Three‐dimensional analysis of sexual dimorphism in ribcage kinematics of modern humans

Abstract

Objectives: Sexual dimorphism is an important biological factor underlying morphological variation in the human skeleton. Previous research found sex-related differences in the static ribcage, with males having more horizontally oriented ribs and a wider lower ribcage than females. Furthermore, a recent study found sex-related differences in the kinematics of the human lungs, with cranio-caudal movements of the caudal part of the lungs accounting for most of the differences between sexes. However, these movements cannot be quantified in the skeletal ribcage, so we do not know if the differences observed in the lungs are also reflected in sex differences in the motion of the skeletal thorax. Materials and methods:To address this issue, we quantified the morphological variation of 42 contemporary human ribcages (sex-balanced) in both maximal inspiration and expiration using 526 landmarks and semilandmarks. Thoracic centroid size differences between sexes were assessed using a t test, and shape differences were assessed using Procrustes shape coordinates, through mean comparisons and dummy regressions of shape on kinematic status. A principal components analysis was used to explore the full range of morphological variation.Results: Our results show significant size differences between males and females both in inspiration and expiration (p < .01) as well as significant shape differences, with males deforming more than females during inspiration, especially in the mediolateral dimension of the lower ribcage. Finally, dummy regressions of shape on kinematic status showed a small but statistically significant difference in vectors of breathing kinematics between males and females (14.78 ; p < .01).Discussion: We support that sex-related differences in skeletal ribcage kinematics are discernible, even when soft tissues are not analyzed. We hypothesize that this differential breathing pattern is primarily a result of more pronounced diaphragmatic breathing in males, which might relate to differences in body composition, metabolism, and ultimately greater oxygen demand in males compared to females. Future research should further explore the links between ribcage morphological variation and basal metabolic rate.

Assessing thoraco‐pelvic covariation in Homo sapiens and Pan troglodytes: A 3D geometric morphometric approach

“…Our results show that torso morphology and thoraco‐pelvic covariation in adult H. sapiens individuals are mainly driven by sexual dimorphism, with relatively wider (lower) thoraces than (upper) pelves in males and the opposite trend in females (Figures 2, 3a and 6a, Table 3). These results replicate the findings of Torres‐Tamayo et al (2018) and are consistent with previous works showing that sexual dimorphism plays an important role in the thorax and pelvis shape variation in H. sapiens (Bellemare et al, 2003; Fischer & Mitteroecker, 2017; García‐Martínez et al, 2016, 2019; Gruss & Schmitt, 2015; K. Rosenberg & Trevathan, 1995).…”

“…They used 3D geometric morphometrics (GMM) of sliding semilandmarks for analyzing 50 torso 3D models segmented from thoraco‐abdominopelvic computed tomography (CT) scans of two different adult populations, and found that modern human females showed mediolaterally narrower (lower) thoraces than their wide (upper) pelves and modern human males showed the opposite trend in both populations. These authors reported that sexual dimorphism would be the main factor driving thoraco‐pelvic covariation in adult H. sapiens , and their results are consistent with those of previous studies analyzing sex‐related variation in the thorax and pelvis separately (Bellemare et al, 2003; Fischer & Mitteroecker, 2015, 2017; García‐Martínez et al, 2016, 2019; LoMauro & Aliverti, 2018; Molgat‐Seon, Peters, & Sheel, 2018). Thus, Torres‐Tamayo et al (2018) proposed a new and more complex torso integration model in H. sapiens (torso integration hypothesis) that called into question the correspondence between a narrow thorax and a narrow pelvis classically reported for this species (Lovejoy, 2005; Middleton, 2013, 2015, 2017; Schultz, 1950, 1961; C. V. Ward, 1993).…”

“…At respiratory skeletal level, male H. sapiens individuals have larger ribcages than females of the same stature and certainly wider lower thoraces (Bellemare et al, 2003; García‐Martínez et al, 2016). Also, sex‐related kinematic differences have been linked to a greater activity of the diaphragm as well as to greater bioenergetic demands and high metabolic rates in males than in females (García‐Martínez et al, 2019; Torres‐Tamayo et al, 2018). With regards to pelvic sexual shape dimorphism, human female pelvis morphology evolved to give birth to large‐brained offspring over the last 2 Ma with the increase of cranial capacity in the genus Homo (Gruss & Schmitt, 2015; Leigh, 1992; Leutenegger, 1987; Rightmire, 2004; K. R. Rosenberg, 1992; K. Rosenberg & Trevathan, 1995; Tague & Lovejoy, 1986).…”

“…Apart from clear locomotor differences, H. sapiens and P. troglodytes may differ in the role that sexual dimorphism plays in the torso skeletal elements. Several studies support the existence of sexual dimorphism in the morphology of the human ribcage, with females showing mediolaterally narrower thoraces than males (Bellemare, Jeanneret, & Couture, 2003; García‐Martínez et al, 2019; García‐Martínez, Torres‐Tamayo, Torres‐Sánchez, García‐Río, & Bastir, 2016). These sex‐related differences have been associated to different bioenergetic demands in females and males (Bitar, Fellmann, Vernet, Coudert, & Vermorel, 1999).…”

Early development of the Neanderthal ribcage reveals a different body shape at birth compared to modern humans

“…Ribcages were segmented through a semi-automatic protocol for Digital Imaging and Communication On Medicine (DICOM) images using the 3D Slicer software (www.slicer.org/) and subsequently reconstructed as 3D models. These 3D models were imported into Viewbox4 software (www.dhal.com) for (semi-) landmarking using existing protocols (60). Thoracic morphology was quantified through 20 homologous 3D landmarks and semilandmarks on ribs 1 to 10 and 19 3D landmarks and semilandmarks on ribs 11 and 12.…”