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Abstract Bite force is a key metric of organismal performance, and expression of masticatory myosin (MHC-M) is associated with high bite force. However, skeletal muscles are multiscale structures, and it remains unclear how adaptations for force production are integrated across scales. We analyzed myosin isoform composition and physiological cross-sectional area of the jaw muscles and measured their dynamic moment armsex vivousing XROMM (X-ray Reconstruction Of Moving Morphology) in six rodent species. We found modifications at all scales in hard biters (grey squirrels) to prioritize force production. Related species (chipmunk, woodchuck and red squirrel) showed a mix of adaptations across scales, with different muscle phenotypes producing equivalent bite force outputs. By contrast, rat and guinea pig showed modifications at all scales consistent with reduced force production. Our results suggest that selection for ecologically relevant traits – including MHC-M expression – occurs at multiple organizational scales within the rodent craniofacial system. 

Traditional work loop studies, that use sinusoidal length trajectories with constant frequencies, lack the complexities of in vivo muscle mechanics observed in modern studies. This study refines methodology of the “avatar” method (a modified work loop) to infer in vivo muscle mechanics using ex vivo experiments with mouse extensor digitorum longus (EDL) muscles. The “avatar” method involves using EDL muscles to replicate in vivo time varying force, as demonstrated by previous studies focusing on guinea fowl lateral gastrocnemius (LG). The present study extends this method by using in vivo length trajectories and electromyographic (EMG) activity from rat medial gastrocnemius (MG) during various gaits on a treadmill. Methodological enhancements from previous work, including adjusted stimulation protocols and systematic variation of starting length, improved predictions of in vivo time varying force production (R2 0.80 – 0.96). The study confirms there are significant influence of length, stimulation, and their interactions on work loop variables (peak force, length at peak force, highest and average shortening velocity, and maximum and minimum active velocity), highlighting the importance of these interactions when muscles produce in vivo forces. We also investigated the limitations of traditional work loops in capturing muscle dynamics in legged locomotion (R2 0.01 – 0.71). While in vivo length trajectories enhanced force prediction, accurately predicting work per cycle remained challenging. Overall, the study emphasizes the utility of the avatar method in elucidating dynamic muscle mechanics and highlights areas for further investigation to refine its application in understanding in vivo muscle function. 

Intra-oral food processing, including chewing, is important for safe swallowing and efficient nutrient assimilation across tetrapods. Gape cycles in tetrapod chewing consist of four phases (fast open and -close, and slow open and -close), with processing mainly occurring during slow close. Basal aquatic-feeding vertebrates also process food intraorally, but whether their chew cycles are partitioned into distinct phases, and how rhythmic their chewing is, remains unknown. Here, we show that chew cycles from sharks to salamanders are as rhythmic as those of mammals, and consist of at least three, and often four phases, with phase distinction occasionally lacking during jaw opening. In fishes and aquatic-feeding salamanders, fast open has the most variable duration, more closely resembling mammals than basal amniotes (lepidosaurs). Across ontogenetically or behaviourally mediated terrestrialization, salamanders show a distinct pattern of the second closing phase (near-contact) being faster than the first, with no clear pattern in partitioning of variability across phases. Our results suggest that distinct fast and slow chew cycle phases are ancestral for jawed vertebrates, followed by a complicated evolutionary history of cycle phase durations and jaw velocities across fishes, basal tetrapods and mammals. These results raise new questions about the mechanical and sensorimotor underpinnings of vertebrate food processing. This article is part of the theme issue ‘Food processing and nutritional assimilation in animals’.