Bite forces, canine strength and skull allometry in carnivores (Mammalia, Carnivora)

Myological and osteological approaches to gape and bite force reconstruction in Smilodon fatalis

Masticatory gape and bite force are important behavioral and ecological variables. While much has been written about the highly derived masticatory anatomy of Smilodon fatalis, there remains a great deal of debate about their masticatory behaviors. To that end, we establish osteological proxies for masticatory adductor fascicle length (FL) based on extant felids and apply these along with previously validated techniques to S. fatalis to provide estimates of fascicle lengths, maximum osteological gapes, and bite force. While the best correlated FL proxies in extant felids do not predict particularly long fascicles, these proxies may be of value for less morphologically distinct felids. A slightly less well correlated proxy predicts a temporalis FL 15% longer than that of Panthera tigris. While angular maximum bony gape is significantly larger in S. fatalis than it is in extant felids, linear gape at the canine tip and carnassial notch were not significantly different from those of extant felids. Finally, we produce anatomical bite force estimates of 1283.74 N at the canine and 4671.41 N at the carnassial, which are similar in magnitude to estimates not of the largest felids but of the much smaller P. onca, with S. fatalis producing slightly less force at the canines and more at the carnassials. These estimates align with previous predictions that S. fatalis may have killed large prey with canine shearing bites produced, in part, by force contributions of the postcranial muscles.

Evolutionary patterns of cat-like carnivorans unveil drivers of the sabertooth morphology

The sabertooth morphology stands as a classic case of convergence, manifesting recurrently across various vertebrate groups, prominently within two carnivorans clades: felids and nimravids. Nonetheless, the evolutionary mechanisms driving these recurring phenotypes remain insufficiently understood, lacking a robust phylogenetic and spatiotemporal framework. We reconstruct the tempo and mode of craniomandibular evolution of Felidae and Nimravidae and evaluate the strength of the dichotomy between conical and saber-toothed species, as well as within saber-toothed morphotypes. To do so, we investigate morphological variation, convergence, phenotypic integration, and evolutionary rates, employing a comprehensive dataset of nearly 200 3D models encompassing mandibles and crania from both extinct and extant feline-like carnivorans, spanning their entire evolutionary timeline. Our results reject the hypothesis of a distinctive sabertooth morphology, revealing instead a continuous spectrum of feline-like phenotypes in both the cranium and mandible, with sporadic instances of unequivocal convergence. Disparity peaked at the end of the Miocene and is usually higher in clades containing taxa with extreme sabertoothed adaptations. We show that taxa with saberteeth exhibit a lower degree of craniomandibular integration, allowing to exhibit a greater range of phenotypes. Those same groups usually show a burst of morphological evolutionary rate at the beginning of their evolutionary history. Consequently, we propose that a reduced degree of integration coupled with rapid evolutionary rates emerge as key components in the development of a sabertooth morphology in multiple clades.

Craniometric Characteristics of Selected Carnivora Species Kept in Captivity in Relation to Bite Force and Bending Strength of the Upper Canines

The aim of this study was to analyse the bite forces of seven species from three carnivore families: Canidae, Felidae, and Ursidae. The material consisted of complete, dry crania and mandibles. A total of 33 measurements were taken on each skull, mandible, temporomandibular joint, and teeth. The area of the temporalis and masseter muscles was calculated, as was the length of the arms of the forces acting on them. Based on the results, the bite force was calculated using a mathematical lever model. This study compared the estimated areas of the masticatory muscles and the bending strength of the upper canines among seven species. A strong correlation was found between cranial size and bite force. The results confirmed the hypothesis that the weight of the animal and the size of the skull have a significant effect on the bite force.

Comparative three-dimensional jaw muscle anatomy of marsupial carnivores (Dasyurus spp.) and the termite-eating numbat (Myrmecobius fasciatus)

Among marsupials, the endangered numbat (Myrmecobius fasciatus) is the only obligate myrmecophage with a diet comprised strictly of termites. Like many other specialised myrmecophagous mammals, numbats have a gracile and highly specialised skull morphology with an elongated rostrum and small braincase. Myrmecobiidae is one of four taxonomic families within the Australasian marsupial order Dasyuromorphia, and to date, the muscular anatomy of any member of this group is relatively poorly known. We utilised microdissection and contrast-enhanced microcomputed tomography scanning to provide the first comprehensive qualitative and quantitative descriptions of jaw muscle anatomy in numbats and quolls (Dasuyrus species). The arrangement of the jaw muscles across these species was conservative, both in gross anatomy and muscle proportions, corresponding to a 'generalised' mammalian pattern. In contrast to Dasyurus, the jaw muscles of the numbat were greatly reduced. Many aspects of the muscle anatomy of the numbat were similar to patterns reported in other myrmecophagous species, particularly a greatly reduced temporalis muscle. Unusually, the digastric muscle in the numbat was comprised of a single, large anterior belly while the posterior belly was absent. We propose that the enlarged anterior belly of the digastric may be linked to jaw stabilisation and coordination of tongue movements during feeding. The lateral insertion and fascial connection of the digastric to the tongue in numbats may also aid in distributing stress evenly across the jaw and minimise muscle fatigue. The muscle descriptions and three-dimensional models provided in this study will facilitate further analysis of musculoskeletal adaptation and evolution within the Dasyuromorphia.

