Musculo-skeletal variation in the forelimb of two highly specialised diggers (genus Talpa)

The forelimbs of mammals are involved in many crucial behaviours for an animal's ecology, including locomotion. It has been shown that forelimb morphology and locomotor mechanisms are greatly impacted by functional constraints induced by the properties of the media across or in which the animal moves. These functional constraints are thought to drive an important part of bone shape, as bone directly remodels in response to both muscle and external forces. Due to its anatomical particularities, the forelimb of fully fossorial moles is of particular interest to better understand fossorial adaptations and has already been studied extensively. Recently, some studies focusing on two European mole species, Talpa europaea and the recently described Talpa aquitania, highlighted inter and intraspecific variations in the inner ear and forelimb bones morphology, which could be linked to locomotor performances. To better understand the specificity of their fossorial adaptations, we focus, in the present study, on these two species' musculature. Performing anatomical dissections, and in accordance with the literature, we provided a redescription of 39 extrinsic and intrinsic muscles inserting on the forelimb bones. We also made a quantitative comparison of muscles features, both at the inter and intraspecific level. Especially, we focus on muscle Physiological Cross-Sectional Area (PCSA), a measure considered as a good estimator of the force-producing capacity of a muscle. Finally, to investigate relationships between bone shape and muscle force, we quantified covariations between shape data and muscle PCSA. Our results highlighted inter and intraspecific (in T. aquitania) variations in muscle force-producing capacity, which is consistent with the studies of the inner ear and forelimb bones. Focusing on the relationship between muscle PCSA and shape of ulna and humerus, we showed that shoulder extensors, carpal/digital extensors, and carpal/digital flexors' PCSA were highly integrated with humerus shape and that elbow extensors' PCSA were highly integrated with the ulna shape. These results are rather consistent with insertion sites of these muscles on bones.