Callum F. Ross

Primate tongue morphology and function are critical to understanding the evolution of feeding, swallowing, and vocalization. In this paper, we examine the primate tongue as a muscular hydrostat with regionally specialized neuromuscular compartments. We integrate anatomical, kinematic, and biomechanical modeling approaches to analyze how muscle architecture and fiber orientation drive complex tongue deformations during functional behaviors. We evaluate the hydraulic mechanisms underlying tongue‐base retraction, highlight species‐specific adaptations in macaques and humans, and review primate tongue kinematics across distinct feeding stages. Finally, we synthesize recent advances in biomechanical modeling and experimental studies of tongue kinematics and their contributions to advancing three‐dimensional analyses of tongue movement during feeding and speech.

The middle ear of mammals is a major functional innovation, distinctive in that it is detached from the mandible and has a tympanic membrane supported by a ring-like ectotympanic. These novelties of the middle ear have enabled modern mammals to develop more sensitive hearing than all other tetrapods, especially at higher frequencies. Fossils from recent decades have clarified the evolution of the detached middle ear from the jaw bones of Paleozoic therapsids and Mesozoic cynodonts, and the evolution of the tympanum. These discoveries make it possible to answer important questions about the functional significance of these features. Here, we evaluate the relative hearing efficacy of a well-known cynodont precursor to mammals, Thrinaxodon liorhinus . Using finite element analysis (FEA), we calculated the harmonic response of the Thrinaxodon ear to bone-conducted and airborne sound and estimated the sound pressure level (SPL) at the stapedial footplate across a broad range of frequencies. We provide evidence that airborne sound received at the tympanum was the most effective mode of sound reception in Thrinaxodon . In contrast, bone conducted sound through the mandibular bones barely met our estimated hearing threshold. Our findings suggest that, like modern mammals, cynodonts were already reliant on a soft tissue tympanum to receive airborne sound, albeit with limited sensitivity to high frequencies. This is a detailed biomechanical evaluation of tympanum function in the cynodont predecessors of mammals and yields insight into the sequence of functional innovations during the evolution of mammal hearing.

Movement at complex joints, such as the ankle, can be challenging to quantify from external kinematics alone. We compared X-ray reconstruction of moving morphology (XROMM) and angles derived from high-speed video footage to study dorsiflexion in the ankle of a rhesus macaque (Macaca mulatta) during stance phase. We found inconsistent correspondence between angles measured on standard videos using five approaches and those measured directly on the talocrural joint using XROMM, indicating that different measurement methods capture different kinematic processes. Many externally measured angles indicate a range of motion of 30 deg during stance phase. The XROMM data, however, demonstrate that the talocrural joint itself only dorsiflexes about 15 deg through stance phase, with much greater mobility during swing phase. This suggests that other parts of the foot, likely the midfoot, contribute substantially to dorsiflexion in stance phase, while the talocrural joint is actively held in place.

Variability in the biomechanics and kinematics of prey capture in vertebrates has been studied extensively, with evidence of multiple strategies for successful feeding in many taxa. Early research into suction-feeding strikes in fishes hypothesized that fish utilize a set of pre-programmed strike kinematics that cannot be altered once initiated. However, more recent evidence has demonstrated that teleost fishes not only deploy unique strike kinematics for different prey types, but they also alter their kinematics in response to a prey item attempting to escape. It has not yet been explicitly investigated whether non-teleost actinopterygians can also modulate the strike in response to different prey types. Here, we examined the kinematics of suction strikes in bowfin, Amia ocellicauda , a holostean fish most closely related to gars. We recorded Amia feeding on both feeder fish and worms, two types of live prey differing in evasiveness, using X-ray reconstruction of moving morphology. We found significant prey-type effects on the magnitude, timing and velocity of jaw opening, hyoid arch depression and pectoral girdle motions. These prey-type effects demonstrate that the ability to modulate feeding strikes evolved early in actinopterygian fishes and is possibly the ancestral state for jawed vertebrates.

The divergence of Homo from gracile australopiths has been described as a trend of decreasing dentognathic size and robusticity, precipitated by stone tool use and/or a shift to softer foods, including meat. Yet, mechanical evidence supporting this narrative is sparse, and isotopic and archaeological data have led to the suggestion that a shift away from a gracile australopith-like diet would not have occurred in the most basal members of Homo but rather only with the appearance of Homo erectus, implying that the origin of our genus is not rooted in dietary change. Here, we provide mechanical evidence that Homo habilis exhibits an australopith-like pattern of facial strain during biting but, unlike most australopiths, was not suited for a diet that required forceful processing by the molar teeth. Homo habilis was at elevated risk of distractive jaw joint forces during those bites, constraining muscle recruitment so as to avoid generating uncomfortable/dangerous levels of tension in the joint. Modern humans have similar limitations. This suggests that selection on skeletal traits favouring forceful postcanine processing was relaxed by the earliest stages in the evolution of our genus, implying that dietary or food processing changes played an important role in the emergence of Homo .

