Masticatory muscle architecture in a water-rat from Australasia (Murinae, Hydromys) and its implication for the evolution of carnivory in rodents

Ontogeny of the masticatory muscles in the opossum Didelphis albiventris (Marsupialia, Didelphimorphia, Didelphidae)

Opossums (marsupials of the Didelphidae family) retain a generalized masticatory apparatus and tribosphenic molars, often used as models to understand the evolution of mastication in early therian mammals. Like all marsupials, their growth goes through a stage when pups complete their development while permanently attached to the mother's teats before weaning and starting feeding on their own. Yet, while the masticatory muscles of adults are known, as is the ontogeny of the cranium and mandible, the ontogenetic changes in the masticatory muscles remain unknown. Here we describe for the first time the changes in the masticatory muscles observed in lactating pups, and weaned juveniles, subadults, and adults in the White-eared opossum, Didelphis albiventris, through dissection of 25 specimens and quantification of relative muscle masses, lines of actions and mechanical advantages whenever possible. We also assessed the scaling patterns of muscle masses and mechanical advantages through ontogeny. The main changes, as expected, were found between suckling and weaned specimens, although some changes still occurred from juveniles to adults. The adult adductor musculature is similar to the other Didelphis species already known, with a dominant m. temporalis that originates on the lateral wall of the skull, up to the sagittal and nuchal crests, and fills the zygomatic arch when inserting into the lateral and medial surfaces of the coronoid process, respectively through the pars superficialis and pars profunda. The m. masseter is also subdivided in superficial and deep bundles which originate posteriorly in the maxilla and zygomatic arch, and insert into the angular process and masseteric fossa in the mandible. The m. pterygoideus medialis originates from the palatine, the pterygoid bone and the alisphenoid, and it inserts on the angular process medially. Suckling pups showed muscles with more restricted attachments, reduced muscle lines of action, and less diversity in the fiber orientation. The absence of the postorbital constriction also resulted in a distinct morphology of the m. temporalis pars profunda, through two bundles, one anterior and one posterior, which insert more inferiorly into the mandible. These major changes can be related to the onset of mastication and to size-related changes in growing weaned age classes. In general, all adductor muscles grew with positive allometry, and increased their fixation areas through, in part, the development of specific regions of the cranium and mandible. Their lines of action also increase and diversify along ontogeny. These changes can be related to the functional requirements for fixation during lactation, which shift to adduction and mastication movements after weaning.

Beyond CREA: Evolutionary patterns of non-allometric shape variation and divergence in a highly allometric clade of murine rodents

The shared functions of the skull are thought to result in common evolutionary patterns in mammalian cranial shape. Craniofacial evolutionary allometry (CREA) is a particularly prominent pattern where larger species display proportionally elongate facial skeletons and smaller braincases. It was recently proposed that CREA arises from biomechanical effects of cranial scaling when diets are similar. Thus, deviations from CREA should occur with changes in cranial biomechanics, for example due to dietary change. Here, we test this using 3D geometric morphometric analysis in a dataset of Australian murine crania, which are highly allometric. We contrast allometric and non-allometric variation in the cranium by comparing evolutionary mode, allometry, ordinations, as well as allometry, integration, and modularity in functional modules. We found evidence of stabilising selection in allometry-containing and size-free shape, and substantial non-allometric variation aligned with dietary specialisation in parallel with CREA. Integration among cranial modules was higher, and modularity lower, with size included, but integration between rostrum and cranial vault, which are involved in the CREA pattern, dropped dramatically after size removal. Our results thus support the hypothesis that CREA is a composite arising from selection on cranial function, with substantial non-allometric shape variation occurring alongside CREA where dietary specialisation impacts selection on gnawing function. This emphasises the need to research mammalian cranial evolution in the context of allometric and non-allometric selection on biomechanical function.

