John. A. Nyakatura

Trabecular bone is known to adapt to mechanical stimuli, but its adaptive signal is understudied in small mammals. Such studies could, for example, inform previous hypotheses about size-specific scaling patterns. Here we looked at muroid rodents to analyse the effect of body size and locomotor behaviour on the trabecular network of the proximal stylopodia. We used µCT scans of humeri and femora to quantify various trabecular parameters. We found that the trabecular network changes with size, characterised by positive allometric scaling of bone volume fraction, connectivity density, and specific bone surface and negative allometric scaling of trabecular thickness and spacing. When compared with the available literature on scaling within various mammalian taxa, our results add to growing evidence that scaling patterns differ between smaller and larger mammals. We detected no significant differences among locomotor groups, which may reflect the limited sample size. However, some descriptive patterns were apparent. Trends in the femur were more in line with the mammalian literature than those observed in the humerus, but more data is needed to confirm these patterns.

Objectives: Beyond non‐biomechanical factors, the trabecular architecture of long bone epiphyses variably underlies functional adaptations to locomotor behavior. A recent study, characterizing tamarin trabecular bone using traditional metrics derived from Volumes of Interest (VOIs), identified a preliminary relationship with leaping distance. We hypothesized that quantifying trabecular topological indices from whole epiphyses in the same system would enhance detection of locomotor signals. Materials and Methods: We analyzed μCT scans of humeral and tibial whole‐epiphyseal trabecular bone in four tamarin species representing short‐ and long‐distance leapers. We quantified topological indices—node density (NodDen), trabecular tortuosity (TrabTort), trabecular length (TrabLen), and fractal dimension (FD)—alongside traditional metrics—degree of anisotropy (DA) and bone volume fraction (BV/TV). We tested relationships between trabecular traits and leaping distance and assessed the effects of non‐functional factors (sex and captivity). Results: Long‐distance leapers exhibited significantly higher NodDen in humeral and tibial epiphyses and increased distal humeral TrabTort. Regions of elevated NodDen were concentrated beneath the humeroscapular joint (proximal humerus) and occurred variably across other epiphyses. In contrast, DA, BV/TV, TrabLen, and FD were more strongly associated with sex and captivity. Discussion: Higher loading magnitudes associated with longer leaps may increase NodDen and, more tentatively, TrabTort in tamarin limb epiphyses. However, locomotion‐related signals were weaker than those linked to sex and captivity, suggesting that leaping distance affects specific trabecular traits, whereas the overall trabecular organization is driven by non‐functional factors. Whole‐epiphysis analyses and topological indices provide a valuable complement to traditional VOI‐based approaches and metrics.

In condylar hyperplasia (CH), one of the condylar processes of the mandible (CPs) grows more than its counterpart, leading to facial asymmetry. Using a mouse model of asymmetric masticatory loading, we investigated a possible mechanical origin of CH by evaluating the association between morphometric parameters of CP cartilage and bone and the mandibular load regime. Thirty-nine adult male and female mouse hemimandibles that underwent different loading regimes were used: underloaded (UL), overloaded (OL), and symmetrically loaded (SL) controls. Micro-CTs were used to assess macroscopic shape and trabecular bone (TB) parameters, and articular cartilage features were studied histologically. Data were compared using multivariate and pairwise statistics. Principal component analysis showed that OL CPs exhibit a wide range of shape variation and show thicker cartilage but display TB features indicative of ongoing remodeling. Female OL tended to show more variable and larger parameters than males. However, differences in traits between sexes and load groups were not always statistically significant. We conclude that an asymmetric increase in mandibular loading may be associated with changes in the macro-/microscopic structure of the CP, affecting males and females differently. Signs of cartilage growth and bone remodeling are found in OL individuals, which are compatible with CH features.

