communities (Landers and Speake 1980, Auffenberg and Franz 1982, Kushlan and Mazzotti 1984, Diemer 1986, 1992a). Prefers open habitats that support a wide variety of herbaceous ground cover vegetation for forage; usually abandons densely canopied areas and frequently can be found in disturbed habitats such as roadsides, fence-rows, old fields, and the edges of overgrown (unburned) uplands (see Diemer 1989, Stewart et al. 1993, Breininger et al. 1994). Upland habitats with extensive canopies reduce the amount of direct sunlight on the ground which may hamper tortoises from reaching minimum thermal requirements for normal daily activities. Also, excessive shade decreases herbaceous vegetation essential for growth, development, and reproduction (Mushinsky and McCoy 1994).

Temporarily abandons marginal habitats during periods of drought; increasing habitat isolation eventually may result in marginal habitats being completely abandoned (Matthews and Moseley 1990). In Georgia, adults congregated on droughty sites in early spring, and many moved to more mesic soils for autumn-winter (McRae et al. 1981).

Densities of gopher tortoises are known to be relatively high in sandhill communities, however, high densities may not be indicative of a healthy population (Mushinsky and McCoy 1994). Mushinsky and McCoy (1994) reported that high densities of some tortoise populations may be the result of tortoises confined to a true or "habitat" island. Tortoises in this situation are unable to move freely to new locations as the quality of the habitat degenerates. More research is needed on the demography of tortoises in confined areas.

Gopher tortoises are highly fossorial and construct extensive burrow systems. They spend much of the time underground. See ecology section for further information on burrows.

Eggs are deposited in a typical flask-shaped nest cavity excavated by the hindlimbs of the female to a depth of about 10-15 cm. Nests may be located in any open sunny area near the burrow of the female, but most often, nests are placed in the spoil mound immediately outside the female's burrow (e.g., Hallinan 1923, Allen and Neil 1951, Arata 1958, Mount 1975, Landers et al. 1980, Butler and Hull 1996).

BURROWING AND BURROW ECOLOGY: Gopher tortoises excavate deep burrows that provide shelter from climatic extremes and refuge from predation. Adult burrows average approximately 4.5 m in length and about 2 m deep (Diemer 1989; see Guyer and Hermann 1997 for information on burrow size and longevity at sites in Georgia and Alabama). Burrows have been found to be significantly shorter in clayey soils than sandy soils which may be a result of respiratory limitations (Ultsch and Anderson 1986); oxygen decrements and carbon dioxide increments were greatest in clayey soils and were positively correlated with burrow length. The high humidity associated with the burrow may offer the tortoise protection from desiccation (Auffenberg and Weaver 1969, Means 1982). At the mouth of each burrow is a mound of subsoil excavated by the burrow resident. Kaczor and Hartnett (1990) found that these soil mounds undergo microsuccession and contribute toward increased plant species diversity in the surrounding habitat.

In northern Florida, Diemer (1992c) found that, on average, adult male tortoises use 5.5 burrows and adult female tortoises use 2.7 burrows per activity season (April-December). In Georgia, tortoises were reported to use 7 and 4 burrows for males and females, respectively (McRae et al. 1981b). Average number of burrows used annually by juvenile tortoises was 1.1 by 0-1- year-olds, 2.2 by 2-year-olds, and 1.7 by 4-5-year-olds in a southern Georgia population (McRae et al. 1981b) and 4.4 by 1-4-year-olds in a central Florida population (Wilson et al. 1994). At the Kennedy Space Center in Florida, Smith et al. (1997) documented the use of individual burrows by several tortoises at different times and occupation of individual burrows by two tortoises at the same time. Suggested reasons for differences in burrow use between populations include differences in ground cover, soil composition, temperature extremes at different latitudes, and number of disturbances to burrows.

Although juvenile tortoises use several burrows they spend most of their time in a primary burrow. Annual use of the primary burrow for juvenile tortoises in a central Florida population was 75% of the use of all burrows (Wilson et al. 1994). The data for estimated use of the primary burrow for adult gopher tortoises are not available. Hatchlings dig burrows (Epperson and Heise 2003) and, in their first year, use multiple burrows (Butler et al. 1995). Several studies have noted that gopher tortoises sometimes use shallow depressions, possibly as resting sites when traveling far from their burrows (Fucigna and Nickerson 1989, Godley 1989, Stout et al. 1989, Diemer 1992c), and windrows, possibly for protection from cattle and machinery (Diemer 1992c). On pine plantations in Alabama, most burrows of juveniles were associated with stumps, fallen logs or tree limbs, or shrub stems, which could interfere with excavation attempts by predators (Aresco 1999). Ashton and Ashton (2001) documented the use of apparently abandoned burrows by juveniles in Citrus County, Florida.