Springhares, flying and flightless scaly-tailed squirrels (Anomaluromorpha, Rodentia) are the squirrely mouse: comparative anatomy of the masticatory musculature and its implications on the evolution of hystricomorphy in rodents

Anomaluromorpha is a particularly puzzling suborder of Rodentia. Endemic to Africa, this clade includes the extant genera Idiurus, Anomalurus, Zenkerella, and Pedetes. These rodents present an hystricomorphous condition of the skull, characterized by a large infraorbital foramen, which evolved independently within the mouse-related clade over a span of approximately 57 million years. They exhibit a high disparity in craniomandibular and dental morphology that has kept their phylogenetic affinities disputed for a long time. Given the past significance of masticatory morphotypes in establishing the classification of Rodentia, we propose to explore variations in the masticatory apparatus of Anomaluromorpha in order to evaluate whether its related features can offer additional data for systematics and contribute to our understanding of the complexity of hystricomorphy. In order to do so, we used traditional dissection and diffusible iodine-based contrast-enhanced computed tomography (diceCT) to accurately describe and compare the anatomy of the specimens. We found that the muscle morphology displays clear differentiation among each anomaluromorph taxonomic unit. Specifically, the masseteric complex of Anomaluromorpha exhibits distinctive synapomorphies such as the infraorbital part of the zygomaticomandibularis muscle being separated into a rostral and orbital part and an absence of a posterior part of the zygomaticomandibularis. Additionally, the orbital portion of the infraorbital part originates from a well-marked ridge and fossa at the level of its area of origin on the anteromedial wall of the orbital cavity, a feature that is absent in other members of the mouse-related clade. This evident bony feature, among others, is strongly associated with muscular anatomy and can contribute to ascertaining the taxonomic status of extinct representatives of the clade. Finally, we showed that the hystricomorphy of Anomaluromorpha largely differs from those of Ctenohystrica and Dipodoidea and that the definition of this morphotype is complex and cannot be reduced simply to the size of the opening of the infraorbital foramen.

Implication of transcription factor FOXD2 dysfunction in syndromic congenital anomalies of the kidney and urinary tract (CAKUT)

Congenital anomalies of the kidney and urinary tract (CAKUT) are the predominant cause for chronic kidney disease below age 30 years. Many monogenic forms have been discovered due to comprehensive genetic testing like exome sequencing. However, disease-causing variants in known disease-associated genes only explain a proportion of cases. Here, we aim to unravel underlying molecular mechanisms of syndromic CAKUT in three unrelated multiplex families with presumed autosomal recessive inheritance. Exome sequencing in the index individuals revealed three different rare homozygous variants in FOXD2, encoding a transcription factor not previously implicated in CAKUT in humans: a frameshift in the Arabic and a missense variant each in the Turkish and the Israeli family with segregation patterns consistent with autosomal recessive inheritance. CRISPR/Cas9-derived Foxd2 knockout mice presented with a bilateral dilated kidney pelvis accompanied by atrophy of the kidney papilla and mandibular, ophthalmologic, and behavioral anomalies, recapitulating the human phenotype. In a complementary approach to study pathomechanisms of FOXD2-dysfunction-mediated developmental kidney defects, we generated CRISPR/Cas9-mediated knockout of Foxd2 in ureteric bud-induced mouse metanephric mesenchyme cells. Transcriptomic analyses revealed enrichment of numerous differentially expressed genes important for kidney/urogenital development, including Pax2 and Wnt4 as well as gene expression changes indicating a shift toward a stromal cell identity. Histology of Foxd2 knockout mouse kidneys confirmed increased fibrosis. Further, genome-wide association studies suggest that FOXD2 could play a role for maintenance of podocyte integrity during adulthood. Thus, our studies help in genetic diagnostics of monogenic CAKUT and in understanding of monogenic and multifactorial kidney diseases.

Gape drives regional variation in temporalis architectural dynamics in tufted capuchins

Dynamic changes in jaw movements and bite forces depend on muscle architectural and neural factors that have rarely been compared within the same muscle. Here we investigate how regional muscle architecture dynamics-fascicle rotation, shortening, lengthening and architectural gear ratio (AGR)-vary during chewing across a functionally heterogeneous muscle. We evaluate whether timing in architecture dynamics relates to gape, food material properties and/or muscle activation. We also examine whether static estimates of temporalis fibre architecture track variation in dynamic architecture. Fascicle-level architecture dynamics were measured in three regions of the superficial temporalis of three adult tufted capuchins (Sapajus apella) using biplanar videoradiography and the XROMM workflow. Architecture dynamics data were paired with regional fine-wire electromyography data from four adult tufted capuchins. Gape accounted for most architectural change across the temporalis, but architectural dynamics varied between regions. Mechanically challenging foods were associated with lower AGRs in the anterior region. The timing of most dynamic architectural changes did not vary between regions and differed from regional variation in static architecture. Collectively these findings suggest that, when modelling temporalis muscle force production in extant and fossil primates, it is important to account for the effects of gape, regionalization and food material properties. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Bayesian Prediction of Multivariate Ecology from Phenotypic Data Yields New Insights into the Diets of Extant and Extinct Taxa

