Righting time versus shell size and shape dimorphism in adult Hermann’s tortoises: Field observations meet theoretical predictions

Abstract

We investigated the relation between righting time (RT) and carapace morphology in 303 adult Hermann’s tortoises (Testudo hermanni Gmelin, 1789) from two geographically close localities. Their size, shape and body mass, adjusted for size, were significantly different between males and females. Righting time differed among the populations and was related to the ambient temperature and the relative body mass and carapace shape. However, analysis showed that the impact of carapace “form” (shape plus relative body mass) alone had only moderate influence on the variation in righting time (“function”). Both “form” and “function” did not contribute much to the segregation of individuals in geometric space, based on either sex or locality. An interesting detail was that tortoises with a height/width ratio of the shell contour higher than 0.75 had quite a short righting time (less than 100 seconds), which is in accordance with the proposed theoretical model of energy balance of righting in chelonians. We suppose that interactions between general carapace “form”, specific components of shell structure, physiological parameters and local environments shape variation in righting response in this species. An interdisciplinary approach combining geometric modeling with traditional biological disciplines would be needed to support this hypothesis.