Sexual dimorphism of body size and shell shape in European tortoises

Abstract

Adult body size and shape were examined in almost 1400 individuals of the tortoises Testudo graeca, T. hermanni and T. marginata from Greece. The size at maturity was greater in females than in males in all three species. Maximum and mean adult sizes were also greater in females than in males in T. graeca and T. hermanni. Males grew to a larger size than females in T. marginata, and mean adult size was similar in the sexes in this species. Sexual dimorphism of shape (adjusted for size covariate) was shown in most of the characters examined, and the degree of this dimorphism differed significantly among the three species. Differences were related to their contrasting courtship behaviours: horizontal head movements and severe biting in T. marginata, vertical head bobs and carapace butting in T. graeca, and mounting and tail thrusting in T. hermanni. There was no difference in the frequency of observations of courtship or fighting among the three species, but courtship was about 10 times more common than combat in males. All species showed greatest courtship activity in autumn; copulation was rarely observed in T. hermanni (only 0.36% of courting males) and not seen in the other species in the field. Observations made throughout the activity season indicated that feeding was equally common in males and females in all three species. Differences in shape were more likely to be the result of sexual selection than of natural selection for fecundity. Detailed predictions are made for sexual dimorphism of other characters in these species.

Morphometric Data of Testudo Hermanni in the Vlora Area

“…The large marginal bottom plate of males is often curved down. The shell form of females is simpler: the lower part of the carapace is narrower than the middle of the shell and the marginal plate almost touches the ground [10]. Changes in the shape and construction of the shell are noticed, and do not appear equally in both genders.…”

“…Gender was determined on the basis of external characteristics. They were distinguished by external shape of the skeleton, the shape of the rear lateral scute, width at anal and supracaudal level, shape and size of the tail [3,10,11]. Adult animals are considered as 13 cm in size for males and over 15 cm for females [10].…”

Sexual Dimorphism in the Greek Tortoise: A Test of the Body Shape Hypothesis

“…Many causal explanations have been suggested for the differences in shell shape between males and females. For example, it has been suggested that females need to develop more voluminous (and hence more spherical) shells using a given amount of shell material to increase clutch size (Brophy, 2006;Tucker et al, 1998;Willemsen and Hailey, 2003). Similarly, the flatter shells and larger limb apertures of males have been explained as adaptations to facilitate locomotion for dispersal or mate searching, and the concave plastra of terrestrial males have been suggested to increase stability during mating (Pritchard, 2008).…”

Geometry and self-righting of turtles

“…Both righting behaviour (Ashe 1970;Wassersug & Izumi-Kurotani 1993;Rivera et al 2004;Stancher et al 2006) and the evolution of shell morphology (Rouault & Blanc 1978;Claude et al 2003;Myers et al 2006) of turtles have been studied recently. An example of their interaction is the sexual dimorphism of species where males are often overturned during combats (Bonnet et al 2001;Willemsen & Hailey 2003;Mann et al 2006), and their shell has adapted to facilitate righting. Here we develop a geometric shell model based on field data to uncover systematically the connections between righting strategies and turtle shell morphology.…”