MODERN TURTLE TRACKS

Several specimens of large chelonid Galapagos tortoises (Geochelone elephantopus) were made available by the Philadelphia Zoo (Figure 4, Figure 5). These animals were in an outdoor pen with broad regions of sandy substrate or fine silty substrate. The moisture level in the substrate varied from dry to saturated. The tortoises were near adult proportions, weighing approximately 130 – 175 kg and had a range in carapace length from approximately 0.75 – 0.84 m and widths ranging from 0.50 to 0.60 m.

Sections of substrate were cleaned of previous tracks with a broom. Tortoises were encouraged to walk across the cleaned surfaces and the resulting tracks were photographed. Tortoises were also encouraged to change gait from a normal walk to a brisk walk by a gentle tapping of a broom handle on the carapace. Such tapping typically increased the pace of the turtles’ walk several fold. Track morphology of the footprints left by the tortoises varied significantly. The variation was largely controlled by the speed of the turtle and to a lesser extent by the substrate composition.

Sandy Substrate Tracks

Tracks in the drier sandy substrate were largely indistinct, slight depressions in the sand. No claw marks were observed in the tracks made by tortoises walking a normal pace across this substrate. When the pace of the tortoise increased several fold, the tracks became widely spaced and were distinct as sets of long grooves made by the claws of each foot of the turtle sweeping across the sand (Figure 6). When walking across a wet sandy substrate, the tracks were well defined, circular to elliptical in shape, and often had a raised lip around the edge (Figure 7). Claw impressions were often seen in these wet sandy tracks, but the claw marks were commonly subtle in definition and typically not all claws left impressions (Figure 7).

Muddy Substrate Tracks

No tracks were observed on a dry silty substrate. Tracks made by tortoises walking at a normal gait on a wet silty substrate were well-defined circular to elliptical depressions 10 to 20 mm deep (Figure 8). As with the tracks made in a sandy substrate, these tracks also exhibited a raised lip. The lips observed on these tracks made on the fine-grained substrate, however, were much wider and rounded in comparison to those tracks observed in a sandy substrate. Claw marks were sometimes evident on these tracks. As the pace of the tortoise increased, the tracks became widely spaced with sets of long grooves made by the claws of each foot sweeping across the mud (Figure 9).