Use of sun compass orientation during natal dispersal in Blanding’s turtles: in situ field experiments with clock-shifting and disruption of magnetoreception

Abstract

When hatchling freshwater turtles emerge from nests and first experience environmental stimuli, they primarily use visual cues to disperse toward nearby open horizons or far dark horizons. Within hours, hatchling Blanding’s turtles (Emydoidea blandingii) develop compass mechanisms to maintain their headings when the initial dispersal targets become invisible. We captured hatchling Blanding’s turtles during dispersal in natural habitat, attached a magnetic or non-magnet disk to each turtle, translocated them into an arena in a visually uniform field of corn, and measured their compass orientation (heading). Hatchlings from the magnet and no-magnet groups were evenly divided into two environmental chambers (6-h clock-shift or non-clock-shift) to create four experimental treatments. After 5 to 11 days hatchlings were re-released in the arena. If hatchlings used a time-compensated sun compass, then clock-shifting would cause a ~90° change in heading. If they used a geomagnetic compass, disruption of magnetoreception would cause wandering. If both compasses were used simultaneously or sequentially, we predicted a range of outcomes. All four treatment groups dispersed directionally during both trials, maintaining their prior headings in natural habitat except for the clock-shift treatment which changed heading ~90° as predicted. The ability of hatchlings to maintain prior headings despite the disruption of geomagnetism supports the absence, inactivity, or disregard of a geomagnetic compass. Only a time-compensated sun compass coupled with an internal clock was necessary and sufficient for hatchlings to maintain compass headings during natal dispersal when relocated from natural prairie habitat to a monoculture habitat with a relatively uniform visual horizon. Individuals moving long distances (e.g., migrating birds or dispersing salamanders) often maintain their movement headings using compass orientation (i.e., a geomagnetic or time-compensated sun compass). When hatchling freshwater turtles emerge from underground terrestrial nests they initiate dispersal in search of wetlands primarily based on visual horizon cues. Because such cues often become obscured by uneven terrain or dense vegetation and because straight-line travel is more efficient and less risky, hatchlings soon develop a mechanism of compass orientation to maintain their dispersal headings. We disrupted their ability to use magnetoreception by attaching magnets to their shells, manipulated their sense of time-of-day with clock-shifting, and then monitored their dispersal in a visually uniform field of mature corn. Our results demonstrated that a sun compass was necessary and sufficient for hatchlings with dispersal experience to maintain their dispersal headings when natural environmental cues were not visible.