Synopsis: During swallowing, a diverse range of mammals—from opossums to humans—propel food boluses out of the oropharynx via tongue base retraction (TBR). The widespread distribution of TBR behavior implies an ancient evolutionary origin, but the biomechanical mechanisms of TBR remain poorly understood. The evolution of TBR behavior is further complicated by the diversity of hyoid and tongue anatomy across mammals: to what extent does hyolingual morphology shape TBR mechanism? Using biplanar videoradiography and the XROMM workflow, we collected high-resolution 3D kinematic data in opossums (Marsupialia), dogs (Placentalia), and macaques (Placentalia) to test hypotheses on the evolutionary conservation of TBR mechanisms. Despite differences in hyolingual morphology and resting hyoid position, both dogs and macaques drive TBR through hyoid movement: hyoid excursions reduce the oral volume and squeeze the tongue base posteriorly, analogous to a hydraulic pump displacing an incompressible fluid. In opossums, however, intrinsic lingual muscles deform the tongue base to initiate TBR, independent of hyoid movement and oral volume change. We suggest that multiple mechanisms are viable for the highly conserved TBR behavior across mammals, and the functional diversity of TBR mechanisms is decoupled from the morphological diversity of the hyolingual system. This decoupling may have facilitated the evolution of novel hyolingual phenotypes while avoiding trade-offs in swallowing performance.

The American Association for Anatomy recently charged a task force with updating and expanding upon best practices and recommendations for human body donation programs. The task force comprised American Association for Anatomy members with specific and detailed knowledge about the legal, ethical, and procedural operations of body donation programs in the United States. The task force developed both foundational and aspirational recommendations. These recommendations emphasize the importance of prioritizing the ethical treatment of all body donors across all education and research sectors. Programs must adhere to several principles to operate an ethical body donation program, the foundation of which is informed consent. The policies and procedures of donation programs must clearly and transparently describe their core operational practices including outreach, registration, custody tracking, use, disposition, and memorialization. Such programs should be governed by a diverse oversight committee whose members are not directly responsible for the program's daily operations. These standards require that all body donation programs follow all national, state, and local laws in the program's jurisdiction. The following recommendations for body donation programs were approved by the Board of Directors of the American Association for Anatomy in 2024 and were conceived as a living document intended to be periodically modified and updated as the ethos and legislation of body donations evolve. Given the importance of these recommendations, translations of this document into Chinese and Spanish are available as supplementary materials S2 and S3.

The primary function of the tetrapod jaw is to transmit jaw muscle forces to bite points. The routes of force transfer in the jaw have never been studied but can be quantified using load paths – the shortest, stiffest routes from regions of force application to support constraints. Here, we use load path analysis to map force transfer from muscle attachments to bite point and jaw joint, and to evaluate how different configurations of trabecular and cortical bone affect load paths. We created three models of the mandible of the Virginia opossum, Didelphis virginiana, each with a cortical bone shell, but with different material properties for the internal spaces: (1) a cortical-trabecular model, in which the interior space is modeled with bulk properties of trabecular bone; (2) a cortical-hollow model, in which trabeculae and mandibular canal are modeled as hollow; and (3) a solid-cortical model, in which the interior is modeled as cortical bone. The models were compared with published in vivo bite force and bone strain data, and the load paths calculated for each model. The trabecular model, which is preferred because it most closely approximates the actual morphology, was best validated by in vivo data. In all three models, the load path was confined to cortical bone, although its route within the cortex varied depending on the material properties of the inner model. Our analysis shows that most of the force is transferred through the cortical, rather than trabecular bone, and highlights the potential of load path analysis for understanding form–function relationships in the skeleton.

Skeletal muscle fibre architecture provides important insights into performance of vertebrate locomotor and feeding behaviours. Chemical digestion and in situ sectioning of muscle bellies along their lengths to expose fibres, fibre orientation and intramuscular tendon, are two classical methods for estimating architectural variables such as fibre length (L_f) and physiological cross‐sectional area (PCSA). It has recently been proposed that L_f estimates are systematically shorter and hence less accurate using in situ sectioning. Here we addressed this hypothesis by comparing L_f estimates between the two methods for the superficial masseter and temporalis muscles in a sample of strepsirrhine and platyrrhine primates. Means or single‐specimen L_f estimates using chemical digestion were greater in 17/32 comparisons (53.13%), indicating the probability of achieving longer fibres using chemical digestion is no greater than chance in these taxonomic samples. We further explored the impact of sampling on scaling of L_f and PCSA in platyrrhines applying a bootstrapping approach. We found that sampling—both numbers of individuals within species and representation of species across the clade significantly influence scaling results of L_f and PCSA in platyrrhines. We show that intraspecific and clade sampling strategies can account for differences between previously published platyrrhine scaling studies. We suggest that differences in these two methodological approaches to assessing muscle architecture are relatively less consequential when estimating L_f and PCSA for comparative studies, whereas achieving more reliable estimates within species through larger samples and representation of the full clade space are important considerations in comparative studies of fibre architecture and scaling.