AbstractMorphology often reflects ecology, enabling the prediction of ecological roles for taxa that lack direct observations, such as fossils. In comparative analyses, ecological traits, like diet, are often treated as categorical, which may aid prediction and simplify analyses but ignores the multivariate nature of ecological niches. Furthermore, methods for quantifying and predicting multivariate ecology remain rare. Here, we ranked the relative importance of 13 food items for a sample of 88 extant carnivoran mammals and then used Bayesian multilevel modeling to assess whether those rankings could be predicted from dental morphology and body size. Traditional diet categories fail to capture the true multivariate nature of carnivoran diets, but Bayesian regression models derived from living taxa have good predictive accuracy for importance ranks. Using our models to predict the importance of individual food items, the multivariate dietary niche, and the nearest extant analogs for a set of data-deficient extant and extinct carnivoran species confirms long-standing ideas for some taxa but yields new insights into the fundamental dietary niches of others. Our approach provides a promising alternative to traditional dietary classifications. Importantly, this approach need not be limited to diet but serves as a general framework for predicting multivariate ecology from phenotypic traits.

From fibre to function: are we accurately representing muscle architecture and performance?

The size and arrangement of fibres play a determinate role in the kinetic and energetic performance of muscles. Extrapolations between fibre architecture and performance underpin our understanding of how muscles function and how they are adapted to power specific motions within and across species. Here we provide a synopsis of how this 'fibre to function' paradigm has been applied to understand muscle design, performance and adaptation in animals. Our review highlights the widespread application of the fibre to function paradigm across a diverse breadth of biological disciplines but also reveals a potential and highly prevalent limitation running through past studies. Specifically, we find that quantification of muscle architectural properties is almost universally based on an extremely small number of fibre measurements. Despite the volume of research into muscle properties, across a diverse breadth of research disciplines, the fundamental assumption that a small proportion of fibre measurements can accurately represent the architectural properties of a muscle has never been quantitatively tested. Subsequently, we use a combination of medical imaging, statistical analysis, and physics-based computer simulation to address this issue for the first time. By combining diffusion tensor imaging (DTI) and deterministic fibre tractography we generated a large number of fibre measurements (>3000) rapidly for individual human lower limb muscles. Through statistical subsampling simulations of these measurements, we demonstrate that analysing a small number of fibres (n < 25) typically used in previous studies may lead to extremely large errors in the characterisation of overall muscle architectural properties such as mean fibre length and physiological cross-sectional area. Through dynamic musculoskeletal simulations of human walking and jumping, we demonstrate that recovered errors in fibre architecture characterisation have significant implications for quantitative predictions of in-vivo dynamics and muscle fibre function within a species. Furthermore, by applying data-subsampling simulations to comparisons of muscle function in humans and chimpanzees, we demonstrate that error magnitudes significantly impact both qualitative and quantitative assessment of muscle specialisation, potentially generating highly erroneous conclusions about the absolute and relative adaption of muscles across species and evolutionary transitions. Our findings have profound implications for how a broad diversity of research fields quantify muscle architecture and interpret muscle function.

Insularity and Aridity as Drivers of Mandibular Disparity in Thylamys elegans (Waterhouse, 1839) from Populations of the Atacama Desert, Chile

Island ecosystems differ in several elements from mainland ecosystems and may induce variations related to natural selection and patterns of adaptation in most aspects of the biology of an organism. Thylamys elegans (Waterhouse, 1839) is a marsupial endemic to Chile, distributed from Loa River to Concepción. Historically, three subspecies have been described: Thylamys elegans elegans, Thylamys elegans coquimbensis and Thylamys elegans soricinus. For this research, two morphometric approaches and a biomechanical model were used to compare the mandible shapes and biomechanics between two Chilean mouse opossum populations belonging to different subspecies: one from the coastal desert of Chile (T. e. coquimbensis) and the other from the central inland region (T. e. elegans). Additionally, mandibles of insular populations found in the Reserva Nacional Pinguino de Humboldt (RNPH)), from which the subspecies association is unknown, were also included. The results showed that insular populations have differences in mandibular shapes, sizes and biomechanical characteristics compared to continental populations, which may be related to environmental variables like aridity and vegetation cover, prey type, insularity effects and/or the founder effect on micromammals, apart from vicariance hypotheses and other selective pressures.