Functional morphology of the Triassic apex predator Saurosuchus galilei (Pseudosuchia: Loricata) and convergence with a post-Triassic theropod dinosaur

Pseudosuchian archosaurs, reptiles more closely related to crocodylians than to birds, exhibited high morphological diversity during the Triassic and are thus associated with hypotheses of high ecological diversity during this time. One example involves basal loricatans which are non-crocodylomorph pseudosuchians traditionally known as "rauisuchians." Their large size (5-8+ m long) and morphological similarities to post-Triassic theropod dinosaurs, including dorsoventrally deep skulls and serrated dentitions, suggest basal loricatans were apex predators. However, this hypothesis does not consider functional behaviors that can influence more refined roles of predators in their environment, for example, degree of carcass utilization. Here, we apply finite element analysis to a juvenile but three-dimensionally well-preserved cranium of the basal loricatan Saurosuchus galilei to investigate its functional morphology and to compare with stress distributions from the theropod Allosaurus fragilis to assess degrees of functional convergence between Triassic and post-Triassic carnivores. We find similar stress distributions and magnitudes between the two study taxa under the same functional simulations, indicating that Saurosuchus had a somewhat strong skull and thus exhibited some degree of functional convergence with theropods. However, Saurosuchus also had a weak bite for an animal of its size (1015-1885 N) that is broadly equivalent to the bite force of modern gharials (Gavialis gangeticus). We infer that Saurosuchus potentially avoided tooth-bone interactions and consumed the softer parts of carcasses, unlike theropods and other basal loricatans. This deduced feeding mode for Saurosuchus increases the known functional diversity of basal loricatans and highlights functional differences between Triassic and post-Triassic apex predators.

How genomic insights into the evolutionary history of clouded leopards inform their conservation

Clouded leopards (Neofelis spp.), a morphologically and ecologically distinct lineage of big cats, are severely threatened by habitat loss and fragmentation, targeted hunting, and other human activities. The long-held poor understanding of their genetics and evolution has undermined the effectiveness of conservation actions. Here, we report a comprehensive investigation of the whole genomes, population genetics, and adaptive evolution of Neofelis. Our results indicate the genus Neofelis arose during the Pleistocene, coinciding with glacial-induced climate changes to the distributions of savannas and rainforests, and signatures of natural selection associated with genes functioning in tooth, pigmentation, and tail development, associated with clouded leopards' unique adaptations. Our study highlights high-altitude adaptation as the main factor driving nontaxonomic population differentiation in Neofelis nebulosa. Population declines and inbreeding have led to reduced genetic diversity and the accumulation of deleterious variation that likely affect reproduction of clouded leopards, highlighting the urgent need for effective conservation efforts.

Different, but the same: Inferring the hunting behaviour of the hypercarnivorous bush dog (Speothos venaticus) through finite element analysis

Cerdocyonina is a clade composed by the South-American canids in which the bush dog (Speothos venaticus) is one of the most elusive species. Known for its unique morphology within the group, this small, bear-like faced canid is the only member of the clade adapted to hypercarnivory, an almost exclusively meat-based diet currently present only in usually large, pack-hunting canids such as the grey wolf (Canis lupus). However, much of the biology of the bush dog is poorly understood, and inferences about its ecology, hunting strategies and diet are usually based on observation of captive individuals and anecdotal records, with reduced quantitative data to offer support. Here, we investigated the craniomandibular functional morphology of the bush dog through finite element analysis (FEA). FEA was employed to model the biting behaviour and to create extrinsic and intrinsic functional scenarios with different loads, corresponding to different bites used to subdue and process the prey. For comparison, the same modelling was applied to the skull of a grey wolf and a grey fox (Urocyon cinereoargenteus). Our analysis showed that the bush dog's responses to loading are more similar to the wolf's than to the fox's in most scenarios, suggesting a convergent craniomandibular functional morphology between these two hypercarnivorous species, despite their distinct phylogenetic positions and body sizes. Differences between the three taxa are noteworthy and suggested to be related to the size of the usual prey. The modelled bite force for the bush dog is relatively strong, about half of that estimated for the wolf and about 40% stronger than the fox's bite. The results strengthen with quantitative data the inferences of the bush dog as a pack-hunting predator with prey size similar to its own, such as large rodents and armadillos, being specialised in subduing and killing its prey using multiple bites. Its similarity to the wolf also confirms anecdotal accounts of predation on mammals that are much larger than itself, such as peccaries and tapirs. These data highlight the ecological specialisation of this small canid in a continent where large, pack-hunting canids are absent.

