CONICET | Buscador de Institutos y Recursos Humanos

As in many other fossorial tetrapods, the most obvious adaptations to scratch-digging in the subterranean tuco-tuco (Rodentia, Ctenomyidae, Ctenomys) are found in the hands, which among other adaptations, present the mesaxonic condition; i.e., the central digits are more developed, and also their claws, which are curved and elongated. The thumb is atrophied and aligned with the rest of the digits, showing a small and flat claw. This configuration of digits and claws seems to be in accordance with which it is expected for rodents: rudimentary movements when handling food items. However, on the palmar side of the hand, tuco-tucos have several pads, being the thenar (located under the thumb) the most developed. In this study we investigated the functional morphology of the thenar pad (TP) through different approaches: osteo-musculoskeletal anatomy, histology and functionality. The analysis of radiographies and clarified and double-stained samples of C. talarum and C. australis hands showed that the TP would be supported by two bony elements separated by a synostosis. The proximal element (elongated and rectangular) articulates proximally with the scapholunar and distally with the distal element (short and conical). Dissections showed that the TP have several associated muscles: the palmaris longus, the abductor pollicis longus, and a massive muscular complex located between the thumb and the TP. By topology it might be inferred that this complex is formed by the abductor pollicis brevis, the pollicis brevis and the adductor pollicis brevis. Longitudinal histological sections of the TP stained with hematoxylin-eosin showed a thick layer of keratin at the distal end, external face, corresponding to the area of the claw in a true digit. The observation of live specimens of C. talarum foraging on two food items of different size and filmed at 300 fps showed that the TP acts as an opposable thumb, with true digit-like movements. Tuco-tucos are able to perform more precise movements than expected, and to grasp and manipulate the food with one hand. In previous studies, it was suggested that the TP was supported by a radial sesamoid bone (= palmar ossicle, = falciform bone, = prepollex). However, a more detailed analysis at the anatomy of this structure in tuco-tucos allows us to propose that its identity as a sesamoid bone is not direct. Our data on the TP skeleton, the muscles, the tendon insertions and the epithelium suggest a much more complex anatomical structure than would be expected for a sesamoid bone. In addition, the results of this study suggest that the TP would actually be a supernumerary digit, with the capacity to manipulate food items. It is suggested to advance on its description and functional analysis, redefining the structure, since the terms used so far to define this structure would not be accurate.