Ovipositor and mouthparts in a fossil insect support a novel ecological role for early orthopterans in 300 million years old forests

A high portion of the earliest known insect fauna is composed of the so-called 'lobeattid insects', whose systematic affinities and role as foliage feeders remain debated. We investigated hundreds of samples of a new lobeattid species from the Xiaheyan locality using a combination of photographic techniques, including reflectance transforming imaging, geometric morphometrics, and biomechanics to document its morphology, and infer its phylogenetic position and ecological role. Ctenoptilus frequens sp. nov. possessed a sword-shaped ovipositor with valves interlocked by two ball-and-socket mechanisms, lacked jumping hind-legs, and certain wing venation features. This combination of characters unambiguously supports lobeattids as stem relatives of all living Orthoptera (crickets, grasshoppers, katydids). Given the herein presented and other remains, it follows that this group experienced an early diversification and, additionally, occurred in high individual numbers. The ovipositor shape indicates that ground was the preferred substrate for eggs. Visible mouthparts made it possible to assess the efficiency of the mandibular food uptake system in comparison to a wide array of extant species. The new species was likely omnivorous which explains the paucity of external damage on contemporaneous plant foliage.

Mitogenomes Reveal Multiple Colonization of Mountains by Rattus in Sundaland

Tropical mountains are cradles of biodiversity and endemism. Sundaland, tropical Southeast Asia, hosts 3 species of Rattus endemic to elevations above 2000 m with an apparent convergence in external morphology: Rattus korinchi and R. hoogerwerfi from Sumatra, and R. baluensis from Borneo. A fourth one, R. tiomanicus, is restricted to lowland elevations across the whole region. The origins of these endemics are little known due to the absence of a robust phylogenetic framework. We use complete mitochondrial genomes from the 3 high altitude Rattus, and several related species to determine their relationships, date divergences, reconstruct their history of colonization, and test for selection on the mitochondrial DNA. We show that mountain colonization happened independently in Borneo (<390 Kya) and Sumatra (~1.38 Mya), likely from lowland lineages. The origin of the Bornean endemic R. baluensis is very recent and its genetic diversity is nested within the diversity of R. tiomanicus. We found weak evidence of positive selection in the high-elevation lineages and attributed the greater nonsynonymous mutations on these branches (specially R. baluensis) to lesser purifying selection having acted on the terminal branches in the phylogeny.

Effects of long-term ethanol storage on muscle architecture

Muscle excursion and force potential can be estimated from architectural variables, including mass, volume, fascicle length, and density. These have been collected from fresh specimens, preserved specimens, and sometimes mixed samples of both. However, preservation alters the gross morphology of muscles. This study aims to quantify the effects of long-term storage on myological properties across a sample of fresh and ethanol preserved Mus musculus specimens ranging in storage time from 16 to 130 years. Masses, volumes, and densities of biceps femoris, quadriceps femoris, and triceps surae were measured, and histological cross-sections of some specimens were used to evaluate the microscale effects of long-term fluid preservation. For the remainder of the sample, chemically dissected fascicle lengths were measured to evaluate the fixation effects on the linear dimensions of muscle architecture. Relative muscle mass, volume, fascicle length, average fiber area, and density, and percent fiber area were regressed against years stored in ethanol. Muscle size dropped steeply between fresh and stored samples, ultimately decreasing by 62 and 60%, respectively. These losses correlate with histologically measured shrinking of average muscle fiber area. Density of stored specimens plateaued 5% below that of fresh ones. Although muscles lost mass and volume during ethanol storage, fascicle lengths did not shorten significantly (presumably because they were preserved attached on either end to bone). This study demonstrates that muscle mass, volume, and density of specimens stored long-term in ethanol should be corrected by factors of 2.64, 2.49, and 1.054 respectively for comparability to fresh specimens.

