34 - Investigating Limb Integument, Osteology, and Myology of the Aldabra Giant Tortoise (Aldabrachelys Gigantea)

Poster Board Number: 34
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Abstract Body : The Aldabra tortoise, Aldabrachelys gigantea, endemic to the Aldabra Atoll of the Seychelles Islands, is the second-largest tortoise species in the world. These giant tortoises can weigh around 250 kg and live more than 150 years. At great sizes and impressive lifespans, the Aldabra likely requires certain adaptations of the integument and musculoskeletal system to facilitate adequate external protection and efficient locomotion of their impressive body masses. The objective of this study was to investigate the functional anatomical adaptations of the integument and musculoskeletal anatomy of the hindlimb apparatus of A. gigantea in comparison to other smaller tortoise species (Centrochelys sulcata and Kinixys erosa).

In this study, comparative anatomical dissections were completed on specimens from three different tortoise species, A. gigantea, C. sulcata, and K. erosa, taking particular note of the muscular, tendinous, and ligamentous relationships in distinct regions of the hindlimb: the thigh, stifle joint, and crus. In the pelvis, the pronounced and elongated lateral processes of the A. gigantea pubis are well appreciated and allow for greater muscular attachments to this region to facilitate stronger movements. Many thigh muscles were identified and found to be consistent with published anatomical references on sea turtles and pond turtles; however, certain muscles in A. gigantea displayed distinct morphologies and were supported by impressive, thick intermuscular septa that were not present in any other dissected specimen or previously documented taxon. In particular, the bulbous muscle belly and focal fibular insertion of m. iliofibularis, homologous to mammalian m. biceps femoris, differed from the pattern observed C. sulcata and K. erosa which had broad and wide insertions along the lateral fibula creating more band-shaped muscle bellies. The predominant function of m. iliofibularis is flexion of the crus, and the focal point of insertion is likely linked to finer control of this muscle action during the forward propelling step of locomotion. In the stifle, no true patella bone or unossified component was appreciated in any tortoise specimen. However, the supporting “patellar” tendon and cruciate and collateral ligaments were observed, suggesting a lack of profound stifle extension during locomotion in any of the tortoise species. As a result, the tendons and ligaments associated with the stifle joint are adapted for knee stability during locomotion, but a limited degree of extension. The m. extensor digitorum communis is the largest extensor of the crus and both its origin and insertion are broad in nature. The osteoderm pattern of A. gigantea revealed the largest osteoderms located on the most caudal region of the pes plantar surface and the most distal regions of the crus. This pattern can likely be explained by the inability of A. gigantea to fully tuck its large pelvic limbs into its shell, unlike smaller tortoise species such as K. erosa, thus serving as a protective mechanism for those vulnerable regions. Ultimately, the results of this comparative tortoise hindlimb study contains useful knowledge to anatomists to highlight interspecies differences and the functional anatomical variation between the locomotor apparatus of aquatic turtles and terrestrial tortoises. Further, the diagnostic imaging and anatomical descriptions are useful for exotic and zoo veterinarians managing tortoise patients with musculoskeletal or integument injuries.