Tunnel-tube and Fourier methods for measuring three-dimensional medium reaction force in burrowing animals

Functional anatomy and disparity of the postcranial skeleton of African mole-rats (Bathyergidae)

The burrowing adaptations of the appendicular system of African mole-rats (Bathyergidae) have been comparatively less investigated than their cranial adaptations. Because bathyergids exhibit different digging modes (scratch-digging and chisel-tooth digging) and social systems (from solitary to highly social), they are a unique group to assess the effects of distinct biomechanical regimes and social organization on morphology. We investigated the morphological diversity and intraspecific variation of the appendicular system of a large dataset of mole-rats (n = 244) including seven species and all six bathyergid genera. Seventeen morpho-functional indices from stylopodial (femur, humerus) and zeugopodial (ulna, tibia-fibula) elements were analyzed with multivariate analysis. We hypothesized that scratch-diggers (i.e., Bathyergus) would exhibit a more specialized skeletal phenotype favoring powerful forelimb digging as compared to the chisel-tooth diggers, and that among chisel-tooth diggers, the social taxa will exhibit decreased limb bone specializations as compared to solitary taxa due to colony members sharing the costs of digging. Our results show that most bathyergids have highly specialized fossorial traits, although such specializations were not more developed in Bathyergus (or solitary species), as predicted. Most chisel tooth-diggers are equally, or more specialized than scratch-diggers. Heterocephalus glaber contrasted significantly from other bathyergids, presenting a surprisingly less specialized fossorial morphology. Our data suggests that despite our expectations, chisel-tooth diggers have a suite of appendicular adaptations that have allowed them to maximize different aspects of burrowing, including shoulder and neck support for forward force production, transport and removal of soils out of the burrow, and bidirectional locomotion. It is probably that both postcranial and cranial adaptations in bathyergids have played an important role in the successful colonization of a wide range of habitats and soil conditions within their present distribution.

Discrete element models for understanding the biomechanics of fossorial animals

The morphological features of fossorial animals have continuously evolved in response to the demands of survival. However, existing methods for animal burrowing mechanics are not capable of addressing the large deformation of substrate. The discrete element method (DEM) is able to overcome this limitation. In this study, we used DEM to develop a general model to simulate the motion of an animal body part and its interaction with the substrate. The DEM also allowed us to easily change the forms of animal body parts to examine how those different forms affected the biomechanical functions. These capabilities of the DEM were presented through a case study of modeling the burrowing process of North American Badger. In the case study, the dynamics (forces, work, and soil displacements) of burrowing were predicted for different forms of badger claw and manus, using the model. Results showed that when extra digits are added to a manus, the work required for a badger to dig increases considerably, while the mass of soil dug only increases gradually. According to the proposed efficiency index (ratio of the amount of soil dug to the work required), the modern manus with 5 digits has indeed biomechanical advantage for their fossorial lifestyle, and the current claw curvature (25.3 mm in radius) is indeed optimal. The DEM is able to predict biomechanical relationships between functions and forms for any fossorial animals. Results can provide biomechanical evidences for explaining how the selective pressures for functions influence the morphological evolution in fossorial animals.

Fossorial adaptations in African mole-rats (Bathyergidae) and the unique appendicular phenotype of naked mole-rats

Life underground has constrained the evolution of subterranean mammals to maximize digging performance. However, the mechanisms modulating morphological change and development of fossorial adaptations in such taxa are still poorly known. We assessed the morpho-functional diversity and early postnatal development of fossorial adaptations (bone superstructures) in the appendicular system of the African mole-rats (Bathyergidae), a highly specialized subterranean rodent family. Although bathyergids can use claws or incisors for digging, all genera presented highly specialized bone superstructures associated with scratch-digging behavior. Surprisingly, Heterocephalus glaber differed substantially from other bathyergids, and from fossorial mammals by possessing a less specialized humerus, tibia and fibula. Our data suggest strong functional and developmental constraints driving the selection of limb specializations in most bathyergids, but more relaxed pressures acting on the limbs of H. glaber. A combination of historical, developmental and ecological factors in Heterocephalus are hypothesized to have played important roles in shaping its appendicular phenotype.