We examined the orientation of 76 naïve painted turtles (Chrysemys picta belli) and 746 snapping turtles (Chelydra serpentina) during initial dispersal from experimental nests in the Weaver Dunes area of southeastern Minnesota. We conducted 15 releases into large circular arenas in 4 natural nesting areas and 2 atypical areas. Hatchling orientation and dispersal for both species were 1) all nonrandom, 2) appeared to be based on vision (i.e., nonpolarized light), and 3) toward nearby, open, and highly illuminated horizons, regardless of whether or not they were associated with the wetlands. A first-order estimate of hatchling snapping turtle perception distance was 55–90 m. We found no evidence that suggests that specular light from the wetlands, olfaction, or humidity gradients were important in orientation. At 2 of 3 locations, substantial changes in orientation direction occurred when hatchling snapping turtles were released in morning vs. late afternoon. Changes in dispersal directions in the morning and afternoon indicated that hatchlings were not orienting toward the sun per se but toward different highly illuminated nearby prairie areas. At one site, hatchling orientation in the afternoon (but not in the morning) was toward a nearby wetland and was consistent with either dispersal toward highly illuminated near horizon or with the perception and use of reflected polarized light from the wetland. Collectively, the results from our study also indicate that 1) hatchlings disperse toward open horizons rather than toward wetlands themselves (i.e., open areas that are not necessarily associated with wetlands), 2) dispersal direction is influenced by time of day, apparently because of changes in the degree of illumination of different horizons, and 3) far horizons apparently were not used because they were beyond the perception distance of hatchlings. The most parsimonious evolutionary explanation of solutions to orientation problems is that, for each species, both adults and hatchlings have similar perception distances and use the same sensory modes and types of environmental cues during terrestrial movements. Comprehensive conservation and management plans for aquatic turtles should include consideration of how habitat changes in nesting areas might alter the environmental cues that determine the initial orientation and successful dispersal of hatchlings. We compared the results from this study with the dispersal patterns of naïve hatchling Blanding's turtles (Emydoidea blandingii) that emerge from nests located much farther from wetlands.
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Vol. 10 • No. 1