In Northern Florida, Diemer (1992b) studied tortoise populations for several years. She found that the number of burrows showing signs of recent activity increased in April, peaked in July, and remained high through October. The burrow surveys showed a continuous cycle of burrow creation and abandonment. The ratio of captured tortoises to burrows (active and inactive) varied among sites and years; the ratio of burrows to tortoises ranged from 0.45-0.69. Percentages of adult individuals in the three populations studied ranged from 40-62%.

Gopher tortoises desiccate more rapidly when deprived of a burrow than any other member of the genus GOPHERUS (Auffenberg and Weaver 1969). They may withstand relatively high body temperatures (Bogert and Cowles 1947) but froth at the mouth and breathe rapidly when heat stressed. Critical thermal maximum is reported as 43.9 C (Hutchinson et al. 1966).

Many vertebrate and invertebrate species have been recorded from gopher tortoise burrows (Young and Goff 1939, Brode 1959, Hansen 1963, Franz 1986, Jackson and Milstrey 1989, Lips 1991, Witz et al. 1991), including protected species such as the eastern indigo snake (DRYMARCHON CORAIS COUPERI) and the gopher frog (RANA CAPITO) (Auffenberg 1969, 1978; Diemer 1986). Some burrow associates prefer active burrows over inactive or abandoned ones (Lips 1991); these can be distinguished by characteristics of the burrow entrance (Auffenberg and Franz 1982, Cox et al. 1987). Eisenberg (1983) found that 73.7% of gopher frogs censused were found in active tortoise burrows. Witz and coworkers (1991) excavated 1019 burrows and found that of the vertebrate symbionts captured only lizards were found significantly more often in active burrows than in either inactive or abandoned burrows. Fecal material and other organic debris in the enlarged area at the bottom of the burrow serves as an important food source for some burrow associates (Milstrey 1986).

POPULATION ECOLOGY:

Often occurs in more or less isolated colonies of up to about 57 individuals; population density within a colony was estimated at 3-27/ha in Florida (Auffenberg and Franz 1982). Folk (1993) estimated density at about 1-2/ha, 3-5/ha, and 16-25/ha on 3 TNC preserves in Florida. At the Kennedy Space Center, Florida, fall densities ranged from a mean of 2.7/ha in disturbed habitat to 0.0/ha in saw palmetto habitat; spring densities ranged from a mean of 2.5/ha in saw palmetto habitat to 0.7/ha in oak-palmetto habitat (Breininger et al. 1994).

A comprehensive study of about 50 populations of gopher tortoises in Florida (McCoy and Mushinsky 1988) found several trends. Gopher tortoise populations residing on sites that had experienced severe area reduction (greater than 25% reduction over the past 20 years), or occurred on sites with greater than 50% tree canopy, or occurred on sites of small size (< 2 ha), tended to have truncated demographic profiles. A truncated profile suggests little recruitment of individuals into the population and abandonment of the site by larger, mature individuals. In contrast, tortoise populations on sites with no or limited area reduction, or sites with less than 50% tree canopy, or relatively large sites (> 2 ha) tended to have a high proportion of large, mature individuals and evidence of recruitment of young into the population (McCoy and Mushinsky 1988).

Comparisons of tortoise populations on true islands with populations on the mainland suggested that tortoises do respond to relatively small, isolated habitats (Mushinsky and McCoy 1994). Both island and mainland tortoise populations show a positive relationship between the number of active and inactive burrows and the area of habitat. Density of burrows, however, decreased as area increased on the mainland, but density of burrows was not related to area on the islands. Also, on the mainland, the ratio of inactive to active burrows (a measure of the tendency of individuals to construct new burrows) increased with area of habitat, and burrow density increased with increasing herbaceous vegetation, but neither of these relations could be demonstrated on islands. Collectively, these findings suggest that tortoises have a greater selection of habitats on the mainland than on islands. Tortoises on islands are confined and may be forced to live in less than ideal conditions. The implications of these findings are profound for tortoises living in small, fragmented "habitat islands" on the mainland. In time, perhaps a few decades, as the quality of their habitat island is degraded, mature adults may be forced to abandon a site in search of better habitat quality. Such individuals, which may be forced to abandon isolated patches of habitat in areas surrounded by human dwellings seem doomed to perish. From a practical perspective, prior to this study (Mushinsky and McCoy 1994), observation of large numbers of active and inactive gopher tortoise burrows in a confined area likely would have been viewed as indicators of a "healthy" population; however, these findings suggest just the opposite. Rather than a signal of a healthy population, large numbers of active and inactive gopher tortoise burrows, relative to the actual number of tortoises, may signal a stressed population (see also Stewart et al. 1993).

Male tortoises seek females for mating from May to July. There is some evidence that dominant males breed with several females (Douglass 1990). When seeking a female, a male moves to the mouth of a burrow occupied by a female and displays a head bobbing behavior (Auffenberg 1966, Wright 1982). If the female exits her burrow the courting male walks in a circle around the female, periodically stopping and performing the head bobbing behavior. When the female approaches the courting male, he bobs his head violently, and bites her on the forelegs, head, anterior edge of the carapace, and gular projection. The female then backs in a semicircle, stops, and extends her hindlimbs. Thereafter, she rotates her body about 180 degrees, so that her posterior end is near the male's head. The courting male then attempts to mount the female; if unsuccessful, he repeats the courting behavior (Auffenberg 1966, Ernst and Barbour 1972).

