The effects of locomotion on the structural characteristics of avian limb bones

Comparative Analysis of Morphometric, Densitometric, and Mechanical Properties of Skeletal Locomotor Elements in Three Duck Species (Anatidae: Anatinae)

Ducks (Anatinae) play a crucial role in wetland ecosystems, contributing to seed dispersal and nutrient cycling. This study investigates the skeletal adaptations of three duck species: the Mallard (Anas platyrhynchos), Tufted Duck (Aythya fuligula), and Green-Winged Teal (Anas crecca). The focus is on the tibiotarsus and humerus bones to understand how these adaptations support their different locomotion and habitat preferences. Bone samples n = 6 of deceased ducks (both male and female) from each species (for a total of 36 samples) were cleaned and measured for length, weight, and density. Dual-energy X-ray absorptiometry was used to determine bone mineral density (BMD) and bone mineral content (BMC), and mechanical properties like yield force and stiffness were tested using a 3-point bending test. The results show significant differences in body weight, bone weight, and bone length among the species, with Mallards being the largest and Teals the smallest. Male Teals displayed higher relative bone weight (RBW) in their tibia compared to male Mallards, and male Mallards had significantly lower RBW in the humerus compared to the other species. Female Teals had higher RBW than the other species. Teals also exhibited much lower BMD in the tibia, whereas female Mallards had lower BMD in the humerus. The Seedor index revealed that male Mallards had the highest values in the tibia, while female Teals had the lowest. Mechanical testing indicated that Teals had lower yield force and breaking force in the tibia, whereas Mallards showed the highest stiffness in both bones. Tufted Ducks had intermediate values, consistent with their diving behaviour. These findings suggest that the Mallard's robust bones support its adaptability to various environments and diverse locomotion and foraging strategies. The Teal's lighter and less dense bones facilitate rapid flight and agility in shallow wetlands. The Tufted Duck's intermediate bone characteristics reflect its specialization in diving, requiring a balance of strength and flexibility. Understanding these skeletal differences may provide valuable insights into the evolutionary biology and biomechanics of these species, aiding in their conservation and enhancing our knowledge of their roles in wetland ecosystems. By exploring the functional morphology of these ducks, this study aims to shed light on the biomechanical mechanisms that underpin their locomotion and foraging behaviours.

Fossil and modern penguin tarsometatarsi: cavities, vascularity, and resilience

Penguin tarsometatarsi are shortened and flattened, and studies devoted to the internal characteristics of these composite bones are very limited. Therefore, we present here a comprehensive, x-ray-microscopy-based analysis based on tarsometatarsi of Eocene stem Sphenisciformes from Seymour Island (Antarctic Peninsula) as well as recent Aptenodytes forsteri, A. patagonicus, and Pygoscelis adeliae penguins. Our study focuses on four aspects: size variability of the medullary cavities, vascularization patterns with emphasis on diaphyseal vessels, cross-sectional anisotropy, and diaphyseal resistance to bending forces. Small-sized Eocene penguins (Delphinornis and Marambiornopsis) show well-developed tarsometatarsal medullary cavities, whereas the cavities of "giant" early Sphenisciformes are either smaller (Palaeeudyptes) or show a conspicuous intermetatarsal size gradient (Anthropornis). Extant penguins exhibit a decrease in cavity dimensions as their body size increases. Distributional tendencies of primary diaphyseal nutrient foramina are quite similar in the smaller Delphinornis, Marambiornopsis, and extant Pygoscelis on one side and in Palaeeudyptes and extant Aptenodytes on the other. Anthropornis shows a unique, plesiomorphic pattern with a prevalence of plantar blood supply to the metatarsals. The diaphyseal nutrient canals diverge in orientation, some obliquely away from the proximal part, others with disparate trajectories. Cross-sectional anisotropy along the tarsometatarsal shaft generally appears to be rather low. Clustering of coherency curves along certain tarsometatarsal segments may reflect a selection process that exerts a significant influence within biomechanically crucial sections. Diaphyseal resistance to mediolateral bending forces is explicitly more efficient in extant penguins than in Eocene Sphenisciformes. This can be interpreted as an adaptation to the waddling gait of extant penguins.

Relative leg-to-arm skeletal strength proportions in orangutans by species and sex

Among extant great apes, orangutans climb most frequently. However, Bornean orangutans (Pongo pygmaeus) exhibit higher frequencies of terrestrial locomotion than do Sumatran orangutans (Pongo abelii). Variation in long bone cross-sectional geometry is known to reflect differential loading of the limbs. Thus, Bornean orangutans should show greater relative leg-to-arm strength than their Sumatran counterparts. Using skeletal specimens from museum collections, we measured two cross-sectional geometric measures of bone strength: the polar section modulus (Zpol) and the ratio of maximum to minimum area moments of inertia (Imax/Imin), at the midshaft of long bones in Bornean (n = 19) and Sumatran adult orangutans (n = 12) using medical CT and peripheral quantitative CT scans, and compared results to published data of other great apes. Relative leg-to-arm strength was quantified using ratios of femur and tibia over humerus, radius, and ulna, respectively. Differences between orangutan species and between sexes in median ratios were assessed using Wilcoxon rank sum tests. The tibia of Bornean orangutans was stronger relative to the humerus and the ulna than in Sumatran orangutans (p = 0.008 and 0.025, respectively), consistent with behavioral studies that indicate higher frequencies of terrestrial locomotion in the former. In three Zpol ratios, adult female orangutans showed greater leg-to-arm bone strength compared to flanged males, which may relate to females using their legs more during arboreal locomotion than in adult flanged males. A greater amount of habitat discontinuity on Borneo compared to Sumatra has been posited as a possible explanation for observed interspecific differences in locomotor behaviors, but recent camera trap studies has called this into question. Alternatively, greater frequencies of terrestriality in Pongo pygmaeus may be due to the absence of tigers on Borneo. The results of this study are consistent with the latter explanation given that habitat continuity was greater a century ago when our study sample was collected.

Radial porosity profiles are a powerful tool for tracing locomotor maturation in developing limb bones

Radial porosity profiles (RPP) are a new quantitative osteohistological parameter designed to capture the dynamic changes in the primary porosity of limb bones through ontogeny, providing insights into skeletal growth and functional development of extant and extinct vertebrates. Previous work hypothesized that RPP channelization-the intraskeletal alignment of RPPs across different bones resulting from similar cortical compaction patterns-indicates increasing locomotor performance of the developing limbs. By investigating RPPs in ontogenetic series of pheasants, pigeons and ducks representing distinct locomotor developmental strategies, we test this hypothesis here and show that RPPs are indeed powerful osteohistological correlates of locomotor ontogeny. Qualitative and quantitative analyses reveal strong association between RPP channelization and fledging, the most drastic locomotor transition in the life history of volant birds. The channelization signal is less clear in precocial leg function; however, when additional intraskeletal and intercohort RPP characteristics are considered, patterns related to leg precocity can also be identified. Thus, we demonstrate that RPPs can be used in future by palaeobiologists to generate breakthroughs in the study of the ontogeny and evolution of flight in fossil birds and pterosaurs. With further baseline data collection from modern terrestrial vertebrates, RPPs could also test hypotheses regarding ontogenetic postural shifts in dinosaurs and other terrestrial archosaurs.

