Orientation in Five Species of Hatchling River Turtles Dispersing from Experimental Nests

Hatchling river turtles (Smooth Softshell Turtles, Apalone mutica; Spiny Softshell Turtles, Apalone spinifera; Northern Map Turtles, Graptemys geographica; False Map Turtles, Graptemys pseudogeographica; and Ouachita Map Turtles, Graptemys ouachitensis) were released in a variety of settings to help place orientation and dispersal from nests in the context of nest site selection by females and juvenile recruitment habitat. Visual cues associated with near open or far dark horizons were the primary environmental cues used during initial orientation and dispersal of the hatchling river turtles. On a river beach, hatchlings of both species of Softshell Turtles dispersed toward the open horizon of the nearby river. In contrast, hatchlings of all 3 species of Map Turtles on the same beach dispersed toward the near dark horizons of a forest that led them away from the river. Hatchling Map Turtles of all 3 species released in autumn and Northern Map Turtles also released in spring at a field arena dispersed toward near dark horizons of pine (Pinus spp.) and deciduous trees to the north and south of the arena (directions that were parallel to the Mississippi River). At a site in upland prairie habitat with no nearby wetlands, hatchlings of all 3 species of Map Turtles dispersed to the north toward near dark horizons of an oak (Quercus spp.) forest rather than toward more distant dark horizons of mixed oak and pine trees. At a lowland prairie site where no near dark horizon was visible, Northern Map Turtles dispersed toward 2 far dark horizons that were ∼ 230 m to the north (a pond surrounded with trees) and to the south (an area of large deciduous trees at the west end of a windrow of pine trees), but not toward the large riparian wetland 280 m to the west. The bimodal dispersal pattern toward 2 equidistant dark horizons within 230 m but not toward the riparian area 280 m to the west suggests that the perception distance for hatchling Northern Map Turtles is between 230 and 280 m. Dispersal of a combined sample of naïve hatchling False Map and Ouachita Map Turtles released in a mature corn field was not different from random, but the directions taken by the majority of hatchlings were to the north and south across corn rows that may be the closest match to dark horizons used for dispersal in typical habitats.