Morphological quantification of the maxillary canine tooth in the domestic dog (Canis lupus familiaris)

Canine tooth shape is known to vary with diet and killing behavior in wild animals and the relationship between form and function is driven in part by selective pressure. However, comparative investigation of the domestic dog (Canis lupus familiaris) is of interest. How do they compare to their wild counterparts? This study sought to quantify and characterize the morphology of the canine tooth in the domestic dog, and to provide a preliminary investigation into the variance in canine tooth morphology across individual dogs of varying breeds. Three-dimensional (3D) models generated from micro-computed tomography (µ-CT) studies of 10 mature maxillary canine teeth from the domesticated dog (Canis lupus familiaris) were used to quantify key morphological features and evaluate variance among dogs. Results show that, utilizing modern imaging and model building software, the morphology of the canine tooth can be comprehensively characterized and quantified. Morphological variables such as second moment of area and section modulus (geometrical parameters related to resistance to bending), as well as aspect ratio, ridge sharpness, cusp sharpness and enamel thickness are optimized in biomechanically critical areas of the tooth crown to balance form and function. Tooth diameter, second moment of area, section modulus, cross sectional area, tooth volume and length as well as enamel thickness are highly correlated with body weight. In addition, we found preliminary evidence of morphological variance across individual dogs. Quantification of these features provide insight into the balance of form and function of the canine tooth in wild and domesticated canids. In addition, results suggest that variance between dogs exist in some morphological features and most morphological features are highly correlated with body weight.

Traumatic injury patterns in humans from large feline predators: A systematic review and descriptive analysis

Background

Traumatic attacks from large feline predators cause a small yet significant burden of disease in rural populations, are increasing in frequency, cause complex injuries and worsen human–wildlife conflicts. Data on the traumatic injury pattern found in victims of these animals is sparse, and this study aimed to collate and synthesise patterns of injury to inform the care and management of these patients.

Methods

A multi-ethnographic literature search (PubMed, Embase, Web of Science, Google Scholar, and other sources) was performed on 12/1/21 to capture all available data describing anatomical injury and the mechanism of injury sustained by humans from attacks by lions, leopards and tigers. Quality and bias assessment was performed using the Joanna Briggs Critical Evaluation tools.

Results

Of 5110 studies identified, 42 were included in this review totalling 84 individual patient cases. A total of 85% of fatal injuries were due to exsanguination combined with neuroaxis injury of the neck. All wounds were susceptible to tissue loss, infection and long-term neuro-vascular complications. Leopards injured anterior-midline structures of the neck more often than did lions and tigers, while the latter caused high-energy fractures. Time lag to treatment for survivors of wild attacks extended to multiple days, and occult injury was common.

Conclusion

In addition to the primary finding of complex neck injury, this study generated specific patterns of injury seen from the included species and highlighted occult injury and healthcare disparity as challenges in providing patient care.

Taking a stab at modelling canine tooth biomechanics in mammalian carnivores with beam theory and finite-element analysis

Canine teeth are vital to carnivore feeding ecology, facilitating behaviours related to prey capture and consumption. Forms vary with specific feeding ecologies; however, the biomechanics that drive these relationships have not been comprehensively investigated. Using a combination of beam theory analysis (BTA) and finite-element analysis (FEA) we assessed how aspects of canine shape impact tooth stress, relating this to feeding ecology. The degree of tooth lateral compression influenced tolerance of multidirectional loads, whereby canines with more circular cross-sections experienced similar maximum stresses under pulling and shaking loads, while more ellipsoid canines experienced higher stresses under shaking loads. Robusticity impacted a tooth's ability to tolerate stress and appears to be related to prey materials. Robust canines experience lower stresses and are found in carnivores regularly encountering hard foods. Slender canines experience higher stresses and are associated with carnivores biting into muscle and flesh. Curvature did not correlate with tooth stress; however, it did impact bending during biting. Our simulations help identify scenarios where canine forms are likely to break and pinpoint areas where this breakage may occur. These patterns demonstrate how canine shape relates to tolerating the stresses experienced when killing and feeding, revealing some of the form-function relationships that underpin mammalian carnivore ecologies.