Exceptional preservation and foot structure reveal ecological transitions and lifestyles of early theropod flyers

Morphology of keratinised toe pads and foot scales, hinging of foot joints and claw shape and size all inform the grasping ability, cursoriality and feeding mode of living birds. Presented here is morphological evidence from the fossil feet of early theropod flyers. Foot soft tissues and joint articulations are qualitatively assessed using laser-stimulated fluorescence. Pedal claw shape and size are quantitatively analysed using traditional morphometrics. We interpret these foot data among existing evidence to better understand the evolutionary ecology of early theropod flyers. Jurassic flyers like Anchiornis and Archaeopteryx show adaptations suggestive of relatively ground-dwelling lifestyles. Early Cretaceous flyers then diversify into more aerial lifestyles, including generalists like Confuciusornis and specialists like the climbing Fortunguavis. Some early birds, like the Late Jurassic Berlin Archaeopteryx and Early Cretaceous Sapeornis, show complex ecologies seemingly unique among sampled modern birds. As a non-bird flyer, finding affinities of Microraptor to a more specialised raptorial lifestyle is unexpected. Its hawk-like characteristics are rare among known theropod flyers of the time suggesting that some non-bird flyers perform specialised roles filled by birds today. We demonstrate diverse ecological profiles among early theropod flyers, changing as flight developed, and some non-bird flyers have more complex ecological roles.

Comparative postcranial osteohistology and bone histovariability of aquatic and terrestrial turtles: the case of the South American Phrynops hilarii , Hydromedusa tectifera (Pleurodira, Chelidae), and Chelonoidis chilensis (Cryptodira, Testudinidae)

This article presents a detailed comparative analysis of the bone microstructure of three extant species of South American turtles. The main histological characteristics of postcranial bones are identified, as well as the intraskeletal, ontogenetic and interspecific variation between aquatic and terrestrial species. For this purpose, thin sections of postcranial bones (seventh cervical vertebra, coracoid, scapula, humerus, radius, ulna, ischium, ilium, pubis, femur, tibia, and fibula) of juvenile and adult specimens of aquatic (Phrynops hilarii and Hydromedusa tectifera) and terrestrial (Chelonoidis chilensis) turtles were analyzed. Bone histology revealed an intraskeletal variation of the microanatomical and microstructural organization in these turtles. The cortical bone is composed of poorly vascularized lamellar and parallel-fibered bone tissue interrupted with lines of arrested growth (LAGs), reflecting a cyclical slow growth rate throughout these turtles' life. Although in the adult specimens a growth rate decrease was observed, none of them have reached somatic maturity. The juvenile and the adult of Chelonoidis chilensis, unlike the aquatic species studied, presented a higher vascularization in their bones, which could imply a faster growth rate in this land specie. The number of LAGs was higher in the stylopodial and zeugopodial bones, which would make these elements suitable for approximate age estimations. Pectoral and pelvic girdle bones also exhibited a good record of LAGs. The information here obtained on extant species represents a powerful tool for the interpretation of paleobiological traits present in closely related fossil forms.

How does bone microanatomy and musculature covary? An investigation in the forelimb of two species of martens (Martes foina, Martes martes)

The long bones and associated musculature play a prominent role in the support and movement of the body and are expected to reflect the associated mechanical demands. But in addition to the functional response to adaptive changes, the conjoined effects of phylogenetic, structural and developmental constraints also shape the animal's body. In order to minimise the effect of the aforementioned constraints and to reveal the biomechanical adaptations in the musculoskeletal system to locomotor mode, we here study the forelimb of two closely related martens: the arboreal pine marten (Martes martes) and the more terrestrial stone marten (Martes foina), focusing on their forelimb muscle anatomy and long bone microanatomy; and, especially, on their covariation. To do so, we quantified muscle data and bone microanatomical parameters and created 3D and 2D maps of the cortical thickness distribution for the three long bones of the forelimb. We then analysed the covariation of muscle and bone data, both qualitatively and quantitatively. Our results reveal that species-specific muscular adaptations are not clearly reflected in the microanatomy of the bones. Yet, we observe a global thickening of the bone cortex in the radius and ulna of the more arboreal pine marten, as well a stronger flexor muscle inserting on its elbow. We attribute these differences to variation in their locomotor modes. Analyses of our 2D maps revealed a shift of cortical thickness distribution pattern linked to ontogeny, rather than species-specific patterns. We found that although intraspecific variation is not negligible, species distinction was possible when taking muscular and bone microanatomical data into consideration. Results of our covariation analyses suggest that the muscle-bone correlation is linked to ontogeny rather than to muscular strength at zones of insertion. Indeed, if we find a correlation between cortical thickness distribution and the strength of some muscles in the humerus, that is not the case for the others and in the radius and ulna. Cortical thickness distribution appears rather linked to bone contact zones and ligament insertions in the radius and ulna, and to some extent in the humerus. We conclude that inference on muscle from bone microanatomy is possible only for certain muscles in the humerus.

Convergent evolution in dippers (Aves, Cinclidae): The only wing-propelled diving songbirds

Of the more than 6,000 members of the most speciose avian clade, Passeriformes (perching birds), only the five species of dippers (Cinclidae, Cinclus) use their wings to swim underwater. Among nonpasserine wing‐propelled divers (alcids, diving petrels, penguins, and plotopterids), convergent evolution of morphological characteristics related to this highly derived method of locomotion have been well‐documented, suggesting that the demands of this behavior exert strong selective pressure. However, despite their unique anatomical attributes, dippers have been the focus of comparatively few studies and potential convergence between dippers and nonpasseriform wing‐propelled divers has not been previously examined. In this study, a suite of characteristics that are shared among many wing‐propelled diving birds were identified and the distribution of those characteristics across representatives of all clades of extant and extinct wing‐propelled divers were evaluated to assess convergence. Putatively convergent characteristics were drawn from a relatively wide range of sources including osteology, myology, endocranial anatomy, integument, and ethology. Comparisons reveal that whereas nonpasseriform wing‐propelled divers do in fact share some anatomical characteristics putatively associated with the biomechanics of underwater “flight”, dippers have evolved this highly derived method of locomotion without converging on the majority of concomitant changes observed in other taxa. Changes in the flight musculature and feathers, reduction of the keratin bounded external nares and an increase in subcutaneous fat are shared with other wing‐propelled diving birds, but endocranial anatomy shows no significant shifts and osteological modifications are limited. Muscular and integumentary novelties may precede skeletal and neuroendocranial morphology in the acquisition of this novel locomotory mode, with implications for understanding potential biases in the fossil record of other such transitions. Thus, dippers represent an example of a highly derived and complex behavioral convergence that is not fully associated with the anatomical changes observed in other wing‐propelled divers, perhaps owing to the relative recency of their divergence from nondiving passeriforms.

The relationship between sternum variation and mode of locomotion in birds

BACKGROUND

The origin of powered avian flight was a locomotor innovation that expanded the ecological potential of maniraptoran dinosaurs, leading to remarkable variation in modern birds (Neornithes). The avian sternum is the anchor for the major flight muscles and, despite varying widely in morphology, has not been extensively studied from evolutionary or functional perspectives. We quantify sternal variation across a broad phylogenetic scope of birds using 3D geometric morphometrics methods. Using this comprehensive dataset, we apply phylogenetically informed regression approaches to test hypotheses of sternum size allometry and the correlation of sternal shape with both size and locomotory capabilities, including flightlessness and the highly varying flight and swimming styles of Neornithes.