Nesting occurs from late April to mid-July (mainly mid-May to mid-June) (Iverson 1980, Landers et al. 1980, Wright 1982, Epperson and Heise 2003)). Clutch size usually is 5-9, averages 3.8 in South Carolina, 5-6 in Florida (e.g., Butler and Hull 1996), 7 in Georgia (see Diemer and Moore 1994), 4.8 in Mississippi (Epperson and Heise 2003). A large female from central Florida produced an unusually large clutch of 25 eggs (Godley 1989). Clutch size increases with increasing female size (Landers et al. 1980, Diemer and Moore 1994). Adult females produce one clutch/year (but some adults do not nest every year). The ovarian cycle was described by Iverson (1980) and Palmer and Guillette (1988). Incubation lasts about 110 days in South Carolina, 80-90 days in northern Florida (Iverson 1980, Landers et al. 1980), mean of 105 days in northeastern Florida (Butler and Hull 1996), mean of 88 dyas in Mississippi (Epperson and Heise 2003). Hatching occurs from August through September. In northeastern Florida, hatchlings emerged from the nest from late August through early October (Butler and Hull 1996). At hatching, and about 24-48 hours prior to emergence, hatchlings exhibit a large external yolk sac (Linley and Mushinsky 1994). The external yolk sac is absorbed as the hatchlings remain in the nest cavity prior to emergence. Just after emergence a deep transverse groove across the plastron is visible; it disappears two to three days after emergence as the anterior- posterior axis of the body becomes straight and the plastron flattens (Ernst and Barbour 1972). This species exhibits temperature-dependent sex determination (Burke et al. 1996, Chelonian Conservation and Biology 2:86-88).

In Georgia, Landers and coworkers (1982) found that pronounced growth occurred through the age of 11 years, after which growth rate gradually decreased. In central Florida, Mushinsky and coworkers (1994) reported an average increase of 18.9 mm/year for ages 1-11, after which time growth slowed to approximately 3%/year until age 20.

Females become sexually mature at a carapace length of 23-24 cm. Body size, rather than age, seems to determine sexual maturity in gopher tortoises. In southern Georgia, it may take from 19-21 years for females to become sexually mature (Landers et al. 1982), while in central Florida females may mature in 9-11 years (Mushinsky et al. 1994). In part, this variation reflects the long activity season available to tortoises in central Florida. In addition to geographic location, however, local conditions also influence the number of years required to achieve sexual maturity. For example, one study of gopher tortoises in central Florida (Godley 1989) found that females attain sexual maturity in 14-16 years, while another study in the same county found that females attain sexual maturity in 9-11 years (Mushinsky et al. 1994). The study area occupied by the faster maturing females was a frequently burned sandhill habitat, whereas the other study area was a mosaic of habitats including pine flatwoods and mixed mesic forests. Males likely mature at a smaller size than females. In north Florida, Diemer and Moore (1994) reported males that were apparently mature at a carapace length of about 18 cm. Potential longevity is several decades.

The level of predation on eggs and young is high. For example, over a two- year period in South Carolina, 17 of 24 (74%) nests were destroyed (Wright 1982). In Georgia, an average female is estimated to produce a successful clutch of eggs (eggs are not destroyed prior to hatching) once a decade (Landers et al. 1980), because about 90% of their nests are destroyed annually. Common predators of eggs are armadillos (DASYPUS NOVEMCINCTUS), raccoons (PROCYON LOTOR), grey foxes (UROCYON CINEREOARGENTEUS), striped skunks (MEPHITIS MEPHITIS), and opossums (DIDELPHIS VIRGINIANUS) (Hallinan 1923, Ernst and Barbour 1972, Douglass and Winegarner 1977, Landers et al. 1980). Hatchling gopher tortoises (individuals in their first year of life) also are subjected to high levels of predation. From egg laying to one year of age, gopher tortoises in northern Florida were estimated to have a mortality rate of 94.2% (Alford 1980). Results from another study in central Florida, which also combined mortality of eggs and hatchlings, suggested an annual mortality rate of 92.3% (Witz et al. 1992). Estimated rates of survivorship of juvenile gopher tortoises (age 1 to 4 years) have been reported from one location in central Florida (Wilson 1991). Wilson (1991) found that predation of juvenile tortoises was higher in October-November and April-May than any other two month interval of the year. Juvenile tortoises are known to bask at the mouths of their burrows more often in the spring and fall of the year than during the summer or winter months (Wilson et al. 1994). It appears that a juvenile tortoise, when positioned at the mouth of the burrow to thermoregulate during the cool months of the year may be quite vulnerable to predation by avian and mammalian predators (Wilson 1991, see also Fitzpatrick and Woolfenden 1978).