Anatomical and ontogenetic influences on muscle density

Physiological cross-sectional area (PCSA), an important biomechanical variable, is an estimate of a muscle's contractile force potential and is derived from dividing muscle mass by the product of a muscle's average fascicle length and a theoretical constant representing the density of mammalian skeletal muscle. This density constant is usually taken from experimental studies of small samples of several model taxa using tissues collected predominantly from the lower limbs of adult animals. The generalized application of this constant to broader analyses of mammalian myology assumes that muscle density (1) is consistent across anatomical regions and (2) is unaffected by the aging process. To investigate the validity of these assumptions, we studied muscles of rabbits (Oryctolagus cuniculus) in the largest sample heretofore investigated explicitly for these variables, and we did so from numerous anatomical regions and from three different age-cohorts. Differences in muscle density and histology as a consequence of age and anatomical region were evaluated using Tukey's HSD tests. Overall, we observed that older individuals tend to have denser muscles than younger individuals. Our findings also demonstrated significant differences in muscle density between anatomic regions within the older cohorts, though none in the youngest cohort. Approximately 50% of the variation in muscle density can be explained histologically by the average muscle fiber area and the average percent fiber area. That is, muscles with larger average fiber areas and a higher proportion of fiber area tend to be denser. Importantly, using the age and region dependent measurements of muscle density that we provide may increase the accuracy of PCSA estimations. Although we found statistically significant differences related to ontogeny and anatomical region, if density cannot be measured directly, the specific values presented herein should be used to improve accuracy. If a single muscle density constant that has been better validated than the ones presented in the previous literature is preferred, then 1.0558 and 1.0502 g/cm³ would be reasonable constants to use across all adult and juvenile muscles respectively.

Insectivory leads to functional convergence in a group of Neotropical rodents

The mandible of vertebrates serves as insertion area for masticatory muscles that originate on the skull, and its functional properties are subject to selective forces related to trophic ecology. The efficiency of masticatory muscles can be measured as mechanical advantage on the mandible, which, in turn, has the property of correlating with bite force and shape. In the present work, we quantify the mechanical advantage of the mandible of akodontine rodents, which present a diverse radiation of insectivorous specialists, to assess their relationship to the estimated bite force and diet. We also tested the degree of morphofunctional convergence in response to insectivory on the group. We found the mechanical advantages to be convergent on insectivorous species, and associated with the estimated bite force, with higher mechanical advantages in species with a stronger bite and short, robust mandibles and lower mechanical advantages in insectivorous species with weaker bites and more elongated, dorso-ventrally compressed mandibles. Insectivorous species of Akodontini are functional specialists for the consumption of live prey and may exploit the resources that shrews, moles and hedgehogs consume elsewhere.

Morphological divergence in giant fossil dormice

Insular gigantism-evolutionary increases in body size from small-bodied mainland ancestors-is a conceptually significant, but poorly studied, evolutionary phenomenon. Gigantism is widespread on Mediterranean islands, particularly among fossil and extant dormice. These include an extant giant population of Eliomys quercinus on Formentera, the giant Balearic genus †Hypnomys and the exceptionally large †Leithia melitensis of Pleistocene Sicily. We quantified patterns of cranial and mandibular shape and their relationships to head size (allometry) among mainland and insular dormouse populations, asking to what extent the morphology of island giants is explained by allometry. We find that gigantism in dormice is not simply an extrapolation of the allometric trajectory of their mainland relatives. Instead, a large portion of their distinctive cranial and mandibular morphology resulted from the population- or species-specific evolutionary shape changes. Our findings suggest that body size increases in insular giant dormice were accompanied by the evolutionary divergence of feeding adaptations. This complements other evidence of ecological divergence in these taxa, which span predominantly faunivorous to herbivorous diets. Our findings suggest that insular gigantism involves context-dependent phenotypic modifications, underscoring the highly distinctive nature of island faunas.

Australian Rodents Reveal Conserved Cranial Evolutionary Allometry across 10 Million Years of Murid Evolution

AbstractAmong vertebrates, placental mammals are particularly variable in the covariance between cranial shape and body size (allometry), with rodents being a major exception. Australian murid rodents allow an assessment of the cause of this anomaly because they radiated on an ecologically diverse continent notably lacking other terrestrial placentals. Here, we use 3D geometric morphometrics to quantify species-level and evolutionary allometries in 38 species (317 crania) from all Australian murid genera. We ask whether ecological opportunity resulted in greater allometric diversity compared with other rodents or whether conserved allometry suggests intrinsic constraints and/or stabilizing selection. We also assess whether cranial shape variation follows the proposed rule of craniofacial evolutionary allometry (CREA), whereby larger species have relatively longer snouts and smaller braincases. To ensure we could differentiate parallel versus nonparallel species-level allometric slopes, we compared the slopes of rarefied samples across all clades. We found exceedingly conserved allometry and CREA-like patterns across the 10-million-year split between Mus and Australian murids. This could support both intrinsic-constraint and stabilizing-selection hypotheses for conserved allometry. Large-bodied frugivores evolved faster than other species along the allometric trajectory, which could suggest stabilizing selection on the shape of the masticatory apparatus as body size changes.