RESULTS

We find evidence for isometry of sternal size relative to body mass and document significant allometry of sternal shape alongside important correlations with locomotory capability, reflecting the effects of both body shape and musculoskeletal variation. Among these, we show that a large sternum with a deep or cranially projected sternal keel is necessary for powered flight in modern birds, that deeper sternal keels are correlated with slower but stronger flight, robust caudal sternal borders are associated with faster flapping styles, and that narrower sterna are associated with running abilities. Correlations between shape and locomotion are significant but show weak explanatory power, indicating that although sternal shape is broadly associated with locomotory ecology, other unexplored factors are also important.

CONCLUSIONS

These results display the ecological importance of the avian sternum for flight and locomotion by providing a novel understanding of sternum form and function in Neornithes. Our study lays the groundwork for estimating the locomotory abilities of paravian dinosaurs, the ancestors to Neornithes, by highlighting the importance of this critical element for avian flight, and will be useful for future work on the origin of flight along the dinosaur-bird lineage.

Differing effects of size and lifestyle on bone structure in mammals

BACKGROUND

Mammals are a highly diverse group, with body mass ranging from 2 g to 170 t, and encompassing species with terrestrial, aquatic, aerial, and subterranean lifestyles. The skeleton is involved in most aspects of vertebrate life history, but while previous macroevolutionary analyses have shown that structural, phylogenetic, and functional factors influence the gross morphology of skeletal elements, their inner structure has received comparatively little attention. Here we analysed bone structure of the humerus and mid-lumbar vertebrae across mammals and their correlations with different lifestyles and body size.

RESULTS

We acquired bone structure parameters in appendicular and axial elements (humerus and mid-lumbar vertebra) from 190 species across therian mammals (placentals + marsupials). Our sample captures all transitions to aerial, fully aquatic, and subterranean lifestyles in extant therian clades. We found that mammalian bone structure is highly disparate and we show that the investigated vertebral structure parameters mostly correlate with body size, but not lifestyle, while the opposite is true for humeral parameters. The latter also show a high degree of convergence among the clades that have acquired specialised (non-terrestrial) lifestyles.

CONCLUSIONS

In light of phylogenetic, size, and functional factors, the distribution of each investigated structural parameter reveals patterns explaining the construction of appendicular and axial skeletal elements in mammalian species spanning most of the extant diversity of the clade in terms of body size and lifestyle. These patterns should be further investigated with analyses focused on specific lifestyle transitions that would ideally include key fossils.

Postcranial Skeletal Pneumaticity in Cuculidae

Postcranial skeletal pneumaticity (i.e., epithelial-lined, air-filled bones) is a condition unique to birds among extant tetrapods. Previous research reveals extensive variation in the expression of this trait in different bird species, from taxa that pneumatize nearly the entire skeleton to others that do not pneumatize a single bone. These studies, however, have primarily focused on aquatic/semi-aquatic birds, specifically Anseriformes (screamers, ducks, geese, swans) and Aequorlitornithes (loons, gulls, penguins, storks, etc.). This is the first clade-centric study of pneumaticity in an exclusively terrestrial clade (i.e., a group without any proclivities for water), Cuculidae. Given the variation in body size and ecology exhibited by cuckoos, they represent an ideal group for evaluating previously established trends in pneumaticity patterns. Similar to previous studies, our results indicate that cuckoos do exhibit extensive postcranial skeletal pneumaticity but with much more limited variation in expression. Of the surveyed species, 30 of 41 display an identical expression pattern, pneumatizing all postaxial vertebrae, the humerus, sternum, and pelvic girdle. The remaining species (11/41) deviate from this pattern by no more than two elements (i.e., the femur or the scapula/coracoid). All variable species expand upon the basic cuckoo pattern, with five species pneumatizing the femur and the remaining six taxa pneumatizing both the scapula and coracoid. Furthermore, most variation occurs in early diverging clades, with distinct subclades associated with specific anatomical expansions in pneumaticity (e.g., pneumatic femora in Neomorphinae and pneumatic scapulae/coracoids in select members of Couinae and Centropodinae). Limited variation noted in Cuculidae may be the result of the relatively high base level of pneumaticity when compared with previously sampled groups of water-oriented birds. Additional analyses indicate a positive relationship between body mass and pneumaticity, with possible (i.e., non-quantifiable) relationships noted between the limited expansions from the basic cuckoo pattern and specific locomotor behaviors (e.g., pneumatic femora present in species with enhanced cursorial behavior). These basic trends have also been observed in other densely sampled neognath clades. Taken together, the data presented herein supports the hypothesis that changes in pneumaticity expression may be correlated with shifts in biomechanical loading regimes rather than solely as a weight saving (i.e., density-altering) mechanism.

Phylogeny and foraging behaviour shape modular morphological variation in bat humeri

Bats show a remarkable ecological diversity that is reflected both in dietary and foraging guilds (FGs). Cranial ecomorphological adaptations linked to diet have been widely studied in bats, using a variety of anatomical, computational and mathematical approaches. However, foraging-related ecomorphological adaptations and the concordance between cranial and postcranial morphological adaptations remain unexamined in bats and limited to the interpretation of traditional aerodynamic properties of the wing (e.g. wing loading [WL] and aspect ratio [AR]). For this reason, the postcranial ecomorphological diversity in bats and its drivers remain understudied. Using 3D virtual modelling and geometric morphometrics (GMM), we explored the phylogenetic, ecological and biological drivers of humeral morphology in bats, evaluating the presence and magnitude of modularity and integration. To explore decoupled patterns of variation across the bone, we analysed whole-bone shape, diaphyseal and epiphyseal shape. We also tested whether traditional aerodynamic wing traits correlate with humeral shape. By studying 37 species from 20 families (covering all FGs and 85% of dietary guilds), we found similar patterns of variation in whole-bone and diaphyseal shape and unique variation patterns in epiphyseal shape. Phylogeny, diet and FG significantly correlated with shape variation at all levels, whereas size only had a significant effect on epiphyseal morphology. We found a significant phylogenetic signal in all levels of humeral shape. Epiphyseal shape significantly correlated with wing AR. Statistical support for a diaphyseal-epiphyseal modular partition of the humerus suggests a functional partition of shape variability. Our study is the first to show within-structure modular morphological variation in the appendicular skeleton of any living tetrapod. Our results suggest that diaphyseal shape correlates more with phylogeny, whereas epiphyseal shape correlates with diet and FG.

