A Roadmap to Reconstructing Muscle Architecture from CT Data

Ecological correlates of three-dimensional muscle architecture within the dietarily diverse Strepsirrhini

Analysis of muscle architecture, traditionally conducted via gross dissection, has been used to evaluate adaptive relationships between anatomical form and behavioral function. However, gross dissection cannot preserve three-dimensional relationships between myological structures for analysis. To analyze such data, we employ diffusible, iodine-based contrast-enhanced computed tomography (DiceCT) to explore the relationships between feeding ecology and masticatory muscle microanatomy in eight dietarily diverse strepsirrhines: allowing, for the first time, preservation of three-dimensional fascicle orientation and tortuosity across a functional comparative sample. We find that fascicle properties derived from these digital analyses generally agree with those measured from gross-dissected conspecifics. Physiological cross-sectional area was greatest in species with mechanically challenging diets. Frugivorous taxa and the wood-gouging species all exhibit long jaw adductor fascicles, while more folivorous species show the shortest relative jaw adductor fascicle lengths. Fascicle orientation in the parasagittal plane also seems to have a clear dietary association: most folivorous taxa have masseter and temporalis muscle vectors that intersect acutely while these vectors intersect obliquely in more frugivorous species. Finally, we observed notably greater magnitudes of fascicle tortuosity, as well as greater interspecific variation in tortuosity, within the jaw adductor musculature than in the jaw abductors. While the use of a single specimen per species precludes analysis of intraspecific variation, our data highlight the diversity of microanatomical variation that exists within the strepsirrhine feeding system and suggest that muscle architectural configurations are evolutionarily labile in response to dietary ecology-an observation to be explored across larger samples in the future.

Computational Approaches and Observer Variation in the 3D Musculoskeletal Modeling of the Heads of Anolis

High-resolution imaging, 3D modeling, and quantitative analyses are equipping evolutionary biologists with new approaches to understanding the variation and evolution of the musculoskeletal system. However, challenges with interpreting DiceCT data and higher order use of modeled muscles have not yet been fully explored, and the error in and accuracy of some digital methods remain unclear. West Indian Anolis lizards are a model clade for exploring patterns in functional adaptation, ecomorphology, and sexual size dimorphism in vertebrates. These lizards possess numerous jaw muscles with potentially different anatomies that sculpt the adductor chamber of the skull. Here we test approaches to quantifying the musculoskeletal shape of the heads of two species of Anolis: A. pulchellus and A. sagrei. We employ comparative approaches such as DiceCT segmentation of jaw muscles, 3D surface attachment mapping, and 3D landmarking with the aim of exploring muscle volumes, 3D muscle fiber architecture, and sexual dimorphism of the skull. We then compare sources of measurement error in these 3D analyses while also presenting new 3D musculoskeletal data from the Anolis feeding apparatus. These findings demonstrate the accessibility and repeatability of these emerging techniques as well as provide details regarding the musculoskeletal anatomy of the heads of A. pulchellus and A. sagrei which show potential for further research of comparative biomechanics and evolution in the clade.

Simulated biomechanical performance of morphologically disparate ant mandibles under bite loading

Insects evolved various modifications to their mouthparts, allowing for a broad exploration of feeding modes. In ants, workers perform non-reproductive tasks like excavation, food processing, and juvenile care, relying heavily on their mandibles. Given the importance of biting for ant workers and the significant mandible morphological diversity across species, it is essential to understand how mandible shape influences its mechanical responses to bite loading. We employed Finite Element Analysis to simulate biting scenarios on mandible volumetric models from 25 ant species classified in different feeding habits. We hypothesize that mandibles of predatory ants, especially trap-jaw ants, would perform better than mandibles of omnivorous species due to their necessity to subdue living prey. We defined simulations to allow only variation in mandible morphology between specimens. Our results demonstrated interspecific differences in mandible mechanical responses to biting loading. However, we found no evident differences in biting performance between the predatory and the remaining ants, and trap-jaw mandibles did not show lower stress levels than other mandibles under bite loading. These results suggest that ant feeding habit is not a robust predictor of mandible biting performance, a possible consequence of mandibles being employed as versatile tools to perform several tasks.

