Literaturdatenbank
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Jackson, D. R., & Ewert, M. A. , Nesting ecology of the alligator snapping turtle (macrochelys temminckii) along the lower apalachicola river, florida, with special reference to hatchling sex-determination. Paper presented at Turtle Survival Alliance 2006 Annual Meeting.
Added by: Admin (13 Dec 2008 16:50:12 UTC) |
| Resource type: Proceedings Article BibTeX citation key: Jackson2006 View all bibliographic details  |
Categories: General Keywords: Chelydridae, Fortpflanzung = reproduction, Habitat = habitat, Macrochelys, Macrochelys temminckii, Nordamerika = North America, Schildkröten = turtles + tortoises, Zeitigung = incubation Creators: Ewert, Jackson Collection: Turtle Survival Alliance 2006 Annual Meeting |
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| Abstract |
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Macrochelys temminckii The alligator snapping turtle, a species with temperature-dependent sex determination, nests along the banks of the lower Apalachicola River, a site of extremely high chelonian diversity on Florida=s Gulf coast. Building from 8 years of observations initiated during the 1970s, we conducted an expanded two-season (1990-1991) field and laboratory study on the distribution and dynamics of the nests. We located 105 nests (63 intact, 42 depredated), 76 of which were in a more intensively surveyed core area of 7.5 km along the Apalachicola mainstem. Eight-three percent of discovered nests occurred on prominent dredged spoil substrates deposited during the early years of channel maintenance for barge traffic (current regulations restrict dumping of spoil to within river banks). For all years of observation, the mean clutch size of 35.1 eggs (range 17-52, n = 130) has remained relatively stable and appears to represent the natural value for a largely undisturbed population. We estimate nesting to occur between 20 April and 11 May but to vary annually by about a week within this period (hence only a single clutch per female). Hatching extends throughout August. In protected nests, hatching success averaged 78% in 1990 (n = 24) and 66% in 1991 (n = 26). The lower rate in 1991 corresponded to abnormally wet conditions that likely led to asphyxiation of some eggs. Estimates of sex ratios in individual nests (derived from samples of six hatchlings) varied from all males to all females. However, overall sex ratios were roughly one male: two females in both years (32.5% male in 1990, 34.8% in 1991). This bias toward females is associated with nesting on the relatively open mounds and beaches of dredged spoil. These sites provide warmer environments for incubation than the more heavily wooded natural levees and consequently produce mostly to exclusively females. Availability of such open sites in presettlement times was probably more limited. Thus, proposals to restore the river floodplain by reducing or eliminating old spoil deposits may affect long-term demographics of this population. Laboratory incubation of eggs elucidated a complex pattern of sex determination. Although warm (29-30 C) temperatures yield exclusively females, all other constant incubation temperatures allow both sexes to develop. However, most hatchlings (59-69%) from 25-27 C are male, whereas most hatchlings from low temperatures (21.5-22.5 C) are female. Thus, two hypothetical pivotal temperatures exist at 24.5 C and 27.2 C, where 1:1 sex ratios are predicted. The apparent production of some all-male clutches in the field can not be explained by constant-temperature laboratory results, which produced at most 2.2 males per female. However, such results were nearly duplicated in one test clutch that produced 97% males (36:1) when incubated in step-shifts of three constant temperatures (24 - 25 - 27 C) that may more closely mimic the regimen experienced by some natural nests, but these results were not duplicated in a larger sample drawn from many clutches. Our data suggest the existence of a temperature response polymorphism, in which some embryos in some clutches might be destined to become females at the same temperatures that produce nearly all males in other clutches.
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