Wing Musculature Reconstruction in Extinct Flightless Auks (Pinguinus and Mancalla) Reveals Incomplete Convergence with Penguins (Spheniscidae) Due to Differing Ancestral States

Despite longstanding interest in convergent evolution, factors that result in deviations from fully convergent phenotypes remain poorly understood. In birds, the evolution of flightless wing-propelled diving has emerged as a classic example of convergence, having arisen in disparate lineages including penguins (Sphenisciformes) and auks (Pan-Alcidae, Charadriiformes). Nevertheless, little is known about the functional anatomy of the wings of flightless auks because all such taxa are extinct, and their morphology is almost exclusively represented by skeletal remains. Here, in order to re-evaluate the extent of evolutionary convergence among flightless wing-propelled divers, wing muscles and ligaments were reconstructed in two extinct flightless auks, representing independent transitions to flightlessness: Pinguinus impennis (a crown-group alcid), and Mancalla (a stem-group alcid). Extensive anatomical data were gathered from dissections of 12 species of extant charadriiforms and 4 aequornithine waterbirds including a penguin. The results suggest that the wings of both flightless auk taxa were characterized by an increased mechanical advantage of wing elevator/retractor muscles, and decreased mobility of distal wing joints, both of which are likely advantageous for wing-propelled diving and parallel similar functional specializations in penguins. However, the conformations of individual muscles and ligaments underlying these specializations differ markedly between penguins and flightless auks, instead resembling those in each respective group's close relatives. Thus, the wings of these flightless wing-propelled divers can be described as convergent as overall functional units, but are incompletely convergent at lower levels of anatomical organization-a result of retaining differing conditions from each group's respective volant ancestors. Detailed investigations such as this one may indicate that, even in the face of similar functional demands, courses of phenotypic evolution are dictated to an important degree by ancestral starting points.

Development of limb bone laminarity in the homing pigeon (Columba livia)

Background

Birds show adaptations in limb bone shape that are associated with resisting locomotor loads. Whether comparable adaptations occur in the microstructure of avian cortical bone is less clear. One proposed microstructural adaptation is laminar bone in which the proportion of circumferentially-oriented vascular canals (i.e., laminarity) is large. Previous work on adult birds shows elevated laminarity in specific limb elements of some taxa, presumably to resist torsion-induced shear strain during locomotion. However, more recent analyses using improved measurements in adult birds and bats reveal lower laminarity than expected in bones associated with torsional loading. Even so, there may still be support for the resistance hypothesis if laminarity increases with growth and locomotor maturation.

Methods

Here, we tested that hypothesis using a growth series of 17 homing pigeons (15–563 g). Torsional rigidity and laminarity of limb bones were measured from histological sections sampled from midshaft. Ontogenetic trends in laminarity were assessed using principal component analysis to reduce dimensionality followed by beta regression with a logit link function.

Results

We found that torsional rigidity of limb bones increases disproportionately with growth, consistent with rapid structural compensation associated with locomotor maturation. However, laminarity decreases with maturity, weakening the hypothesis that high laminarity is a flight adaptation at least in the pigeon. Instead, the histological results suggest that low laminarity, specifically the relative proportion of longitudinal canals aligned with peak principal strains, may better reflect the loading history of a bone.

Differential locomotor and predatory strategies of Gondwanan and derived Laurasian dromaeosaurids (Dinosauria, Theropoda, Paraves): Inferences from morphometric and comparative anatomical studies

Tetrapod limbs morphology is a reliable proxy of locomotor capacities. Beyond this, other aspects of life habits, such as predation abilities, can also be relevant to determine main morphofunctional appendicular properties, which ultimately reflect a compromise between different factors of the biological role. Dromaeosauridae is a dinosaur clade belonging to Theropoda, a group of bipedal predators. Dromaeosaurids represent an interesting study case, in which the hindlimbs have been proposed to be involved in both locomotion and predation activity. A peculiar feature characterizing all dromaeosaurids is a modified second pedal digit, which is typically related to predation. This theropod group is closely related to birds and diversified during the Cretaceous Period, mainly in the Northern Hemisphere (Laurasia). However, a subclade of dromaeosaurids, the Unenlagiinae, was recently recognized for Gondwana. Nevertheless, there are morphological differences between derived Laurasian dromaeosaurids (eudromaeosaurs) and unenlagiines. Such differences are observed in the proportions between hindlimb bones and in the presence of a subarctometatarsalian condition in unenlagiines, which is mainly characterized by a proximally constricted metatarsal III. To evaluate the function of these divergent morphologies, we conducted morphometric analyses and comparisons of qualitative morphological aspects, encompassing unenlagiines, other dromaeosaurids, as well as taxa from other theropod groups, including extant birds. The former approach consisted of two phylogenetic principal component analyses, one based on the main measurements of the hindlimb, and the other focused on the lengths of the pedal phalanges. The first analysis drew the unenlagiines close to taxa with long tibiae, as well as long and slender metatarsi. Instead, eudromaeosaurs are closer to taxa with shorter tibiae, and shorter and wider metatarsi. The second analysis showed that eudromaeosaurs and unenlagiines have similar phalangeal proportions, including the elongation of distal phalanges. However, the shorter second phalanx of the pedal digit II of eudromaeosaurs could have increased the force generated by this digit, which was the main predatory tool of the autopodium. This, together with a shorter and wider metatarsus, and a marked hinge-like morphology of the articular surfaces of metatarsals and phalanges, possibly allowed eudromaeosaurs to exert a great gripping strength and hunt large prey. Conversely, the longer and slender subarctometatarsus, and less well-marked hinge joints of unenlagiines possibly gave them greater cursorial capacities. Additionally, the longer second phalanx of digit II allowed unenlagiines fast movements of this digit to hunt smaller and elusive prey. Thus, the distinctive morphological evolutionary pathways of these two dromaeosaurid clades seem to have been influenced by the particular locomotor and predatory specializations that characterized each of these lineages.

Morphological Disparity of the Humerus in Modern Birds

From a functional standpoint, the humerus is a key element in the skeleton of vertebrates as it is the forelimb’s bone that connects with the pectoral girdle. In most birds, the humerus receives both the forces exerted by the main flight muscles and the aerodynamical stresses exerted upon the wing during locomotion. Despite this functional preeminence, broad scale studies of the morphological disparity of the humerus in the crown group of birds (Neornithes) are lacking. Here, we explore the variation in shape of the humeral outline in modern birds and its evolutionary relationship with size and the evolution of different functional regimes, including several flight strategies, wing propelled diving and complete loss of wing locomotory function. Our findings suggest that most neornithines evolved repeatedly towards a general humeral morphology linked with functional advantages related with more efficient flapping. Lineages evolving high-stress locomotion such as hyperaeriality (e.g., swifts), hovering (e.g., hummingbirds) and wing-propelled diving (e.g., penguins) greatly deviate from this general trend, each exploring different morphologies. Secondarily flightless birds deviate to a lesser degree from their parent clades in humeral morphology likely as a result of the release from constraints related with wing-based locomotion. Furthermore, these taxa show a different allometric trend that flighted birds. Our results reveal that the constraints of aerial and aquatic locomotion are main factors shaping the macroevolution of humeral morphology in modern birds.