Parallel And Divergent Morphological Adaptations Underlying The Evolution of Jumping Ability in Ants

Jumping is a rapid locomotory mode widespread in terrestrial organisms. However, it is a rare specialization in ants. Forward jumping has been reported within four distantly related ant genera: Gigantiops, Harpegnathos, Myrmecia, and Odontomachus. The temporal engagement of legs/body parts during jump, however, varies across these genera. It is unknown what morphological adaptations underlie such behaviors and whether jumping in ants is solely driven directly by muscle contraction or additionally relies on elastic recoil mechanism. We investigated the morphological adaptations for jumping behavior by comparing differences in the locomotory musculature between jumping and non-jumping relatives using X-ray micro-CT and 3D morphometrics. We found that the size-specific volumes of the trochanter depressor muscle (scm6) of the middle and hind legs are 3-5 times larger in jumping ants, and that one coxal remotor muscle (scm2) is reduced in volume in the middle and/or hind legs. Notably, the enlargement in the volume of other muscle groups is directly linked to the legs or body parts engaged during the jump. Furthermore, a direct comparison of the muscle architecture revealed two significant differences between jumping vs. non-jumping ants: First, the relative Physiological Cross-Sectional Area (PCSA) of the trochanter depressor muscles of all three legs were larger in jumping ants, except in the front legs of Odontomachus rixosus and Myrmecia nigrocincta; second, the relative muscle fiber length was shorter in jumping ants compared to non-jumping counterparts, except in the front legs of O. rixosus and M. nigrocincta. These results suggest that the difference in relative muscle volume in jumping ants is largely invested in the area (PCSA), and not in fiber length. There was no clear difference in the pennation angle between jumping and non-jumping ants. Additionally, we report that the hind leg length relative to body length was longer in jumping ants. Based on direct comparison of the observed vs. possible work and power output during jumps, we surmise that direct muscle contractions suffice to explain jumping performance in three species, except for O. rixosus, where the lack of data on jumping performance prevents us from drawing definitive conclusions for this particular species. We suggest that increased investment in jumping-relevant musculature is a primary morphological adaptation that separates jumping from non-jumping ants. These results elucidate the common and idiosyncratic morphological changes underlying this rare adaptation in ants. まとぅみ (Okinawan language-Uchinaaguchi) (Japanese) РЕЗЮМЕ (Kazakh) ZUSAMMENFASSUNG (German).

A biomechanical model for the relation between bite force and mandibular opening angle in arthropods

Bite forces play a key role in animal ecology: they affect mating behaviour, fighting success, and the ability to feed. Although feeding habits of arthropods have a significant ecological and economical impact, we lack fundamental knowledge on how the morphology and physiology of their bite apparatus controls bite performance, and its variation with mandible gape. To address this gap, we derived a biomechanical model that characterizes the relationship between bite force and mandibular opening angle from first principles. We validate this model by comparing its geometric predictions with morphological measurements on the muscoloskeletal bite apparatus of Atta cephalotes leaf-cutter ants, using computed tomography (CT) scans obtained at different mandible opening angles. We then demonstrate its deductive and inductive utility with three examplary use cases: Firstly, we extract the physiological properties of the leaf-cutter ant mandible closer muscle from in vivo bite force measurements. Secondly, we show that leaf-cutter ants are specialized to generate extraordinarily large bite forces, equivalent to about 2600 times their body weight. Thirdly, we discuss the relative importance of morphology and physiology in determining the magnitude and variation of bite force. We hope that a more detailed quantitative understanding of the link between morphology, physiology, and bite performance will facilitate future comparative studies on the insect bite apparatus, and help to advance our knowledge of the behaviour, ecology and evolution of arthropods.

New frontiers in imaging, anatomy, and mechanics of crocodylian jaw muscles

New imaging and biomechanical approaches have heralded a renaissance in our understanding of crocodylian anatomy. Here, we review a series of approaches in the preparation, imaging, and functional analysis of the jaw muscles of crocodylians. Iodine-contrast microCT approaches are enabling new insights into the anatomy of muscles, nerves, and other soft tissues of embryonic as well as adult specimens of alligators. These imaging data and other muscle modeling methods offer increased accuracy of muscle sizes and attachments without destructive methods like dissection. 3D modeling approaches and imaging data together now enable us to see and reconstruct 3D muscle architecture which then allows us to estimate 3D muscle resultants, but also measurements of pennation in ways not seen before. These methods have already revealed new information on the ontogeny, diversity, and function of jaw muscles and the heads of alligators and other crocodylians. Such approaches will lead to enhanced and accurate analyses of form, function, and evolution of crocodylians, their fossil ancestors and vertebrates in general.