Cartilage thickness in mammalian joints increases with higher body masses. Contradicting previous studies found this increase to be positive allometric or negative allometric. Since approaches like computational modelling of animal locomotion, muscle moment arms, and joint dynamics rely on estimates of joint spacing (JS), it is important to accurately estimate an animal's cartilage thickness based on body mass. Here, we measured ex vivo internal joint distances (IJDs) or bone‐to‐bone distances on CT scans of fresh cadaveric forelimbs in a sample of small‐ to medium‐sized mammals. IJDs were measured in the humero‐ulnar and humero‐radial joint. We find IJDs to scale isometrically in both joints across the entire sample, with positive allometric tendencies only within a subsample of cursorial species and only in the humero‐ulnar articulation. The previously reported positive allometry can be linked to a cursorial sampling bias, and negative allometry results from a size constraint, acting on larger mammals than sampled here. Additionally, the IJDs were not affected by limb poses (i.e., flexed to extended, supinated, pronated). In rats and guinea pigs of varying sizes, we observed intraspecific isometric scaling with slight positive trends. This suggests that theoretically greater absolute forces—resulting from increased body mass with similar posture—only marginally contribute to relatively thicker cartilage in small‐ to medium‐sized mammals. Further, we conducted a “range of motion” (ROM) analysis in the humero‐ulnar joint of rats, guinea pigs, and maras, thus species of increasing body mass and level of cursoriality. ROM was assessed with varying estimated JS. Differences in the results are most biologically reasonable when allowing all six degrees of freedom (DOF, three rotational and three translational). Mobility decreases with increasing body mass and level of cursoriality, facilitated by increased restriction of movement to a single axis of flexion and extension. Such a trend persisted regardless of whether JS thresholds were estimated using intraspecific or interspecific regression models. The results suggest that variations in elbow mobility are less influenced by applied JS than by the morphological characteristics of the bones forming the joint. This observation has implications for future comparative studies of mammalian elbow function. Still, more research is needed to separate body mass, degree of cursoriality, or locomotor type as factors for elbow mobility.

Ascending and descending sloping and vertical branches is critical for arboreal locomotion and likely played a major role in early primate evolution. While most studies have focused on ascent, descending behaviors also provide insight into the functional significance of arboreal adaptations. To test how descending vertical supports of varying diameters affects locomotor abilities, we quantified postural and kinematic features during descents and ascents on vertical supports in 21 eutherian and metatherian mammals, and examined their relation to morphology. Primates showed greater variability in descent behaviors, using tail-first and side postures more often than other mammals, which predominantly descended head-first. Overall, animals adopted several kinematic adjustments to enhance stability during descent compared to ascent, including slower speeds, higher duty factors, and greater use of asymmetrical gaits. Additionally, vertical descent strategies reflected trade-offs among body mass, limb proportions, and head mass. Using a morphology-based model, we then inferred possible descent behaviors in 13 extinct euarchontoglires. Our results suggest that ancestral adaptations for vertical locomotion may have promoted frequent upright (head-up) postures in early primates.

Humans are drawn to patterns and hierarchies in nature, mimicking them particularly in decoration and architecture. Natural patterns, however, are never purely esthetic and, since evolution works on a variety of factors simultaneously, natural structural systems are intrinsically multifunctional. In order to understand the roles that structural patterns play in biology (and therefore their potential capabilities and utilization in design, architecture and engineering), we need to catalog and encapsulate the diversity of examples and the materials involved. Here, we provide a first classification of biological “tilings,” tessellated natural architectures that involve the repeated pattern of geometric, discrete elements bound by a joint material. By examining 100 examples across the Tree of Life, we reveal this natural structural motif is unexpectedly prevalent: we cover a huge taxonomic diversity, eight orders of magnitude in size scale, and myriad morphologies and functions ranging from optics to armor, allowing us to construct a hierarchical system of eight variables to classify form, function, and materiality in biological tilings. Using diverse means of data analysis (including multiple correspondence analysis), we show this database can be explored to reveal fundamental links among anatomical characteristics and functions as well as connections among and within taxonomic groups. Our resulting collection of “tessellated materials” and its companion website act therefore as a multidisciplinary meeting point (e.g. for biologists, designers, engineers, architects). In this way, our database offers windows for exploring selective pressures and trade-offs and a launchpad for future research and collaborative, cross-disciplinary, bioinspired projects.

