Sexual dimorphism in steppe tortoises (Testudo horsfieldii): influence of the environment and sexual selection on body shape and mobility

Abstract

Selective forces shape sexes differently, with male body proportions facing strong selection to enhance mate searching and male-to-male combat traits, and female fitness being influenced by the ability to assimilate large amounts of nutrients necessary for vitellogenesis (and/or gestation), and their ability to carry the eggs or embryos. We evaluated the sexual dimorphism of body proportion of more than 800 wild steppe tortoises (Testudo horsfieldii) in Uzbekistan. The thick, well-developed shell offers protection from predators but pronounced digging habits probably also constrain body shape (e.g. a shell that is dorso-ventrally flattened, although round from a dorsal view helps to penetrate into, and move within the soil). Thus, in this species, natural selection might favour a heavy and flat shell that is 'closed' with small openings for appendages. In males, these environmental influences appear to be countered by sexual selection. Compared to females, they weigh less (absolutely and relative to shell dimensions), have longer legs, have shell structure allowing wider movements for their legs, and they walk faster. Males were also able to right themselves more quickly than females did in experimental tests. This quick righting ability is critical because intra-sexual combats frequently result in males being flipped onto their backs and becoming prone to hyperthermia or predation. Females are heavily built, with wide shells (relative to male shells), which may provide space for carrying eggs. From our results, a number of simple hypotheses can be tested on a wide range of chelonian species.

Ontogenetic shift of antipredator behaviour in Hermann’s tortoises

“…When tested regardless of body size, only few morphological traits (MaxCW, ANW and ASJ) had an effect on male RT. This result came as a surprise since some morphological traits, such as bridge length and shell height, were previously recognized to have an effect on self-righting (Bonnet et al 2001;Zuffi and Plaitano 2007;Golubović et al 2013a) but were not confirmed by this study.…”

“…Males can also lose their equilibrium while mounting a female or during combats with rivals, which consequently can result with better self-righting performances in males (Bonnet et al 2001;Golubović et al 2013a). Overturned on their backs, tortoises are not able to quickly escape, since they have to go through a self-righting process first.…”

“…Several studies of tortoise self-righting performance found considerable effects of morphology (Bonnet et al 2001;Golubović et al 2013aGolubović et al , 2015Stojadinović et al 2013). Therefore, I considered functional-morphological aspects on this stage of antipredator behaviour.…”

“…When overturned on its dorsum, which can easily occur when attacked by a predator, a tortoise/turtle perceives a situation as highly threatening Ibáñez et al 2014). Furthermore, the righting performance itself is quite challenging for tortoises due to morphological constraintsrelatively short limbs and a rigid armour that precludes flexion of the torso (Bonnet et al 2001;Zuffi and Plaitano 2007;Golubović et al 2013aGolubović et al , 2015.…”

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.…”