Comparative masticatory myology in anteaters and its implications for interpreting morphological convergence in myrmecophagous placentals

BACKGROUND

Ecological adaptations of mammals are reflected in the morphological diversity of their feeding apparatus, which includes differences in tooth crown morphologies, variation in snout size, or changes in muscles of the feeding apparatus. The adaptability of their feeding apparatus allowed them to optimize resource exploitation in a wide range of habitats. The combination of computer-assisted X-ray microtomography (µ-CT) with contrast-enhancing staining protocols has bolstered the reconstruction of three-dimensional (3D) models of muscles. This new approach allows for accurate descriptions of muscular anatomy, as well as the quick measurement of muscle volumes and fiber orientation. Ant- and termite-eating (myrmecophagy) represents a case of extreme feeding specialization, which is usually accompanied by tooth reduction or complete tooth loss, snout elongation, acquisition of a long vermiform tongue, and loss of the zygomatic arch. Many of these traits evolved independently in distantly-related mammalian lineages. Previous reports on South American anteaters (Vermilingua) have shown major changes in the masticatory, intermandibular, and lingual muscular apparatus. These changes have been related to a functional shift in the role of upper and lower jaws in the evolutionary context of their complete loss of teeth and masticatory ability.

METHODS

We used an iodine staining solution (I2KI) to perform contrast-enhanced µ-CT scanning on heads of the pygmy (Cyclopes didactylus), collared (Tamandua tetradactyla) and giant (Myrmecophaga tridactyla) anteaters. We reconstructed the musculature of the feeding apparatus of the three extant anteater genera using 3D reconstructions complemented with classical dissections of the specimens. We performed a description of the musculature of the feeding apparatus in the two morphologically divergent vermilinguan families (Myrmecophagidae and Cyclopedidae) and compared it to the association of morphological features found in other myrmecophagous placentals.

RESULTS

We found that pygmy anteaters (Cyclopes) present a relatively larger and architecturally complex temporal musculature than that of collared (Tamandua) and giant (Myrmecophaga) anteaters, but shows a reduced masseter musculature, including the loss of the deep masseter. The loss of this muscle concurs with the loss of the jugal bone in Cyclopedidae. We show that anteaters, pangolins, and aardvarks present distinct anatomies despite morphological and ecological convergences.

Proximate and ultimate drivers of variation in bite force in the insular lizards Podarcis melisellensis and Podarcis sicula

Bite force is a key performance trait in lizards because biting is involved in many ecologically relevant tasks, including foraging, fighting and mating. Several factors have been suggested to impact bite force in lizards, such as head morphology (proximate factors), or diet, intraspecific competition and habitat characteristics (ultimate factors). However, these have been generally investigated separately and mostly at the interspecific level. Here we tested which factors drive variation in bite force at the population level and to what extent. Our study includes 20 populations of two closely related lacertid species, Podarcis melisellensis and Podarcis sicula, which inhabit islands in the Adriatic. We found that lizards with more forceful bites have relatively wider and taller heads, and consume more hard prey and plant material. Island isolation correlates with bite force, probably by driving resource availability. Bite force is only poorly explained by proxies of intraspecific competition. The linear distance from a large island and the proportion of difficult-to-reduce food items consumed are the ultimate factors that explain most of the variation in bite force. Our findings suggest that the way in which morphological variation affects bite force is species-specific, probably reflecting the different selective pressures operating on the two species.

Review of the methods used for calculating physiological cross-sectional area (PCSA) for ecological questions

This review examines literature that used physiological cross-sectional area (PCSA) as a representative measure of an individual muscle's maximal isometric force production. PCSA is used to understand the muscle architecture and how a trade-off between muscle force and muscle contractile velocity reflect adaptations of the musculoskeletal system as a reflection of functional demands. Over the decades, methods have been developed to measure muscle volume, fascicle lengths, and pennation angle to calculate PCSA. The advantages and limitations of these methods (especially the inclusion/elimination of pennation angle) are discussed frequently; however, these method descriptions are scattered throughout the literature. Here, we reviewed and summarised the different approaches to collecting and recording muscle architectural properties to subsequently calculate PCSA. By critically discussing the advantages and limitations of each methodology, we aim to provide readers with an overview of repeatable methods to assess muscle architecture. This review may serve as a guide to facilitate readers searching for the appropriate techniques to calculate PCSA and measure muscle architecture to be applied in ecomorphology research. RESEARCH HIGHLIGHTS: Discuss the theories behind PCSA in a synthesised review to inform researchers about PCSA methodology.