Performance of wild animals with “broken” traits: Movement patterns in nature of moose with leg injuries

Organismal traits are presumed to be well suited for performance in the tasks required for survival, growth, and reproduction. Major injuries to such traits should therefore compromise performance and prevent success in the natural world; yet some injured animals can survive for long periods of time and contribute to future generations. We here examine 3 years of camera trap observations along a remote trail through old‐growth forest in northern British Columbia, Canada. The most common observations were of moose (2966), wolves (476), and brown bears (224). The moose overwhelmingly moved in one direction along the trail in the late fall and early winter and in the other direction in the spring. This movement was clustered/contagious, with days on which many moose traveled often being interspersed with days on which few moose traveled. On the video recordings, we identified 12 injured moose, representing 1.4% of all moose observations. Seven injuries were to the carpus, three were to the antebrachium, and two were to the tarsus—and they are hypothesized to reflect damage to ligaments, tendons, and perhaps bones. The injured moose were limping in all cases, sometimes severely; and yet they did not differ noticeably from uninjured moose in the direction, date, contagiousness, or speed of movement along the trail. We discuss the potential relevance of these findings for the action of natural selection in the evolution of organismal traits important for performance.

Dietary niches of creodonts and carnivorans of the late Eocene Cypress Hills Formation

Modern North American carnivorous mammal assemblages consist of species from a single clade: the Carnivora. Carnivorans once coexisted with members of other meat-eating clades, including the creodonts (Hyaenodontida and Oxyaenida). Creodonts, however, went extinct in North America during the late Eocene and early Oligocene, potentially due to niche overlap and resource competition with contemporary carnivorans. In this study, we employ a community ecology approach to understand whether the dietary niches of coexisting creodonts and carnivorans overlapped during the late Eocene (Chadronian North American Land Mammal Age), a time when creodonts were dwindling and carnivorans were diversifying. We quantify niche overlap based on inferences of diet from carnassial tooth shape estimated using Orientation Patch Count, Dirichlet's Normal Surface Energy, and linear dental measurements as well as from body mass for all species in the Calf Creek Local Fauna of Cypress Hills, Saskatchewan (Treaty 4 land). Although creodonts and carnivorans shared characteristics of their carnassial tooth shape, suggesting similar chewing mechanics and feeding habits, we find that marked differences in body size likely facilitated niche partitioning, at least between the largest creodonts and carnivorans. Calculations of prey focus masses and prey mass spectra indicate that only the smallest creodont may have experienced significant competition for prey with the coeval carnivorans. We suggest that the ultimate extinction of creodonts from North America during the late Eocene and Oligocene was unlikely to have been driven by factors related to niche overlap with carnivorans.

The canine counts! Significance of a craniodental measure to describe sexual dimorphism in canids: Golden jackals (Canis aureus) and African wolves (Canis lupaster)

Sexual dimorphism is a widespread phenomenon among mammals, including carnivorans. While sexual dimorphism in golden jackals (Canis aureus) has been analysed in the past, in the related and apparently convergent canid, the African wolf (Canis lupaster), it is poorly studied and showed to be relatively small. Previously, sexual size dimorphism (SSD) research in these species was mostly based on skull and body measurements. In our study, we also included dental measurements, namely the diameter of the canine. We used 11 measured sections of 104 adult specimens, comprising 61 golden jackal and 43 African wolf skulls. Data analyses were carried out through logistic regression and conditional inference trees (CIT). To compare the results of SSD to other species, sexual dimorphism indices (SDI) were calculated. Golden jackals and African wolves show significant sexual size dimorphism, both in cranial and dental size. The logistic regression revealed that the mesiodistal diameter of the upper canine is most effective in discerning the sexes. The difference in the calculated SDI of the canine diameter between the sexes amounted to 8.71 in golden jackals and 14.11 in African wolves, respectively—with regional diversity. Thus, the canine diameter is an important measure to investigate SSD as well as an easy tool to apply in the field.

Cranial shape variation in mink: Separating two highly similar species

European and American minks (Mustela lutreola and Neovison vison, respectively) are very similar in their ecology, behavior, and morphology. However, the American mink is a generalist predator and seems to adapt better to anthropized environments, allowing it to outcompete the European mink in areas where it has been introduced, threatening the survival of the native species. To assess whether morphological differences may be contributing to the success of the American mink relative to the European mink, we analyzed shape variation in the cranium of both species using 3D geometric morphometrics. A set of 38 landmarks and 107 semilandmarks was used to study shape variation between and within species, and to assess how differences in size factored into that variation. Sexual dimorphism in both size and shape was also studied. Significant differences between species were found in cranial shape, but not in size. Relative to American mink, European mink have a shorter facial region with a rounder forehead and wider orbits, a longer neurocranium with less developed crests and processes, and an antero-medially placed tympanic bullae with an anteriorly expanded cranial border. Within species, size-related sexual dimorphism is highly significant, but sexual dimorphism in shape is only significant in American mink, not in European mink. Additionally, two trends common to both species were discovered, one related to allometric changes and another to sexual size dimorphism. Shape changes related to increasing size can be subdivided into two, probably related, groups: increased muscle force and growth. The first group somewhat parallels the differences between both mink species, while the second group of traits includes an anterodorsal expansion of the face, and the neurocranium shifting from a globous shape in small individuals to a dorsoventrally flattened ellipse in the largest ones. Finally, the sexual dimorphism trend, while also accounting for differences in muscle force, seems to be related to the observed dietary differences between males and females. Overall, differences between species and sexes, and shape changes with increasing size, seem to mainly relate to differences in masticatory-muscle volume and therefore muscle force and bite force, which, in turn, relate to a wider range of potential prey (bigger prey, tougher shells). Thus, muscle force (and dietary range) would be larger in American mink than in European mink, in males than in females, and in larger individuals than in smaller ones.

