Abstract
Simple SummaryWhen turned upside down, terrestrial turtles have no active control on self-righting if the animal has been flipped over. Turtles belonging to Testudo genus present high-domed carapaces and short neck and legs. Using geometric morphometric techniques, we studied left-right frontal symmetry among some Testudo species to study if carapaces’ geometry may serve as the tool to roll right side up. A clear right directionality was detected in the studied sample. This fact. more that easing the self-righting potential (“kinematic instability”, understood as the ability to self-right without effort), would make the stable ventral turning difficult (“static stability”, understood as the ability to resist passively turning of the body produced by destabilizing forces).The aim of this research was to contribute to the study of the doming geometry of Testudo carapace as an unstable point of equilibrium when animals are overturned. We performed this research using geometric morphometric using a sample of 64 Testudo individuals belonging to different species (T. hermanni n = 30, T. graeca n = 3, T. marginata n = 13 and T. horsfieldii n = 18), sexes and ages. A set of four sagittal landmarks (discrete homologous points) and 15 pairs of semi-landmarks, on the frontal doming of the carapace, were digitized on individual carapace pictures. Significative fluctuating asymmetry was detected, defined as small, completely random departures from bilateral symmetry, but much less than directional asymmetry, which appeared highly significative. Anti-symmetry did not appear. Carapace asymmetry was dominated by a clear right directionality. A possible biological speculation could be that this asymmetry more that easing the self-righting potential (“kinematic instability”, understood as the ability to self-right without effort), makes stable ventral turning difficult (“static stability”, understood as the ability to resist passively turning the body produced by destabilizing forces). This asymmetry is present among both sexes but more marked among males. An explanation for this sexually differentiated pattern could be the higher locomotion and the fight for mating in males, making them consequently more prone to losing their balance and falling on their back. These data may be useful in studying adaptative traits in Testudo species as well as establishing a seminal base for future studies. This research is the first attempt to explore a suitable method to assess doming asymmetry which could be useful in future, more extensive investigations, on a larger interspecific sample.
Highlights
Turtles have an anatomy dominated by the shell, a structure derived from elements of the axial skeleton [1]
As carapace asymmetry did not increase with size, it is assumed that turtle shells do not become increasingly asymmetrical with age
A possible speculative biological explanation for the detected asymmetrical pattern in Testudo carapace is that this asymmetry makes turning difficult (“static stability”, understood as the ability to resist passively turning of the body produced by destabilizing forces that damp stability forces), rather than having an effect on the easiness of self-righting potential (“kinematic instability”)
Summary
Turtles have an anatomy dominated by the shell, a structure derived from elements of the axial skeleton [1]. A shell is composed of the carapace (dorsal elements), and the plastron (ventral plate), enclosing the locomotor elements in a situation unique among tetrapods [1]. Terrestrial turtles can self-right [2] but with their short limbs and neck, they have no active control on self-righting if the animal has been flipped over. A carapace’s geometry may serve as the tool to roll right-side up [2,3]. This must be especially important for species that have high-domed carapaces and short legs. If a domed carapace is not dorsally symmetrical, it seems that this useful instability would help animals in self-righting, so asymmetrical dome-backed tortoises practically would roll back into position almost passively. Much of the literature has been devoted to documenting specific variations of shell [4,5,6,7,8], but there is a scarcity of studies that explicitly test the carapace form from this functional point of view [9,10]
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