Dietary morphology of two island-endemic murid rodent clades is consistent with persistent, incumbent-imposed competitive interactions

A lineage colonizing a geographic region with no competitors may exhibit rapid diversification due to greater ecological opportunity. The resultant species diversity of this primary-colonizing (incumbent) clade may limit subsequent lineages' ability to persist unless these non-incumbent lineages are ecologically distinct. We compare the diversity in diet-related mandibular morphology of two sympatric murid rodent clades endemic to Luzon Island, Philippines-incumbent Phloeomyini and secondary-colonizing Chrotomyini-to the mandibular morphological diversity of Sahul Hydromyini, the sister clade of Chrotomyini and the incumbent murid lineage on the supercontinent of Sahul. This three-clade comparison allows us to test the hypothesis that incumbent lineages can force persistent ecological distinction of subsequent colonists at the time of colonization and throughout the subsequent history of the two sympatric clades. We find that Chrotomyini forms a subset of the diversity of their clade plus Sahul Hydromyini that minimizes overlap with Phloeomyini. We also infer that this differentiation extends to the stem ancestor of Chrotomyini and Sahul Hydromyini, consistent with a biotic filter imposed by Phloeomyini. Our work illustrates that incumbency has the potential to have a profound influence on the ecomorphological diversity of colonizing lineages at the island scale even when the traits in question are evolving at similar rates among independently colonizing clades.

Morphometric models for estimating bite force in Mus and Rattus: mandible shape and size do better than lever-arm ratios

Morphological traits are frequently used as proxies for functional outputs such as bite force performance. This allows researchers to infer and interpret the impacts of functional variation, notably in adaptive terms. Despite their mechanical bases, the predictive power of these proxies for performance is not always tested. In particular, their accuracy at the intraspecific level is rarely assessed, and they have sometimes been shown to be unreliable. Here, we compare the performance of several morphological proxies in estimating in vivo bite force, across five species of murine rodents, at the interspecific and intraspecific levels. Proxies used include the size and shape of the mandible, as well as individual and combined muscular mechanical advantages (temporal, superficial masseter and deep masseter). Maximum voluntary bite force was measured in all individuals included. To test the accuracy of predictions allowed by the proxies, we combined linear regressions with a leave-one-out approach, estimating an individual bite force based on the rest of the dataset. The correlations between estimated values and the in vivo measurements were tested. At the interspecific and intraspecific levels, size and shape were better estimators than mechanical advantages. Mechanical advantage showed some predictive power at the interspecific level, but generally not within species, except for the deep masseter in Rattus. In few species, size and shape did not allow us to predict bite force. Extrapolations of performance based on mechanical advantage should therefore be used with care, and are mostly unjustified within species. In the latter case, size and shape are preferable.

Divergent in shape and convergent in function: Adaptive evolution of the mandible in Sub-Antarctic mice

Convergent evolution in similar environments constitutes strong evidence of adaptive evolution. Transported with people around the world, house mice colonized even remote areas, such as Sub-Antarctic islands. There, they returned to a feral way of life, shifting towards a diet enriched in terrestrial macroinvertebrates. Here, we test the hypothesis that this triggered convergent evolution of the mandible, a morphological character involved in food consumption. Mandible shape from four Sub-Antarctic islands was compared to phylogeny, tracing the history of colonization, and climatic conditions. Mandible shape was primarily influenced by phylogenetic history, thus discarding the hypothesis of convergent evolution. The biomechanical properties of the jaw were then investigated. Incisor in-lever and temporalis out-lever suggested an increase in the velocity of incisor biting, in agreement with observations on various carnivorous and insectivorous rodents. The mechanical advantage related to incisor biting also revealed an increased functional performance in Sub-Antarctic populations, and appears to be an adaptation to catch prey more efficiently. The amount of change involved was larger than expected for a plastic response, suggesting microevolutionary processes were evolved. This study thus denotes some degree of adaptive convergent evolution related to changes in habitat-related changes in dietary items in Sub-Antarctic mice, but only regarding simple, functionally relevant aspects of mandible morphology.