Masticatory muscle architectural correlates of dietary diversity in Canidae, Ursidae, and across the order Carnivora

Carnivorans represent extreme ecomorphological diversity, encompassing remarkable variation in form, habitat, and diet. The relationship between the masticatory musculature and dietary ecology has been explored in a number of carnivoran lineages, including felids and the superfamily Musteloidea. In this study, we present novel architectural data on two additional carnivoran families-Ursidae and Canidae-and supplement these previous studies with additional felid, musteloid, herpestid, hyaenid, and viverrid taxa (a total of 53 species across 10 families). Gross dissection data were collected following a standardized protocol-sharp dissection followed by chemical digestion. Summed jaw adductor forces were also transformed into bite force estimates (BF) using osteologically calculated leverages. All data were linearized, log-transformed, and size-adjusted using two proxies for each taxon-body mass (BM) and cranial geometric mean-to assess relative scaling trends. These architectural data were then analyzed in the context of dietary ecology to examine the impact of dietary size (DS) and dietary mechanical properties (DMP). Muscle mass, physiological cross-sectional area, and BF scaled with isometry or positive allometry in all cases, whereas fascicle lengths (FLs) scaled with isometry or negative allometry. With respect to diet, BM-adjusted FLs were strongly correlated with DS in musteloids, but not in any other lineage. The relationship between size-adjusted BF and DMP was also significant within musteloids, and across the sample as a whole, but not within other individual lineages. This interfamilial trend may reflect the increased morphological and dietary diversity of musteloids relative to other carnivoran groups.

3D Insights into the Effects of Captivity on Wolf Mastication and Their Tooth Marks; Implications in Ecological Studies of Both the Past and Present

Human populations have been known to develop complex relationships with large carnivore species throughout time, with evidence of both competition and collaboration to obtain resources throughout the Pleistocene. From this perspective, many archaeological and palaeontological sites present evidence of carnivore modifications to bone. In response to this, specialists in the study of microscopic bone surface modifications have resorted to the use of 3D modeling and data science techniques for the inspection of these elements, reaching novel limits for the discerning of carnivore agencies. The present research analyzes the tooth mark variability produced by multiple Iberian wolf individuals, with the aim of studying how captivity may affect the nature of tooth marks left on bone. In addition to this, four different populations of both wild and captive Iberian wolves are also compared for a more in-depth comparison of intra-species variability. This research statistically shows that large canid tooth pits are the least affected by captivity, while tooth scores appear more superficial when produced by captive wolves. The superficial nature of captive wolf tooth scores is additionally seen to correlate with other metric features, thus influencing overall mark morphologies. In light of this, the present study opens a new dialogue on the reasons behind this, advising caution when using tooth scores for carnivore identification and contemplating how elements such as stress may be affecting the wolves under study.

Evaluating bony predictors of bite force across the order Carnivora

In carnivorans, bite force is a critical and ecologically informative variable that has been correlated with multiple morphological, behavioral, and environmental attributes. Whereas in vivo measures of biting performance are difficult to obtain in many taxa-and impossible in extinct species-numerous osteological proxies exist for estimating masticatory muscle size and force. These proxies include both volumetric approximations of muscle dimensions and direct measurements of muscular attachment sites. In this study, we compare three cranial osteological techniques for estimating muscle size (including 2D-photographic and 3D-surface data approaches) against dissection-derived muscle weights and physiological cross-sectional area (PCSA) within the jaw adductor musculature of 40 carnivoran taxa spanning eight families, four orders of magnitude in body size, and the full dietary spectrum of the order. Our results indicate that 3D-approaches provide more accurate estimates of muscle size than do surfaces measured from 2D-lateral photographs. However, estimates of a muscle's maximum cross-sectional area are more closely correlated with muscle mass and PCSA than any estimates derived from muscle attachment areas. These findings highlight the importance of accounting for muscle thickness in osteological estimations of the masticatory musculature; as muscles become volumetrically larger, their larger cross-sectional area does not appear to be associated with a proportional increase in the attachment site area. Though volumetric approaches approximate muscle dimensions well across the order as a whole, caution should be exercised when applying any single method as a predictor across diverse phylogenies.

