Muscle architecture and functional anatomy of the extrinsic thoracic limb muscles in a semiarboreal xenarthran (Tamandua mexicana)

Abstract

Tamandua mexicana is a mammal belonging to Xenarthrans, with high interest in medicine, conservation, and evolutionary studies. It has semiarboreal locomotion with specific adaptations to allow its movement on different substrates. To carry out this type of locomotion, the extrinsic muscles that allow scapular stabilization as well as retraction and protraction movements are essential. Despite their importance, no studies have analyzed the anatomy and muscle architecture of this species. Therefore, the objective of this study was to analyze the macroscopic anatomy and muscle architecture of the extrinsic thoracic limb muscles in T. mexicana . Four specimens were dissected and fixed in 10% diluted formalin. Once the origins and insertions were described, the muscles were removed and immersed in 30% nitric acid for 96 h and 50% glycerin for 2 h. Photographs were taken to measure the length of the fibers and the muscle belly via ImageJ software, and the Lf/Lm ratio was then calculated. Finally, for statistical analysis, the muscles were classified according to their primary function (retractors, protractors, and adductors), and descriptive statistics (mean ratio and SD) were calculated for each muscle group and individual muscle. The trapezius and brachiocephalicus muscles were found to be fused, as were the serratus ventralis thoracis and rhomboideus cervicis muscles. Two pectoralis profundus muscles were found. In terms of the ratio, all muscles and groups had ratios >0.5. Thus, the anatomy and architecture of the extrinsic thoracic limb muscles in T. mexicana could respond to ontogenetic and evolutionary factors that make it a better adaptation for movement in trees.

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