To Hop or Not to Hop? The Answer Is in the Bird Trees

Birds can use different types of gaits to move on the ground: they either walk, hop, or run. Although velocity can easily explain a preference for running, it remains unclear what drives a bird species to favor hopping over walking. As many hopping birds are relatively small and arboreal, we wanted to test the link between size, arboreality, and hopping ability. First, we carried out ancestral character state reconstructions of size range, hopping ability, and habitat traits on over 1000 species of birds. We found that both hopping ability and arboreality were derived and significantly correlated traits in avian evolution. Second, we tested the influence of hopping ability on the morphology of the lower appendicular skeleton by quantifying the shape differences of the pelvis and the three long bones of the hind limbs in 47 avian species with different habitats and gait preferences. We used geometric morphometrics on 3D landmarks, digitized on micro–computed tomography (micro-CT) and surface scans of the pelvis, femur, tibiotarsus, and tarsometatarsus. Locomotion habits significantly influence the conformation of the pelvis, especially at the origin of hip and knee muscle extensors. Interestingly, habitat, more than locomotion habits, significantly changed tarsometatarsus conformation. The morphology of the distal part of the tarsometatarsus constrains digit orientation, which leads to a greater ability to perch, an advantageous trait in arboreality. The results of this work suggest an arboreal origin of hopping and illuminate the evolution of avian terrestrial locomotion.[Anatomy; avian; gait; leg; lifestyle; pelvis; tree-dwelling.]

Growth rate and locomotor performance tradeoff is not universal in birds

Though a tradeoff between growth rate and locomotor performance has been proposed, empirical data on this relationship are still limited. Here we statistically analyze the associations of growth rate and flight ability in birds by assessing how growth rate is correlated with three wing parameters of birds: flight muscle ratio, wing aspect ratio, and wing loading. We find that fast-growing birds tended to have higher flight muscle ratios and higher wing loadings than slow-growing birds, which suggests that fast-growing birds may have better takeoff performance, but lower efficiency in maneuvering flight. Accordingly, our findings suggest that the relationship between growth rate and flight ability is more complex than a simple tradeoff. Since the hindlimbs also contribute greatly to the locomotion of birds, future investigations on the relationship between growth rate and hindlimb performance will provide more insights into the evolution of birds.

The evolutionary diversity of locomotor innovation in rodents is not linked to proximal limb morphology

Rodents are the most species-rich order within Mammalia and have evolved disparate morphologies to accommodate numerous locomotor niches, providing an excellent opportunity to understand how locomotor innovation can drive speciation. To evaluate the connection between the evolutionary success of rodents and the diversity of rodent locomotor ecologies, we used a large dataset of proximal limb CT scans from across Myomorpha and Geomyoidea to examine internal and external limb shape. Only fossorial rodents displayed a major reworking of their proximal limbs in either internal or external morphology, with other locomotor modes plotting within a generalist morphospace. Fossorial rodents were also the only locomotor mode to consistently show increased rates of humerus/femur morphological evolution. We propose that these rodent clades were successful at spreading into ecological niches due to high behavioral plasticity and small body sizes, allowing them to modify their locomotor mode without requiring major changes to their proximal limb morphology.

Identifying medullary bone in extinct avemetatarsalians: challenges, implications and perspectives

Medullary bone (MB) is a sex-specific tissue produced by female birds during the laying cycle, and it is hypothesized to have arisen within Avemetatarsalia, possibly outside Avialae. Over the years, researchers have attempted to define a set of criteria from which to evaluate the nature of purported MB-like tissues recovered from fossil specimens. However, we argue that the prevalence, microstructural and chemical variability of MB in Neornithes is, as of yet, incompletely known and thus current diagnoses of MB do not capture the extent of variability that exists in modern birds. Based on recently published data and our own observations of MB distribution and structure using computed tomography and histochemistry, we attempt to advance the discourse on identifying MB in fossil specimens. We propose: (i) new insights into the phylogenetic breadth and structural diversity of MB within extant birds; (ii) a reevaluation and refinement of the most recently published list of criteria suggested for confidently identifying MB in the fossil record; (iii) reconsideration of some prior identifications of MB-like tissues in fossil specimens by taking into account the newly acquired data; and (iv) discussions on the challenges of characterizing MB in Neornithes with the goal of improving its diagnosis in extinct avemetatarsalians. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Functional Morphology and Morphological Diversification of Hind Limb Cross-Sectional Traits in Mustelid Mammals

Locomotor habits in mammals are strongly tied to limb bones’ lengths, diameters, and proportions. By comparison, fewer studies have examined how limb bone cross-sectional traits relate to locomotor habit. Here, we tested whether climbing, digging, and swimming locomotor habits reflect biomechanically meaningful differences in three cross-sectional traits rendered dimensionless— cross-sectional area (CSA), second moments of area (SMA), and section modulus (MOD)—using femora, tibiae, and fibulae of 28 species of mustelid. CSA and SMA represent resistance to axial compression and bending, respectively, whereas MOD represents structural strength. Given the need to counteract buoyancy in aquatic environments and soil’s high density, we predicted that natatorial and fossorial mustelids have higher values of cross-sectional traits. For all three traits, we found that natatorial mustelids have the highest values, followed by fossorial mustelids, with both of these groups significantly differing from scansorial mustelids. However, phylogenetic relatedness strongly influences diversity in cross-sectional morphology, as locomotor habit strongly correlates with phylogeny. Testing whether hind limb bone cross-sectional traits have evolved adaptively, we fit Ornstein–Uhlenbeck (OU) and Brownian motion (BM) models of trait diversification to cross-sectional traits. The cross-sectional traits of the femur, tibia, and fibula appear to have, respectively, diversified under a multi-rate BM model, a single rate BM model, and a multi-optima OU model. In light of recent studies on mustelid body size and elongation, our findings suggest that the mustelid body plan—and perhaps that of other mammals—is likely the sum of a suite of traits evolving under different models of trait diversification.

Ontogenetic changes of geometrical and mechanical characteristics of the avian femur: a comparison between precocial and altricial birds

The mechanical performance of limb bones is closely associated with an animal's locomotor capability and is thus important to our understanding of animal behaviour. This study combined a geometrical analysis and three-point bending tests to address the question of how the mechanical performance of the femurs of Japanese quail (Coturnix coturnix japonica) and pigeon (Columba livia domestica) respond to changing functional demands during ontogeny. Results showed that hatchling quails had stiff bone tissues, and the femoral ultimate loads scaled negatively with body mass, corresponding to high functional demands during early growth. The hatchling pigeon femora had weak material properties but they showed a dramatic increase in Young's modulus during growth. Consequently, although femoral cross-sectional geometry showed negative allometry, the ultimate loads scaled positively with body mass. Older pigeons had more circular bone cross-sections than younger pigeons, probably due to load stimulation changes occurred shortly after the onset of locomotion. Negative allometry and isometry of the cross-sectional geometry of hind limb bones were observed in flying birds and ground-dwelling birds, respectively. The correspondence between geometrical change and locomotor pattern suggests that ontogenetic changes in cross-sectional geometry may be an effective indicator of avian locomotor behaviour.

Morphological diversification of biomechanical traits: mustelid locomotor specializations and the macroevolution of long bone cross-sectional morphology

BACKGROUND

Morphological diversity of limb bone lengths, diameters, and proportions in mammals is known to vary strongly with locomotor habit. It remains less well known how different locomotor habits are correlated with cross-sectional traits of the limb skeleton, such as cross-sectional area (CSA), second moments of area (SMA), and section modulus (MOD) and whether these traits have evolved adaptively. CSA and SMA represent the bone's resistance to axial compression and bending, respectively, whereas MOD represents bone structural strength related to shape. Sampling 28 species of mustelids, a carnivoran lineage with diverse locomotor habits, we tested for differences in humeral, radial, and ulnar cross-sectional traits among specialists for climbing, digging, and swimming, in addition to generalists. Given that the limbs of digging specialists function in the dense substance of soil, and that swimming specialists need to counteract buoyancy, we predicted that these mustelids with these specializations should have the greatest values of cross-sectional traits.