Back to the bones: do muscle area assessment techniques predict functional evolution across a macroevolutionary radiation?

Measures of attachment or accommodation area on the skeleton are a popular means of rapidly generating estimates of muscle proportions and functional performance for use in large-scale macroevolutionary studies. Herein, we provide the first evaluation of the accuracy of these muscle area assessment (MAA) techniques for estimating muscle proportions, force outputs and bone loading in a comparative macroevolutionary context using the rodent masticatory system as a case study. We find that MAA approaches perform poorly, yielding large absolute errors in muscle properties, bite force and particularly bone stress. Perhaps more fundamentally, these methods regularly fail to correctly capture many qualitative differences between rodent morphotypes, particularly in stress patterns in finite-element models. Our findings cast doubts on the validity of these approaches as means to provide input data for biomechanical models applied to understand functional transitions in the fossil record, and perhaps even in taxon-rich statistical models that examine broad-scale macroevolutionary patterns. We suggest that future work should go back to the bones to test if correlations between attachment area and muscle size within homologous muscles across a large number of species yield strong predictive relationships that could be used to deliver more accurate predictions for macroevolutionary and functional studies.

Novel Integrative Modeling of Molecules and Morphology across Evolutionary Timescales

Evolutionary models account for either population- or species-level processes but usually not both. We introduce a new model, the FBD-MSC, which makes it possible for the first time to integrate both the genealogical and fossilization phenomena, by means of the multispecies coalescent (MSC) and the fossilized birth–death (FBD) processes. Using this model, we reconstruct the phylogeny representing all extant and many fossil Caninae, recovering both the relative and absolute time of speciation events. We quantify known inaccuracy issues with divergence time estimates using the popular strategy of concatenating molecular alignments and show that the FBD-MSC solves them. Our new integrative method and empirical results advance the paradigm and practice of probabilistic total evidence analyses in evolutionary biology.[Caninae; fossilized birth–death; molecular clock; multispecies coalescent; phylogenetics; species trees.]

Dietary ecology of Alaskan polar bears (Ursus maritimus) through time and in response to Arctic climate change

Arctic climate change poses serious threats to polar bears (Ursus maritimus) as reduced sea ice makes seal prey inaccessible and marine ecosystems undergo bottom-up reorganization. Polar bears' elongated skulls and reduced molar dentition, as compared to their sister species the grizzly bear (Ursus arctos), are adaptations associated with hunting seals on sea ice and a soft, lipid-rich diet of blubber and meat. With significant declines in sea ice, it is unclear if and how polar bears may be altering their diets. Clarifying polar bear dietary responses to changing climates, both today and in the past, is critical to proper conservation and management of this apex predator. This is particularly important when a dietary strategy may be maladaptive. Here, we test the hypothesis that hard-food consumption (i.e., less preferred foods including bone), inferred from dental microwear texture analysis, increased with Arctic warming. We find that polar bears demonstrate a conserved absence of hard-object feeding in Alaska through time (including approximately 1000 years ago), until the 21st century, consistent with a highly conserved and specialized diet of soft blubber and flesh. Notably, our results also suggest that some 21st-century polar bears may be consuming harder foods (e.g., increased carcass utilization, terrestrial foods including garbage), despite having skulls and metabolisms poorly suited for such a diet. Prior to the 21st century, only polar bears with larger mandibles demonstrated increased hard-object feeding, though to a much lower degree than closely related grizzly bears which regularly consume mechanically challenging foods. Polar bears, being morphologically specialized, have biomechanical constraints which may limit their ability to consume mechanically challenging diets, with dietary shifts occurring only under the most extreme scenarios. Collectively, the highly specialized diets and cranial morphology of polar bears may severely limit their ability to adapt to a warming Arctic.

Canines and carnassials as indicators of sociality in durophagous hyaenids: analyzing the past to understand the present

We analyzed the lower and upper dentition of the family Hyaenidae along its evolutionary history from a multivariate point of view. A total of 13,103 individual measurements of the lengths and widths of canines and the main post-canine teeth (lower third and fourth premolar, lower first molar, and upper second, third, and fourth premolars) were collected for 39 extinct and extant species of this family. We analyzed these measurements using principal component analyses. The multivariate structure characterized the main groups of previously defined hyaenid ecomorphs. Strikingly, our analyses also detected differences between social hunting durophages (such as Crocuta crocuta) and solitary scavengers (such as Hyaena hyaena or Parahyaena brunnea). Concerning the hyaenid bauplan, social hunters have large carnassials and smaller canines, whereas solitary scavengers show the exact opposite morphological adaptations. Additionally, scavengers exhibited upper canines larger than lower ones, whereas hunters have upper and lower canines of similar size. It is hypothesized that sociality has led to an increase in carnassial length for hunting durophages via scramble competition at feeding. Such competition also penalizes adults from bringing food to cubs, which are consequently breastfed. On the other hand, it is also hypothesized that natural selection has led to solitary scavengers having large canines to transport carcasses to cubs. Our results indicate that these functional aspects are also better reflected by lower teeth than the upper dentition, which leads to a mosaic evolution.

