Quantitative shape analysis of the deltoid tuberosity of modern humans (Homo sapiens) and common chimpanzees (Pan troglodytes)

Purpose To identify anatomical differences in the deltoid tuberosity of Homo sapiens and Pan troglodytes, potentially relating to the different uses of the forelimb in these two phylogenetically related species. Basic procedures We have used three-dimensional geometric morphometrics (3D GM) to analyze the deltoid tuberosity of scanned humeri from 30 H. sapiens and 27 P. troglodytes. We also used the 3D scans of the humeri to calculate the surface area of the deltoid tuberosity. Finally, we dissected the deltoid muscles of three H. sapiens and three P. troglodytes to determine the relative mass and the physiological cross-sectional area (PCSA) of each part of the muscle. Main findings The 3D GM analysis of the deltoid tuberosity identified an anteroposterior enlargement of the P. troglodytes tuberosity, with a lateral displacement of the middle segment, whereas in H. sapiens, there was a distal displacement of the middle segment. Muscle architecture analysis indicated higher normalized values ​​of the PCSA of the clavicular and acromial deltoid in P. troglodytes. Principal conclusions The anatomical features observed in our P. troglodytes specimens serve to strengthen the three parts of the deltoid muscle. This fact can be related to the use of the forelimb in locomotion, both arboreal and knuckle-walking, in this species. Humans use the forelimb mainly in manipulative tasks, so they do not develop – as do chimpanzees – the anatomical features that increase the deltoid force. Our findings have shown that the different uses of the forelimb in modern humans and common chimpanzees can affect both muscle architecture and bone morphology, either jointly or separately.