Research Topic
In their study, Gaudin and Nyakatura aimed to investigate the evolution of the epaxial musculature in animals such as sloths, opossums, and armadillos from a functional perspective. During research, a dissection of fresh specimens of the North American Common long-nosed armadillo, Dasypus novemcinctus, and a marsupial, the Virginia opossum, Didelphis virginiana, was performed. For comparison, the scholars used available data on the two-toed sloth, Choloepus didactylus, because it is a xenarthran that exhibits a highly derived locomotor mode. In addition, the opossum’s sample was used to show a more generalized mammalian condition. During the dissection, each mammalian epaxial muscle group, such as the transversospinalis, iliocostalis, and longissimus dorsi, was removed to determine their mass, with all data corrected for the length and mass of the samples’ bodies.
Goals
The researchers conducted their experiment for several reasons, grounded in available studies. For instance, while the significance of xenarthrales in ecosystems has been widely recognized, the functional role of these joints remains poorly understood. In previous studies, it was found that xenarthrales do indeed stiffen the vertebral column in Dasypus. However, an increase in mobility in the post-diaphragmatic region was also observed, suggesting that xenarthrous articulations serve a dual function. Therefore, Gaudin and Nyakatura set several goals for their current study, the first of which was to ascertain the quantitative significance of the observed differences in epaxial muscle mass between the opossum and armadillo.
The second goal was to determine whether the epaxial musculature, like the axial skeleton, exhibits clear adaptations for fossoriality in xenarthrans. In simpler terms, the researchers aimed to determine whether any notable differences in muscle mass could be linked to the armadillo’s digging behavior. Since early xenarthrans likely engaged in scratch digging, similar to the modern Common long-nosed armadillo (Dasypus), the study aimed to illuminate the early evolution of the group’s back muscles. Additionally, comparing this to the condition observed in the highly derived modern sloth Choloepus didactylus enhances our understanding of the evolution of back muscles within Xenarthra.
Methodology
During the dissection, muscles were extracted in three distinct blocks from the thoracic, lumbar, and sacral regions of the vertebral column in each armadillo and opossum specimen. Subsequently, the weight of each block was measured separately for the iliocostalis, longissimus dorsi, and transversospinalis groups. To perform the statistical analysis accurately, the muscles were contained in fifteen milliliter tubes, frozen overnight at −70 °C, then lyophilized for one to three days until fully dehydrated.
As the muscle samples were completely dry, the researchers accurately weighed them to the nearest tenth of a gram. It was crucial to address the size discrepancies among the opossums, armadillos, and the sloth; the raw data were normalized before statistical analysis, using three methods. The first was dividing muscle mass by total body mass, by total body length, and by total epaxial muscle mass.
Findings
Based on the analysis, Didelphis and Dasypus are approximately comparable in size, with the opossum slightly longer and the armadillo somewhat heavier. The discrepancy in the latter measurement is primarily due to the carapace on the trunk, which accounts for almost 14% of the armadillo’s body mass. Despite differences in size, t-test outcomes for measurements adjusted by both total weight and body length were identical to those obtained with unadjusted values. All points of analysis showed a statistically significant difference (p < 0.01) in muscle mass between the two animals, with armadillos exhibiting greater muscle mass.
The transversospinalis and iliocostalis muscles of the armadillo are 3.5–4.5 times larger than those of the opossum. Despite the small sample size in this study, the p-values obtained in statistical tests for both raw and normalized data were below 0.01. Furthermore, these muscle groups constitute a greater portion of the total epaxial muscle mass in the armadillo compared to the opossum. Such a difference has been statistically significant only for the transversospinalis.
Considering their slender, strap-like structure, it is unsurprising that the epaxial muscles of Choloepus appear smaller compared to those of Dasypus when adjusted for both body length and mass. However, the researchers could not predict that both the iliocostalis and transversospinalis muscles are notably larger in the opossum than their counterparts when adjusted for body mass and length. Additionally, it should be noted that the iliocostalis muscles represent a larger percentage of the total epaxial muscle mass in the sloth compared to either the opossum or the armadillo.
Implications
The study’s significance lies in its contribution to understanding the functional adaptations and evolutionary patterns of back muscles in Xenarthra. By comparing the anatomy of epaxial muscles across armadillos, sloths, and opossums, the study sheds light on how these muscles have changed over time to support varied locomotor behaviors and ecological roles within this diverse group of mammals. Through this investigation, the study provides valuable insights into the biomechanical adaptations associated with fossoriality, arboreality, and terrestrial locomotion in Xenarthra, thus enriching the scholarly community’s knowledge of the evolutionary history and ecological diversity of such a-mammalian lineage.
Reference
Gaudin, T. J., & Nyakatura, J. A. (2018). Epaxial musculature in armadillos, sloths, and opossums: Functional significance and implications for the evolution of back muscles in the Xenarthra. Journal of Mammalian Evolution, 25(4), 565-572.