Legler, 1959
Bolson tortoise
Recognition
The oblong carapace (to 40 cm; but fossils may have reached 1 m, Morafka et al., 1989) is rather flat topped, low arched, widest behind the center, and lacks a cervical indentation. The posterior marginals are flared and slightly serrate. The cervical scute is about as broad as long. Vertebrals are broader than long; the 1st is usually the narrowest, the 3rd broadest, and the 5th laterally expanded. Well-defined growth annuli surround the slightly raised vertebral and pleural areolae, but become worn smooth with age. There are usually 11 marginals on each side, and the single supracaudal scute is undivided and downturned. The carapace varies from pale greenish yellow or lemon yellow to straw colored or brown. Carapacial areolae are dark brown or black; lateral marginals contain much yellow pigment and are always paler than the rest of the carapace. Some individuals may also have some dark radial stripes on the carapaces. The plastron is well-developed and covers most of the carapacial opening. Its forelobe is as broad as or slightly narrower than the hindlobe, which bears a well-defined posterior notch. The plastral formula is: abd > gul > fem >< hum > pect >< an. The paired gulars are thickened and may project beyond the carapacial rim, especially in males. The bridge is broad with a large axillary scute and one or two inguinals. The plastron is yellow with a dark-brown or black blotch on each scute; these blotches fade with age. The head is broad to moderate with a nonprojecting snout, and the upper jaw may be slightly hooked. The scales on the top of the head are large and irregular; the prefrontal is longitudinally divided, and the following frontal scale may also be subdivided. The iris is yellow to brown. Head, limbs, and tail are yellow to brown; the jaws are tan to brown. The anterior foreleg surface is covered with large, slightly overlapping, brown-centered scales in seven or eight longitudinal rows. The hindlimbs contain two large dark-tipped spurs on each thigh.
Diploid chromosomes number 52 (26 macrochromosomes and 26 microchromosomes): 20 metacentric and submetacentric, 10 subtelocentric, and 22 acrocentric and telocentric (Stock, 1972; Dowler and Bickham, 1982).
Sexual dimorphism is inconspicuous, though, generally, males are slightly smaller than females, have slightly more concave plastrons, longer more projecting gular scutes, more flare to the carapace marginals, larger better developed integumentary chin glands, and slightly longer, thicker tails.
Distribution
Gopherus flavomarginatus is restricted in north central Mexico almost entirely to the valleys and low foothills of the internally drained basin (Bolson de Mapimi) which borders on southeastern Chihuahua, southwestern Coahuila, and northeastern Durango, Mexico, at elevations from 1000 to 1300 m.
Geographic Variation
Morafka (in Bury, 1982) reported that individuals from Durango have much more yellow on the carapace and much less sexual dimorphism in plastral concavity than tortoises from the Sierra del Diablo, Chihuahua.
Habitat
The area is a rather xeric steppe to desert with low humidity and rainfall, and wide temperature fluctuations; the soil is usually sandy. Vegetation includes grass (Hilaria mutica is virtually always present; other genera present: Aristidia, Bouteloua, Scleropogon, Sporobolus, Tridens) and dry shrubs, dominated by the creosote bush (Larrea divaricata) and mesquite (Prosopis juliaflora), but tarbush (Flourensia cernua) and quayule (Parthenium incanum) are also present.
Natural History
Sexual maturity probably requires 15-20 years (12-17 years; according to Germano, 1994), and individuals larger than 25 cm are regarded capable of reproduction (Morafka et al., 1989). Germano (1994) reported that the smallest reproducing captive female was 28.5 cm. The male and female gametic cycles have not been described.
Courtship and mating take place from April through August. Species and sex recognition is by head bobbing. Males trail the females and mount immediately when the females are caught. Females may also initiate courtship and pursue males.
Nesting occurs from April to September, with a peak in June. Egg deposition in captives has been in a hole about 20 cm deep, outside but near the entrance of the burrow, and captive females have become aggressive toward humans at these nest sites.
As many as three clutches (average 1.3) may be oviposited a year (Morafka et al., 1989). Typically clutches hold 5-6 (3-9) white, brittle, almost spherical eggs (Morafka et al., 1989; Germano, 1994). Seven of eight clutches are destroyed by predators each year, so an average female may produce only 3.4 young over an eight year period (Morafka et al., 1989). With probable survivorship to adulthood (20 years) of less than 5%, replacement of an adult may require 50 years. Hatchlings emerge from late July to October in 75-100 days; they typically have a carapace length of 45-70 mm.
Like other Gopherus, the species is herbivorous; 21 plant species have been identified as food items (Morafka et al., 1989). Grasses comprise 64% of the diet, shrubs 14%, and annual herbs 5%. The most important species in the diet (dry weight percentage) are: Bouteloua barbata (22.7%), Hilaria mutica (15.2%), Tridens pulchellus (14.9%), Sida leporosa (13.5%), Sphaeralcea angustifolia (9.8%) and Chloris virgata (5%). Bouteloua is the most important food during the late summer, and Hilaria and Sida are the main foods during the rest of the year.
Bolson tortoises, like their relative, Gopherus polyphemus, are burrowers, digging a single-entranced hole which slopes steeply downward to a depth of 1.5 to 2.5 m and a length of up to 10 m (Morafka, in Bury, 1982). Here they may aestivate for periods in summer, and hibernate during cold winters (Lieberman and Morafka, 1988). Each tortoise, particularly hatchlings, excavates multiple burrow and pallet sites in the wet season, but uses only one burrow during the dry season (Tom, 1994). Regardless of the season, hatchlings prefer to excavate or opportunistically inhabit burrows under prickly pear cactus (Opuntia rastrera).
IUCN Red List Status (1996)
Vulnerable (A1d, B1+2bc, C1). Since populations are relatively small and mostly adult, and their habitat is one which has been slowly drying, G. flavomarginatus is considered endangered (Bury et al., 1988).