RESULTS

We analyzed cross-sectional traits (calculated via μCT scanning and rendered dimensionless) in 5% increments along a bone's length and found significant differences among locomotor habits, though differences in ulnar cross-sectional traits were fewer than those for the humerus and radius. Swimming specialists had the greatest values of cross-sectional traits, followed by digging specialists. Climbing specialists had the lowest values of cross-sectional traits. However, phylogenetic affinity underlies these results. Fitting models of trait evolution to CSA and SMA revealed that a multi-rate Brownian motion model and a multi-optima Ornstein-Uhlenbeck model are the best-fitting models of evolution for these traits. However, inspection of α-values uncovered that many of the OU models did not differ from a Brownian motion model.

CONCLUSIONS

Within Mustelidae, differences in limb function and locomotor habit influence cross-sectional traits in ways that produce patterns that may diverge from adaptive patterns exhibited by external traits (e.g., bone lengths) of the mammalian limb skeleton. These results suggest that not all the traits of a single organ evolve under a single evolutionary process and that models of trait evolution should be fit to a range of traits for a better understanding of the evolution of the mammalian locomotor system.

A well-preserved partial scapula from Japan and the reconstruction of the triosseal canal of plotopterids

The discovery of a well-preserved cranial end of a plotopterid scapula from the Early Oligocene Jinnobaru Formation in southwestern Japan has provided a fine example of its bone structure and has enabled the reconstruction of the triosseal canal (canalis triosseus) of the unique extinct penguin-like bird. It is believed that plotopterids performed penguin-like underwater propulsion using wings that were similar to those of penguins. Until this discovery, the lack of well-preserved plotopterid scapulae hindered reconstruction of the canalis triosseus, which is an important structure for the wing-upstroke. We reconstructed a composite model of the canalis triosseus based on the new scapula. The reconstructed size of the canal is as large as that in Emperor Penguins (Aptenodytes forsteri), suggesting that the bird had a large and powerful m. supracoracoideus, which is the essential muscle for the powered upstroke required for wing-propelled diving. Plotopterids likely have had the same functional requirement as penguins, the powerful wing-upstroke in the water. They must have also been capable swimmers. This scapula accounts for the structural difference between plotopterids and penguins in terms of the canalis triosseus. The large canalis triosseus of plotopterids was composed of the elongated acromion of the scapula, while penguins have a long processus acromialis claviculae for the same function.

Wing bone geometry reveals active flight in Archaeopteryx

Archaeopteryx is an iconic fossil taxon with feathered wings from the Late Jurassic of Germany that occupies a crucial position for understanding the early evolution of avian flight. After over 150 years of study, its mosaic anatomy unifying characters of both non-flying dinosaurs and flying birds has remained challenging to interpret in a locomotory context. Here, we compare new data from three Archaeopteryx specimens obtained through phase-contrast synchrotron microtomography to a representative sample of archosaurs employing a diverse array of locomotory strategies. Our analyses reveal that the architecture of Archaeopteryx's wing bones consistently exhibits a combination of cross-sectional geometric properties uniquely shared with volant birds, particularly those occasionally utilising short-distance flapping. We therefore interpret that Archaeopteryx actively employed wing flapping to take to the air through a more anterodorsally posteroventrally oriented flight stroke than used by modern birds. This unexpected outcome implies that avian powered flight must have originated before the latest Jurassic.

3D quantitative comparative analysis of long bone diaphysis variations in microanatomy and cross-sectional geometry

Long bone inner structure and cross-sectional geometry display a strong functional signal, leading to convergences, and are widely analyzed in comparative anatomy at small and large taxonomic scales. Long bone microanatomical studies have essentially been conducted on transverse sections but also on a few longitudinal ones. Recent studies highlighted the interest in analyzing variations of the inner structure along the diaphysis using a qualitative as well as a quantitative approach. With the development of microtomography, it has become possible to study three-dimensional (3D) bone microanatomy and, in more detail, the form-function relationships of these features. This study focused on the selection of quantitative parameters to describe in detail the cross-sectional shape changes and distribution of the osseous tissue along the diaphysis. Two-dimensional (2D) virtual transverse sections were also performed in the two usual reference planes and results were compared with those obtained based on the whole diaphysis analysis. The sample consisted in 14 humeri and 14 femora of various mammalian taxa that are essentially terrestrial. Comparative quantitative analyses between different datasets made it possible to highlight the parameters that are strongly impacted by size and phylogeny and the redundant ones, and thus to estimate their relevance for use in form-function analyses. The analysis illustrated that results based on 2D transverse sections are similar for both sectional planes; thus if a strong bias exists when mixing sections from the two reference planes in the same analysis, it would not problematic to use either one plane or the other in comparative studies. However, this may no longer hold for taxa showing a much stronger variation in bone microstructure along the diaphysis. Finally, the analysis demonstrated the significant contribution of the parameters describing variations along the diaphysis, and thus the interest in performing 3D analyses; this should be even more fruitful for heterogeneous diaphyses. In addition, covariation analyses showed that there is a strong interest in removing the size effect to access the differences in the microstructure of the humerus and femur. This methodological study provides a reference for future quantitative analyses on long bone inner structure and should make it possible, through a detailed knowledge of each descriptive parameter, to better interpret results from the multivariate analyses associated with these studies. This will have direct implications for studies in vertebrate anatomy, but also in paleontology and anthropology.

Intraskeletal histovariability, allometric growth patterns, and their functional implications in bird-like dinosaurs

With their elongated forelimbs and variable aerial skills, paravian dinosaurs, a clade also comprising modern birds, are in the hotspot of vertebrate evolutionary research. Inferences on the early evolution of flight largely rely on bone and feather morphology, while osteohistological traits are usually studied to explore life-history characteristics. By sampling and comparing multiple homologous fore- and hind limb elements, we integrate for the first time qualitative and quantitative osteohistological approaches to get insight into the intraskeletal growth dynamics and their functional implications in five paravian dinosaur taxa, Anchiornis, Aurornis, Eosinopteryx, Serikornis, and Jeholornis. Our qualitative assessment implies a considerable diversity in allometric/isometric growth patterns among these paravians. Quantitative analyses show that neither taxa nor homologous elements have characteristic histology, and that ontogenetic stage, element size and the newly introduced relative element precocity only partially explain the diaphyseal histovariability. Still, Jeholornis, the only avialan studied here, is histologically distinct from all other specimens in the multivariate visualizations raising the hypothesis that its bone tissue characteristics may be related to its superior aerial capabilities compared to the non-avialan paravians. Our results warrant further research on the osteohistological correlates of flight and developmental strategies in birds and bird-like dinosaurs.

Neither slim nor fat: estimating the mass of the dodo (Raphus cucullatus, Aves, Columbiformes) based on the largest sample of dodo bones to date

The dodo (Raphus cucullatus) might be the most enigmatic bird of all times. It is, therefore, highly remarkable that no consensus has yet been reached on its body mass; previous scientific estimates of its mass vary by more than 100%. Until now, the vast amount of bones stored at the Natural History Museum in Mauritius has not yet been studied morphometrically nor in relation to body mass. Here, a new estimate of the dodo’s mass is presented based on the largest sample of dodo femora ever measured (n = 174). In order to do this, we have used the regression method and chosen our variables based on biological, mathematical and physical arguments. The results indicate that the mean mass of the dodo was circa 12 kg, which is approximately five times as heavy as the largest living Columbidae (pigeons and doves), the clade to which the dodo belongs.