Phenotypic integration in feliform carnivores: Covariation patterns and disparity in hypercarnivores versus generalists

The skeleton is a complex arrangement of anatomical structures that covary to various degrees depending on both intrinsic and extrinsic factors. Among the Feliformia, many species are characterized by predator lifestyles providing a unique opportunity to investigate the impact of highly specialized hypercarnivorous diet on phenotypic integration and shape diversity. To do so, we compared the shape of the skull, mandible, humerus, and femur of species in relation to their feeding strategies (hypercarnivorous vs. generalist species) and prey preference (predators of small vs. large prey) using three-dimensional geometric morphometric techniques. Our results highlight different degrees of morphological integration in the Feliformia depending on the functional implication of the anatomical structure, with an overall higher covariation of structures in hypercarnivorous species. The skull and the forelimb are not integrated in generalist species, whereas they are integrated in hypercarnivores. These results can potentially be explained by the different feeding strategies of these species. Contrary to our expectations, hypercarnivores display a higher disparity for the skull than generalist species. This is probably due to the fact that a specialization toward high-meat diet could be achieved through various phenotypes. Finally, humeri and femora display shape variations depending on relative prey size preference. Large species feeding on large prey tend to have robust long bones due to higher biomechanical constraints.

Dry versus wet and gross: Comparisons between the dry skull method and gross dissection in estimations of jaw muscle cross-sectional area and bite forces in sea otters

Bite force is a measure of feeding performance used to elucidate links between animal morphology, ecology, and fitness. Obtaining live individuals for in vivo bite-force measurements or freshly deceased specimens for bite force modeling is challenging for many species. Thomason's dry skull method for mammals relies solely on osteological specimens and, therefore, presents an advantageous approach that enables researchers to estimate and compare bite forces across extant and even extinct species. However, how accurately the dry skull method estimates physiological cross-sectional area (PCSA) of the jaw adductor muscles and theoretical bite force has rarely been tested. Here, we use an ontogenetic series of southern sea otters (Enhydra lutris nereis) to test the hypothesis that skeletomuscular traits estimated from the dry skull method accurately predicts test traits derived from dissection-based biomechanical modeling. Although variables from these two methods exhibited strong positive relationships across ontogeny, we found that the dry skull method overestimates PCSA of the masseter and underestimates PCSA of the temporalis. Jaw adductor in-levers for both jaw muscles and overall bite force are overestimated. Surprisingly, we reveal that sexual dimorphism in craniomandibular shape affects temporalis PCSA estimations; the dry skull method predicted female temporalis PCSA well but underestimates male temporalis PCSA across ontogeny. These results highlight the importance of accounting for sexual dimorphism and other intraspecific variation when using the dry skull method. Together, we found the dry skull method provides an underestimation of bite force over ontogeny and that the underlying anatomical components driving bite force may be misrepresented.

The pre-Pleistocene fossil thylacinids (Dasyuromorphia: Thylacinidae) and the evolutionary context of the modern thylacine

The thylacine is popularly used as a classic example of convergent evolution between placental and marsupial mammals. Despite having a fossil history spanning over 20 million years and known since the 1960s, the thylacine is often presented in both scientific literature and popular culture as an evolutionary singleton unique in its morphological and ecological adaptations within the Australian ecosystem. Here, we synthesise and critically evaluate the current state of published knowledge regarding the known fossil record of Thylacinidae prior to the appearance of the modern species. We also present phylogenetic analyses and body mass estimates of the thylacinids to reveal trends in the evolution of hypercarnivory and ecological shifts within the family. We find support that Mutpuracinus archibaldi occupies an uncertain position outside of Thylacinidae, and consider Nimbacinus richi to likely be synonymous with N. dicksoni. The Thylacinidae were small-bodied (< ~8 kg) unspecialised faunivores until after the ~15-14 Ma middle Miocene climatic transition (MMCT). After the MMCT they dramatically increase in size and develop adaptations to a hypercarnivorous diet, potentially in response to the aridification of the Australian environment and the concomitant radiation of dasyurids. This fossil history of the thylacinids provides a foundation for understanding the ecology of the modern thylacine. It provides a framework for future studies of the evolution of hypercarnivory, cursoriality, morphological and ecological disparity, and convergence within mammalian carnivores.