Objectives: The intricate trabecular architecture of long-bone epiphyses underpins functional adaptations for diverse mammalian locomotion. Despite extensive study in other mammals, tamarin trabecular structure and fine-grained differences among leaping taxa remain poorly characterized. Materials and Methods: We examined humeral and tibial trabecular networks in four tamarin species representing short- and long-distance leapers using µCT-scans and a whole-epiphysis approach. We quantified network complexity with topological indices (node density, tortuosity, trabecular length, fractal dimension) alongside traditional metrics (degree of anisotropy [DA], bone volume fraction [BV/TV]) to capture both geometric and topological features. Results: Long leapers exhibit significantly higher node density in both humeral and tibial epiphyses and increased trabecular tortuosity in the distal humerus. Elevated node density localizes beneath the humero-scapular joint, within the proximal humerus, and in variable regions across other epiphyses. Other parameters (DA, BV/TV, trabecular length, fractal dimension) showed no leaping-related differences, instead correlating with sex and captivity. Discussion: Increased mechanical strain during longer leaps likely drives higher node density and, to a lesser extent, tortuosity in humeral and tibial epiphyses, with node density showing the strongest functional signal. While sex and captivity influence other trabecular traits, patterns in these key metrics support locomotor adaptation. Integrating whole-epiphysis analyses with novel topological indices enhances detection of subtle functional signals and complements VOI-based and traditional frameworks in comparative trabecular bone studies.

Ascending and descending sloping and vertical branches is critical for arboreal locomotion and likely played a major role in early primate evolution. While most studies have focused on ascent, descending behaviors also provide insight into the functional significance of arboreal adaptations. To test how descending vertical supports of varying diameters affects locomotor abilities, we quantified postural and kinematic features during descents and ascents on vertical supports in 21 eutherian and metatherian mammals, and examined their relation to morphology. Primates showed greater variability in descent behaviors, using tail-first and side postures more often than other mammals, which predominantly descended head-first. Overall, animals adopted several kinematic adjustments to enhance stability during descent compared to ascent, including slower speeds, higher duty factors, and greater use of asymmetrical gaits. Additionally, vertical descent strategies reflected trade-offs among body mass, limb proportions, and head mass. Using a morphology-based model, we then inferred possible descent behaviors in 13 extinct euarchontoglires. Our results suggest that ancestral adaptations for vertical locomotion may have promoted frequent upright (head-up) postures in early primates.

Analyses of the musculoskeletal system of callitrichid primates contribute to the understanding of the specializations of an apparently highly conserved body plan exhibited by this radiation of New World primates. This pilot study provides data from computational modeling of muscle function of five hip extensor muscles in four species of Callitrichidae to identify potential adaptations to previously documented differential leaping behaviors. Based on microCT scans of fresh cadavers, we reconstructed the muscle topology to inform the modeling of instantaneous muscle moment arms (MMAs) contributing to hip extension and accompanying muscle strains. Generally, muscle properties of the four species were surprisingly similar despite documented differences in leaping behavior. However, all extensors of Goeldi's marmoset (except for the semimembranosus) had the longest instantaneous MMAs. This may result in a greater capacity to generate hip torques in these marmosets (assuming identical force provided by the muscles), beneficial to their specialization in long‐distance trunk‐to‐trunk leaps. The shorter instantaneous MMAs of the extensors of the three other studied species indicate specialization toward more rapid hip extension. Strain analysis showed that, in all four species, the two glutei optimally generate force during the entire extension of the hip from a strongly crouched leg position to take off with an almost entirely extended leg. For the other three muscles (biceps femoris, semimembranosus and semitendinosus), we found optimal strains for force generation only at 50°–140° hip extension. We tentatively conclude that a relatively generalized musculoskeletal system for hip extension, coupled with moderate biomechanical adaptations favoring either joint torque or rotational speed, enables callitrichids to achieve remarkable locomotor versatility within highly intricate arboreal environments.