A geometric morphometric study of sex differences in the scapula, humerus and ulna of Chaetophractus villosus (Xenarthra, Dasypodidae)

Abstract

ABSTRACT. Sexual diff erences in some of the components of the pectoral girdle and forelimb of Chaetophractus villosus (Desmarest, 1804) were investigated by means of geometric morphometrics. A total of 15 scapulae (7 males, 8 females) and 50 humeri-ulnae complexes (24 males, 26 females) were examined. No size diff erences were detected between sexes for any of the bones, but shape diff erences were found for the humerus and the ulna that enhance the in-forces or the related in-levers. Females had a more robust humerus, with an enlarged deltoid tuberosity, a more spherical head, higher tubercular and epicondylar widths, and more developed supracondylar crests. The ulna of the females was characterised by a more pronounced curvature along its main axis, a caudal displacement of the lateral fossa, a longer olecranon, and a deeper trochlear notch. For both bones, the discriminant analysis classifi ed correctly all the specimens according to their sex. A small percentage of the variance (5.2% for the humerus; 6.8% for the ulna) was explained by allometry. The morphological traits found for the humerus and ulna of the females suggest an increased structural strength that could result in powerful digging strokes while keeping shoulder and elbow stabilisation, suggesting a fossorial advantage over males.KEYWORDS. Large hairy armadillo, fossorial mammals, shape-based dimorphism, appendicular skeleton.RESUMEN. Estudio del dimorfi smo sexual en la escápula, húmero y ulna de Chaetophractus villosus (Xenarthra, Dasypodidae) mediante técnicas de morfometría geométrica. Se investigaron las diferencias sexuales en algunos de los elementos de la cintura escapular y del miembro anterior de Chaetophractus villosus (Desmarest, 1804) utilizando técnicas de morfometría geométrica. La muestra estuvo constituida por 15 escápulas (7 machos, 8 hembras) y 50 complejos húmero-ulna (24 machos, 26 hembras). No se detectaron diferencias de tamaño entre sexos para ninguno de los tres huesos. Sin embargo, se observaron diferencias de forma para el húmero y la ulna, con mayor desarrollo en las hembras de estructuras que favorecen el incremento de fuerzas internas o de las palancas internas relacionadas. En ellas, el húmero se caracterizó por una morfología general más robusta, con mayor desarrollo de la tuberosidad deltoidea, cabeza más esférica, mayores anchos intertubercular y epicondilar, y mayor desarrollo de las crestas supracondilares. Para la ulna se observó una curvatura más pronunciada a lo largo de su eje mayor, desplazamiento de la fosa lateral hacia caudal, olecranon de mayor desarrollo, e incisura troclear más profunda. Para ambos huesos, el análisis discriminante clasifi có correctamente al total de los ejemplares de acuerdo al sexo. Un pequeño porcentaje de la varianza (5,2% para el húmero; 6,8% para la ulna) se debió a efectos alométricos. Las características morfológicas del húmero y la ulna de las hembras sugieren una ventaja fosorial sobre los machos, debido a una mayor fuerza estructural durante los movimientos involucrados...

Functional and morphological divergence in the forelimb musculoskeletal system of scratch‐digging subterranean mammals (Rodentia: Bathyergidae)

“…A similar protuberance, but even more developed than in bathyergids has been observed in armadillos, e.g. Chaetophractus villosus and Zaedyus pichiy (Acuña et al, 2017;Marshall et al, 2021). On the medial side of the scapula, the subscapular fossa is relatively flat and similar among all four species.…”

Anatomical and radiographic study of the scapula in juveniles and adults of Tamandua mexicana (Xenarthra: Myrmecophagidae)

“…However, these are different to Tamandua, because this surface is not so great as to give origin to the m. subscapular, similar to what occurs in armadillos, where it is observed that this surface is wide but only at the level of the caudal angle (Acuña, Sidorkewicj, Popp, & Casanave, 2017;Miles, 1941;Olson, Womble, Thomas, Glenn, & Butcher, 2016;Taylor, 1978), which is considered by Monteiro and Abe (1999) as an expansion of the infraspinous fossa in armadillos and developed due to the greater retraction required by the thoracic limb in order to dig-where a major part of the force is contributed by the m. teres major for their fossorial habits (Olson et al, 2016).…”

“…Even in non‐human primates, this surface has already been previously described (Mivart, ), and current studies have indicated its presence such as in Lemur catta (Makungu, Groenewald, Plessis, Barrows, & Koeppel, ), cercopithecids (Dunham, Kane, & McGraw, ) and Saguinus leucopus (Vélez‐García, Monroy‐Cendales, & Castañeda‐Herrera, ). However, these are different to Tamandua, because this surface is not so great as to give origin to the m. subscapular, similar to what occurs in armadillos, where it is observed that this surface is wide but only at the level of the caudal angle (Acuña, Sidorkewicj, Popp, & Casanave, ; Miles, ; Olson, Womble, Thomas, Glenn, & Butcher, ; Taylor, ), which is considered by Monteiro and Abe () as an expansion of the infraspinous fossa in armadillos and developed due to the greater retraction required by the thoracic limb in order to dig—where a major part of the force is contributed by the m. teres major for their fossorial habits (Olson et al, ).…”

Comparative morpho‐functional analysis of the humerus and ulna in three Western European moles species of the genus Talpa, including the newly described T. aquitania

“…Geometric morphometric analyses on armadillo species have shown that adaptations to digging ability seem to be mostly related to a more secure elbow joint and more powerful muscles that control the forearm and hand (Milne et al, 2009 ). The ulna is one of the main bones undergoing stress during burrowing (Acuña et al, 2016 ). Moreover, digging ability has been previously shown to be well characterized by the relative length of the olecranon of the ulna, in other burrowing mammals such as armadillos (Vizcaíno et al, 1999 ).…”