Computed tomographic analysis of the internal structure of the metacarpals and its implications for hand use, pathology, and surgical intervention

The variation of bone structure and biomechanics between the metacarpals is not well characterized. It was hypothesized that their structure would reflect their common patterns of use (i.e., patterns of hand grip), specifically that trabecular bone density would be greater on the volar aspect of all metacarpal bases, that this would be most pronounced in the thumb, and that the thumb diaphysis would have the greatest bending strength. Cross-sections at basal and mid-diaphyseal locations of 50 metacarpals from 10 human hands were obtained by peripheral quantitative computed tomography. The volar and dorsal trabecular densities of each base were measured and characterized using the volar/dorsal density ratio. The polar stress-strain index (SSIp), a surrogate measure of torsional/bending strength, was measured for each diaphysis and standardized for bone length and mass. Comparisons were made using mixed-model analyses of variance (ANOVAs) and post hoc tests. Volar/dorsal trabecular density ratios showed even distribution in all metacarpal bases except for the thumb, which showed greater values on the volar aspect. The thumb, second, and third metacarpals all had high bending strength (SSIp), but the thumb's SSIp relative to its length and trabecular mass was much higher than those of the other metacarpals. Trabecular density of the metacarpal bases was evenly distributed except in the thumb, which also showed higher bending strength relative to its length and mass. Understanding of how these indicators of strength differ across metacarpals may improve both fracture diagnosis and treatment and lays the groundwork for investigating changes with age, hand dominance, and occupation.

Adaptive evolution of a derived radius morphology in manakins (Aves, Pipridae) to support acrobatic display behavior

The morphology of the avian skeleton is often studied in the context of adaptations for powered flight. The effects of other evolutionary forces, such as sexual selection, on avian skeletal design are unclear, even though birds produce diverse behaviors that undoubtedly require a variety of osteological modifications. Here, we investigate this issue in a family of passerine birds called manakins (Pipridae), which have evolved physically unusual and elaborate courtship displays. We report that, in species within the genus Manacus, the shaft of the radius is heavily flattened and shows substantial solidification. Past work anecdotally notes this morphology and attributes it to the species' ability to hit their wings together above their heads to produce loud mechanical sonations. Our results show that this feature is unique to Manacus compared to the other species in our study, including a variety of taxa that produce other sonations through alternate wing mechanisms. At the same time, our data reveal striking similarities across species in total radius volume and solidification. Together, this suggests that supposedly adaptive alterations in radial morphology occur within a conserved framework of a set radius volume and solidness, which in turn is likely determined by natural selection. Further allometric analyses imply that the radius is less constrained by body size and the structural demands that underlie powered flight, compared to other forelimb bones that are mostly unmodified across taxa. These results are consistent with the idea that the radius is more susceptible to selective modification by sexual selection. Overall, this study provides some of the first insight into the osteological evolution of passerine birds, as well as the way in which opposing selective forces can shape skeletal design in these species. J. Morphol. 277:766-775, 2016. © 2016 Wiley Periodicals, Inc.

Challenges and advances in the study of pterosaur flight

Pterosaurs have fascinated scientists and nonscientists alike for over 200 years, as one of the three known clades of vertebrates to have evolved flapping flight. The smallest pterosaurs were comparable in size to the smallest extant birds and bats, but the largest pterosaurs were vastly larger than any extant flier. This immense size range, coupled with poor preservation and adaptations for flight unknown in extant vertebrates, have made interpretations of pterosaur flight problematic and often contentious. Here we review the anatomical, evolutionary, and phylogenetic history of pterosaurs, as well as the views, perspectives, and biases regarding their interpretation. In recent years, three areas of pterosaur biology have faced challenges and made advances: structure of the wing membrane, function of the pteroid, body size and mass estimates, as well as flight mechanics and aerodynamics. Comparative anatomical and fossil study, simulated bone loading, and aerodynamic modeling have all proved successful in furthering our understanding of pterosaur flight. We agree with previous authors that pterosaurs should be studied as pterosaurs, a diverse but phylogenetically, anatomically, and mechanically constrained clade that can offer new insights into the diversity of vertebrate flight.

Bone histology in extant and fossil penguins (Aves: Sphenisciformes)

Substantial changes in bone histology accompany the secondary adaptation to life in the water. This transition is well documented in several lineages of mammals and non-avian reptiles, but has received relatively little attention in birds. This study presents new observations on the long bone microstructure of penguins, based on histological sections from two extant taxa (Spheniscus and Aptenodytes) and eight fossil specimens belonging to stem lineages (†Palaeospheniscus and several indeterminate Eocene taxa). High bone density in penguins results from compaction of the internal cortical tissues, and thus penguin bones are best considered osteosclerotic rather than pachyostotic. Although the oldest specimens sampled in this study represent stages of penguin evolution that occurred at least 25 million years after the loss of flight, major differences in humeral structure were observed between these Eocene stem taxa and extant taxa. This indicates that the modification of flipper bone microstructure continued long after the initial loss of flight in penguins. It is proposed that two key transitions occurred during the shift from the typical hollow avian humerus to the dense osteosclerotic humerus in penguins. First, a reduction of the medullary cavity occurred due to a decrease in the amount of perimedullary osteoclastic activity. Second, a more solid cortex was achieved by compaction. In extant penguins and †Palaeospheniscus, most of the inner cortex is formed by rapid osteogenesis, resulting an initial latticework of woven-fibered bone. Subsequently, open spaces are filled by slower, centripetal deposition of parallel-fibered bone. Eocene stem penguins formed the initial latticework, but the subsequent round of compaction was less complete, and thus open spaces remained in the adult bone. In contrast to the humerus, hindlimb bones from Eocene stem penguins had smaller medullary cavities and thus higher compactness values compared with extant taxa. Although cortical lines of arrested growth have been observed in extant penguins, none was observed in any of the current sampled specimens. Therefore, it is likely that even these 'giant' penguin taxa completed their growth cycle without a major pause in bone deposition, implying that they did not undergo a prolonged fasting interval before reaching adult size.

Qualitative skeletal correlates of wing shape in extant birds (Aves: Neoaves)

BACKGROUND

Among living fliers (birds, bats, and insects), birds display relatively high aspect ratios, a dimensionless shape variable that distinguishes long and narrow vs. short and broad wings. Increasing aspect ratio results in a functional tradeoff between low induced drag (efficient cruise) and increased wing inertia (difficult takeoff). Given the wide scope of its functional effects, the pattern of aspect ratio evolution is an important factor that contributes to the substantial ecological and phylogenetic diversity of living birds. However, because the feathers that define the wingtip (and hence wingspan and aspect ratio) often do not fossilize, resolution in the pattern of avian wing shape evolution is obscured by missing information. Here I use a comparative approach to investigate the relationship between skeletal proxies of flight feather attachment and wing shape.

