Homeward orientation mechanisms in three species of turtles (Trionyx spinifer, Chrysemys picta, and Terrapene carolina)

Abstract

1. Adults of three species of turtles (Trionyx spinifer, Chrysemys picta and Terrapene carolina) were tested in a variety of situations in order to establish the relative orientation abilities and the mechanisms utilized by each. Following capture, displacement, and release, individuals of all three species exhibited a significant degree of orientation in the homeward direction when tested in a large circular terrestrial arena on clear days (Fig. 1 and Table 1). This ability was not evident when the animals were tested on overcast days. Additionally it was demonstrated that the orienting behavior was contingent upon solar cues in conjunction with an internal biological clock. When tested on clear moonless nights, on nights when the moon was fully visible, and immediately after sunset (when the plane of polarized light is most pronounced directly overhead) (Fig. 2 and Table 2), random orientation responses were obtained for C. picta and T. carolina. Trionyx spinifer, however, failed to move from the release sites in any of these three test situations (Table 2). 2. Individuals of all three species exhibited a positive geotaxis at a 2.5° incline (Fig. 3 and Table 3). Conflicting geotactic and celestial cues were then juxtaposed in order to determine the minimum angle of inclination required to overcome the animal's motivation to orient in the homeward direction. This minimum angle was found to be 10.0° for C. picta, 7.5° for T. carolina, and 2.5° for T. spinifer (Fig. 4 and Table 3). 3. The results of this study demonstrate that C. picta, T. carolina, and T. spinifer possess Type III (true navigation) orientation ability.