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.

An assessment of the role of the falx cerebri and tentorium cerebelli in the cranium of the cat (Felis silvestris catus)

The falx cerebri and the tentorium cerebelli are two projections of the dura mater in the cranial cavity which ossify to varying degrees in some mammalian species. The idea that the ossification of these structures may be necessary to support the loads arising during feeding has been proposed and dismissed in the past, but never tested quantitatively. To address this, a biomechanical model of a domestic cat (Felis silvestris catus) skull was created and the material properties of the falx and tentorium were varied for a series of loading regimes incorporating the main masticatory and neck muscles during biting. Under these loading conditions, ossification of the falx cerebri does not have a significant impact on the stress in the cranial bones. In the case of the tentorium, however, a localized increase in stress was observed in the parietal and temporal bones, including the tympanic bulla, when a non-ossified tentorium was modelled. These effects were consistent across the different analyses, irrespective of loading regime. The results suggest that ossification of the tentorium cerebelli may play a minor role during feeding activities by decreasing the stress in the back of the skull.

Bite Forces and Their Measurement in Dogs and Cats

Bite force is generated by the interaction of the masticatory muscles, the mandibles and maxillae, the temporomandibular joints (TMJs), and the teeth. Several methods to measure bite forces in dogs and cats have been described. Direct in vivo measurement of a bite in dogs has been done; however, bite forces were highly variable due to animal volition, situation, or specific measurement technique. Bite force has been measured in vivo from anesthetized dogs by electrical stimulation of jaw adductor muscles, but this may not be reflective of volitional bite force during natural activity. In vitro bite forces have been estimated by calculation of the force produced using mechanical equations representing the jaw adductor muscles and of the mandible and skull structure Bite force can be estimated in silico using finite element analysis (FEA) of the computed model of the anatomical structures. FEA can estimate bite force in extinct species; however, estimates may be lower than the measurements in live animals and would have to be validated specifically in domestic dogs and cats to be reliable. The main factors affecting the bite forces in dogs and cats are body weight and the skull's morphology and size. Other factors such as oral pain, TMJ disorders, masticatory muscle atrophy, and malocclusion may also affect bite force. Knowledge of bite forces in dogs and cats is essential for various clinical and research fields such as the development of implants, materials, and surgical techniques as well as for forensic medicine. This paper is a summary of current knowledge of bite forces in dogs and cats, including the effect of measurement methods and of other factors.

CRANIOMETRIA EM LOBOS-GUARÁ Chrysocyon brachyurus ILLIGER, 1815 (CARNIVORA, CANIDAE)

Resumo Os objetivos do presente trabalho foram obter mensurações em crânios de Chrysocyon brachyurus adultos, caracterizar valores craniométricos para a espécie e observar se a classificação craniométrica de cães domésticos é adequada para o lobo-guará. Foram utilizados sete crânios de lobo-guará adultos, sem distinção de sexo. Determinou-se a localização de nove pontos craniométricos utilizados em cães domésticos para o lobo-guará. Os pontos foram referência para as 18 medidas craniométricas utilizadas para o cálculo de seis índices craniométricos. Os índices craniométricos são: cefálico 96,147 ± 3,89 mm, crânio facial 1,06 ± 0,20 mm, do neurocrânio 61,68 ± 1,83 mm, facial 100,06 ± 3,36 mm, basal 29,79 ± 1,18 mm, e do forame magno 89,25 ± 7,82 mm. A classificação em braquicefálico, dolicocefálico ou mesaticefálico não se adequa a Chrysocyon brachyurus.

Scaling and Accommodation of Jaw Adductor Muscles in Canidae

The masticatory apparatus amongst closely related carnivoran species raises intriguing questions about the interplay between allometry, function, and phylogeny in defining interspecific variations of cranial morphology. Here we describe the gross structure of the jaw adductor muscles of several species of canid, and then examine how the muscles are scaled across the range of body sizes, phylogenies, and trophic groups. We also consider how the muscles are accommodated on the skull, and how this is influenced by differences of endocranial size. Data were collected for a suite of morphological metrics, including body mass, endocranial volume, and muscle masses and we used geometric morphometric shape analysis to reveal associated form changes. We find that all jaw adductor muscles scale isometrically against body mass, regardless of phylogeny or trophic group, but that endocranial volume scales with negative allometry against body mass. These findings suggest that head shape is partly influenced by the need to house isometrically scaling muscles on a neurocranium scaling with negative allometry. Principal component analysis suggests that skull shape changes, such as the relatively wide zygomatic arches and large sagittal crests seen in species with higher body masses, allow the skull to accommodate a relative enlargement of the jaw adductors compared with the endocranium. Anat Rec, 299:951-966, 2016. © 2016 The Authors The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology Published by Wiley Periodicals, Inc.