Antipredator response and agility in terrestrial and aquatic Chelonians - steady as a rock or running back to water

Abstract

The hape of Chelonian’s shell, i.e. the length of bony bridges between plastron and carapace, physically limits movements and thus affects agility (Bonnet et al. 2001). Besides these function-morphological restrictions, behavioural characteristics may additionally influence mobility. To test this hypothesis, I compared speed and efficiency of European pond turtles (Emys orbicularis) and Hermann’s tortoises (Testudo hermanni) in two field experiments: (1) self righting when overturned on the back and (2) releasing from a rope stuck on front part of their plastron, which mimics coping with vegetation. Both tests represent situations with which these animals deal in everyday movements. European pond turtles were faster and more efficient in both tests. They primarily used their long necks as a lever to flip back on legs, while Hermann’s tortoises relied on vigorous legs movements (Domokos & Varkonyi 2008). Additionally, European pond turtles were bolder to start moving shortly after handling. These findings are in accordance with antipredator tactics observed in these two species. When close to water, E. orbicularis promptly dive in at first sign of potential areal or terrestrial predator (Martín et al. 2005). On land, further from water body, E. orbicularis keeps the swiftness of reaction and does not rely on hiding inside the shell as much as T. hermanni. Adult European pond turtles are more prone to terrestrial predators such as dogs and boars, due to flat shell and narrow bridges, compared to adult tortoises with more domed and enclosed shells (Dosik & Stayton 2016).