Number of tables: 2Abbreviated title: 3D morphometrics of human rib cage.Key words: rib cage; geometric morphometrics; sex differences. This is the peer reviewed version of the following article: American Journal of Physical Anthropology 161(3): 467-477 (2016), which has been published in final form at http://dx.doi.org/10.1002/ajpa.23051. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. *Corresponding author:2 Title: Morphological and functional implications of sexual dimorphism in the human skeletal thorax. ABSTRACT Objectives:The human respiratory apparatus is characterized by sexual dimorphism, the cranial airways of males being larger (both absolutely and relatively) than those of females. These differences have been linked to sex-specific differences in body composition, bioenergetics and respiratory function. However, whether morpho-functional variation in the thorax is also related to these features is less clear. We apply 3D geometric morphometrics to study these issues and their implications for respiratory function. Material and methods:402 landmarks and semilandmarks were measured in CT-reconstructions of rib cages from adult healthy subjects (N male =18; N female =24) in maximal inspiration (MI) and maximal expiration (ME). After Procrustes registration, size and shape data were analyzed by mean comparisons and regression analysis. Respiratory function was quantified through functional size, which is defined as the difference of rib cage size between MI and ME.Results: Males showed significantly larger thorax size (p<0.01) and functional size (p<0.05) than females. In addition, the 3D-shape differed significantly between sexes (p<0.01). Male rib cages were wider (particularly caudally) and shorter, with more horizontally oriented ribs when compared to females. While thorax widening and rib orientation were unrelated to allometry, thorax shortening showed a slight allometric signal. Conclusions:Our findings are in line with previous research on sexual dimorphism of the respiratory system. However, we add that thorax shortening observed previously in males is the only feature caused by allometry. The more horizontally oriented ribs and the wider thorax of males may indicate a greater diaphragmatic contribution to rib cage kinematics than in females, and differences in functional size fit with the need for greater oxygen intake in males.3

Sexual dimorphism in the human respiratory system has been previously reported at the skeletal (cranial and thoracic) level, but also at the pulmonary level. Regarding lungs, foregoing studies have yielded sex-related differences in pulmonary size as well as lung shape details, but different methodological approaches have led to discrepant results on differences in respiratory patterns between males and females. The purpose of this study is to analyse sexual dimorphism in human lungs during forced respiration using 3D geometric morphometrics. Eighty computed tomographies (19 males and 21 females) were taken in maximal forced inspiration (FI) and expiration (FE), and 415 (semi)landmarks were digitized on 80 virtual lung models for the 3D quantification of pulmonary size, shape and kinematic differences. We found that males showed larger lungs than females (P < 0.05), and significantly greater size and shape differences between FI and FE. Morphologically, males have pyramidal lung geometry, with greater lower lung width when comparing with the apices, in contrast to the prismatic lung shape and similar widths at upper and lower lungs of females. Multivariate regression analyses confirmed the effect of sex on lung size (36.26%; P < 0.05) and on lung shape (7.23%; P < 0.05), and yielded two kinematic vectors with a small but statistically significant angle between them (13.22°; P < 0.05) that confirms sex-related differences in the respiratory patterns. Our 3D approach shows sexual dimorphism in human lungs likely due to a greater diaphragmatic action in males and a predominant intercostal muscle action in females during breathing. These size and shape differences would lead to different respiratory patterns between sexes, whose physiological implications need to be studied in future research.

ObjectivesSeveral studies have analyzed the sexual dimorphism of the skeletal cranial airways. This study aimed to quantify the three‐dimensional (3D) morphology of the soft tissues of the upper airways in a human population. We addressed hypotheses about morphological features related to respiratory and energetic aspects of nasal sexual dimorphism.MethodsWe reconstructed 3D models of 41 male and female soft tissue nasal airways from computed tomography data. We measured 280 landmarks and semilandmarks for 3D‐geometric morphometric analyses to test for differences in size and 3D morphology of different functional compartments of the soft tissue airways.ResultsWe found statistical evidence for sexual dimorphism: Males were larger than females. 3D features indicated taller and wider inflow tracts, taller outflow tracts and slightly taller internal airways in males. These characteristics are compatible with greater airflow in males.DiscussionThe differences in 3D nasal airway morphology are compatible with the respiratory‐energetics hypothesis according to which males differ from females because of greater energetic demands. Accordingly, structures related to inflow and outflow of air show stronger signals than structures relevant for air‐conditioning.

The human ribcage expands and contracts during respiration as a result of the interaction between the morphology of the ribs, the costo-vertebral articulations and respiratory muscles. Variations in these factors are said to produce differences in the kinematics of the upper thorax and the lower thorax, but the extent and nature of any such differences and their functional implications have not yet been quantified. Applying geometric morphometrics we measured 402 three-dimensional (3D) landmarks and semilandmarks of 3D models built from computed tomographic scans of thoraces of 20 healthy adult subjects in maximal forced inspiration (FI) and expiration (FE). We addressed the hypothesis that upper and lower parts of the ribcage differ in kinematics and compared different models of functional compartmentalization. During inspiration the thorax superior to the level of the sixth ribs undergoes antero-posterior expansion that differs significantly from the medio-lateral expansion characteristic of the thorax below this level. This supports previous suggestions for dividing the thorax into a pulmonary and diaphragmatic part. While both compartments differed significantly in mean size and shape during FE and FI the size changes in the lower compartment were significantly larger. Additionally, for the same degree of kinematic shape change, the pulmonary thorax changes less in size than the diaphragmatic thorax. Therefore, variations in the form and function of the diaphragmatic thorax will have a strong impact on respiratory function. This has important implications for interpreting differences in thorax shape in terms of respiratory functional differences within and among recent humans and fossil hominins. Anat Rec, 300:255-264, 2017. © 2016 Wiley Periodicals, Inc.