Bone histological evidence for posture-related load transmission in the talpid pectoral girdle.

The pectoral girdle of terrestrial tetrapods functions as a critical interface for transmitting loads between the trunk and forelimbs. While mammals with a parasagittal forelimb posture mainly rely on the scapula as the principal element for load bearing, talpids (Talpidae) exhibit a markedly specialised pectoral girdle related to a hyper-abducted forelimb posture. In talpids, the humero-clavicular articulation plays an important functional role, suggesting that load-transmission mechanisms within the pectoral girdle may differ from those of typical terrestrial mammals. However, because the pectoral girdle of talpids is highly developed as a fossorial adaptation, it is difficult to infer the mechanical environments experienced by each skeletal element from external morphology alone, and the mechanical roles of the pectoral girdle elements are poorly understood. To examine whether posture-related reorganisation of load-transmission pathways is reflected in bone microstructure, collagen fibre orientation weighted mean greyscale level (CFO-WMGL) and cortical bone robusticity (relative robusticity indicator: RRI) in the clavicle and scapula were evaluated in fossorial talpids (Mogera imaizumii, Mogera wogura and Urotrichus talpoides) and compared with those of the parasagittal outgroup Suncus murinus. Talpids exhibited high RRI values in both the clavicle and scapula, indicating overall mechanical reinforcement of the pectoral girdle. In contrast, CFO-WMGL showed element-specific differences among taxa. In Mogera, CFO-WMGL values were consistently higher in the clavicle than in the scapula, suggesting that the clavicle played a relatively greater load-bearing role within the pectoral girdle. Conversely, S. murinus exhibited histological characters consistent with the scapula functioning as the principal element for load bearing. These distinct histological patterns indicate that the pectoral girdle is not a mechanically uniform structure, but rather an integrated functional system in which each skeletal element performs distinct mechanical roles. These findings suggest that the evolution of the forelimb posture in talpids was accompanied by a reorganisation of load-transmission pathways within the pectoral girdle, involving a relative increase in the load-bearing role of the clavicle compared with the scapula. This study demonstrates that bone histological characters can provide insights into skeletal function that are not apparent from external morphology alone. This approach may offer a useful framework for reconstructing load-transmission mechanisms in both extant and extinct tetrapods and for investigating how the mechanical demands imposed on each skeletal element have changed during evolution. Future comparative studies across a broader range of taxa will be essential for evaluating the generality of these relationships and for advancing our understanding of the mechanical reorganisation of the vertebrate musculoskeletal system throughout evolutionary history.