The interplay between morphological specialization and kinematic flexibility is important for organisms that move between habitats within different substrates. Burrowing is energetically expensive and requires substantial interaction with soil to dislodge and transport it. True moles (Talpidae) have extraordinary forelimb morphologies and a unique ability to dig in loose as well as compact soils, yet we know little of how moles coordinate their forelimb joint kinematics when digging in soils of different compactness. Using marker-based X-ray Reconstruction of Moving Morphology (XROMM), we tested the hypothesis that moles burrow using different forelimb kinematics in loose and compact substrates. We predicted that moles raise mounds of loose soil by performing powerful compacting strokes mainly with long-axis rotation of the humerus (i.e. pronation/ supination), but shear compact soil away by performing scratching strokes involving amplified elbow extension, similar to most scratching diggers. We also predicted that in both types of substrate, moles displace soil rearward like other mammalian diggers. Our results support our hypothesis but not the predictions. Eastern moles (Scalopus aquaticus) move substrates upward using compacting strokes in loose substrates and outward from the body midline using scratching strokes in compact substrates; unlike the digging strokes of most mammalian forelimb diggers, the powerstroke of moles itself does not displace substrates directly rearward. Compacting and scratching strokes involve similar ranges of humeral pronation and retraction at the scapulohumeral (shoulder) joint, yet the movements at the elbow and carpal joints differ. Our results demonstrate that the combination of stereotypic movements of the shoulder joint, where the largest digging muscles are located, and flexibility in the elbow and carpal joints makes moles extremely effective diggers in both loose and compact substrates.
Burrowing is an energetically costly form of locomotion that has evolved multiple times in mammals. Burrowing performance is associated with varying degrees of morphological specialization and is also affected by the characteristics of the substrate that animals burrow in. Moles (Talpidae) are well known for their specialized forelimbs and extreme dedication to life underground. In this study, we investigated how soil compactness affects burrowing performance in two sympatric mole species, eastern moles (Scalopus aquaticus) and hairy‐tailed moles (Parascalops breweri). We measured burrowing speed, the amount of soil moved, rate of soil transport, tunnel length, activity level and the tendency to burrow over long distances, and tested how these variables changed in response to loose, intermediate and compact soils. We found that increasing soil compactness impedes tunneling performance as evidenced by reduced burrowing speed, increased soil transport, shorter tunnels, shorter activity time and less time spent burrowing continuously. Eastern moles built longer tunnels than hairy‐tailed moles as soil compactness increased. This difference is linked to burrowing for longer times and distances, not higher burrowing speeds or rates of soil transport. Differences in performance between the two species may be associated with differences in the structure and extent of their burrow systems or the morphology of their forelimbs. They may also reflect preferences for loose (hairy‐tailed moles) or compact soils (eastern moles).
A recurring theme in the evolution of tetrapods is the shift from sprawling posture with laterally orientated limbs to erect posture with the limbs extending below the body. However, in order to invade particular locomotor niches, some tetrapods secondarily evolved a sprawled posture. This includes moles, some of the most specialized digging tetrapods. Although their forelimb anatomy and posture facilitates burrowing, moles also walk long distances to forage for and transport food. Here, we use X-ray Reconstruction Of Moving Morphology (XROMM) to determine if the mole humerus rotates around its long axis during walking, as it does when moles burrow and echidnas walk, or alternatively protracts and retracts at the shoulder in the horizontal plane as seen in sprawling reptiles. Our results reject both hypotheses and demonstrate that forelimb kinematics during mole walking are unusual among those described for tetrapods. The humerus is retracted and protracted in the parasagittal plane above, rather than below the shoulder joint and the ‘false thumb’, a sesamoid bone (os falciforme), supports body weight during the stance phase, which is relatively short. Our findings broaden our understanding of the diversity of tetrapod limb posture and locomotor evolution, demonstrate the importance of X-ray-based techniques for revealing hidden kinematics and highlight the importance of examining locomotor function at the level of individual joint mobility.
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