RESULTS

An accessory lobe of the internal index process of digit II-1, a bony correlate of distal primary attachment, shows weak but statistically significant relationships to aspect ratio and mass independent of other skeletal morphology. The dorsal phalangeal fossae of digit II-1, which house distal primaries VIII and IX, also show a trend of increased prominence with higher aspect ratio. Quill knobs on the ulna are examined concurrently, but do not show consistent signal with respect to wing shape.

CONCLUSIONS

Although quill knobs are cited as skeletal correlates of flight performance in birds, their relationship to wing shape is inconsistent among extant taxa, and may reflect diverging selection pressures acting on a conserved architecture. In contrast, correlates of distal primary feather attachment on the major digit show convergent responses to increasing aspect ratio. In light of the diversity of musculoskeletal and integumentary mophology that underlies wing shape in different avian clades, it is unlikely that a single skeletal feature will show consistent predictive power across Neoaves. Confident inference of wing shape in basal ornithurine birds will require multiple lines of evidence, together with an understanding of clade-specific evolutionary trends within the crown.

Wings versus legs in the avian bauplan: development and evolution of alternative locomotor strategies

Wings have long been regarded as a hallmark of evolutionary innovation, allowing insects, birds, and bats to radiate into aerial environments. For many groups, our intuitive and colloquial perspective is that wings function for aerial activities, and legs for terrestrial, in a relatively independent manner. However, insects and birds often engage their wings and legs cooperatively. In addition, the degree of autonomy between wings and legs may be constrained by tradeoffs, between allocating resources to wings versus legs during development, or between wing versus leg investment and performance (because legs must be carried as baggage by wings during flight and vice versa). Such tradeoffs would profoundly affect the development and evolution of locomotor strategies, and many related aspects of animal ecology. Here, we provide the first evaluation of wing versus leg investment, performance and relative use, in birds-both across species, and during ontogeny in three precocial species with different ecologies. Our results suggest that tradeoffs between wing and leg modules help shape ontogenetic and evolutionary trajectories, but can be offset by recruiting modules cooperatively. These findings offer a new paradigm for exploring locomotor strategies of flying organisms and their extinct precursors, and thereby elucidating some of the most spectacular diversity in animal history.

Microstructure and cross-sectional shape of limb bones in Great Horned Owls and Red-tailed Hawks: how do these features relate to differences in flight and hunting behavior?

The Red-tailed Hawk and Great Horned Owl are two species of raptor that are similar in body size, diet, and habitat. Both species use their hindlimbs during hunting, but differ in foot morphology, how they approach and immobilize prey, and the average size of prey captured. They also differ in primary flight style: the Red-tailed Hawk uses static soaring and the Great Horned Owl uses flap-gliding. The objectives of this study were to characterize the microstructure and cross-sectional shape of limb bones of these species and examine the relationship with flight and hunting behaviors. The mid-shaft of six limb bones from six individuals of each species was sampled. The degree of bone laminarity (proportion of circular primary vascular canals) and cross-sectional geometric parameters were calculated. In both species, the humerus and femur exhibited features that suggest high resistance to torsional loading, whereas the tibiotarsus and phalanges had a shape more likely to resist compression and bending in a specific plane. The femur of the Red-tailed Hawk exhibited higher laminarity and larger polar moment of area than that of the Great Horned Owl. The tibiotarsus was more elliptical than that of the Great Horned Owl. The hawk approaches prey from a more horizontal axis, takes prey of greater mass, and is more likely to pursue prey on the ground, which could potentially be causing more torsional loads on the femur and bending loads on the tibiotarsus. In addition, differences in polar moment of area of the phalanges between the species could relate to differences in foot morphology or digit length. The humerus and ulna of the flap-gliding Great Horned Owl are more elliptical than the static soaring Red-tailed Hawk, a shape that may better resist the bending loads associated with a larger amount of flapping.

Blood flow for bone remodelling correlates with locomotion in living and extinct birds

Nutrient arteries enter limb bones through discrete foramina on the shafts. They are required for bone remodelling in response to mechanical loading and dynamic forces imposed by locomotion. The cross-sectional area of the nutrient foramen of the femur represents an index of blood flow rate to the shaft and thus provides insight into the animal's level of activity. Morphometric data on femoral length, mass and foramen size from 100 extant bird species and eight extinct moa species were analysed allometrically and phylogenetically. The nutrient foramen blood flow index (Qi) and femur mass (Mf) increase with body mass (Mb). At 1 kg body mass, cursorial species have approximately 2.1 times higher Qi and 1.9 times heavier Mf than volant species. The scaling of Qi on Mf is independent of the primary mode of locomotion, but the ratio Qi/Mf decreases significantly in larger birds, although absolute Qi increases. The overall avian equation for Qi on Mb is not significantly different from previous data from mammals, but when differences in blood pressure are accounted for, estimated blood flow to the femur is approximately 1.9 times higher in cursorial birds than in mammals, possibly in relation to bipedalism and quadrupedalism, respectively. Femoral bone blood flow in both endothermic groups is estimated to be 50-100 times higher than in ectothermic reptiles.

Ecology and caudal skeletal morphology in birds: the convergent evolution of pygostyle shape in underwater foraging taxa

Birds exhibit a specialized tail that serves as an integral part of the flight apparatus, supplementing the role of the wings in facilitating high performance aerial locomotion. The evolution of this function for the tail contributed to the diversification of birds by allowing them to utilize a wider range of flight behaviors and thus exploit a greater range of ecological niches. The shape of the wings and the tail feathers influence the aerodynamic properties of a bird. Accordingly, taxa that habitually utilize different flight behaviors are characterized by different flight apparatus morphologies. This study explores whether differences in flight behavior are also associated with variation in caudal vertebra and pygostyle morphology. Details of the tail skeleton were characterized in 51 Aequornithes and Charadriiformes species. Free caudal vertebral morphology was measured using linear metrics. Variation in pygostyle morphology was characterized using Elliptical Fourier Analysis, a geometric morphometric method for the analysis of outline shapes. Each taxon was categorized based on flight style (flap, flap-glide, dynamic soar, etc.) and foraging style (aerial, terrestrial, plunge dive, etc.). Phylogenetic MANOVAs and Flexible Discriminant Analyses were used to test whether caudal skeletal morphology can be used to predict flight behavior. Foraging style groups differ significantly in pygostyle shape, and pygostyle shape predicts foraging style with less than 4% misclassification error. Four distinct lineages of underwater foraging birds exhibit an elongate, straight pygostyle, whereas aerial and terrestrial birds are characterized by a short, dorsally deflected pygostyle. Convergent evolution of a common pygostyle phenotype in diving birds suggests that this morphology is related to the mechanical demands of using the tail as a rudder during underwater foraging. Thus, distinct locomotor behaviors influence not only feather attributes but also the underlying caudal skeleton, reinforcing the importance of the entire caudal locomotor module in avian ecological diversification.

Walking like dinosaurs: chickens with artificial tails provide clues about non-avian theropod locomotion

Birds still share many traits with their dinosaur ancestors, making them the best living group to reconstruct certain aspects of non-avian theropod biology. Bipedal, digitigrade locomotion and parasagittal hindlimb movement are some of those inherited traits. Living birds, however, maintain an unusually crouched hindlimb posture and locomotion powered by knee flexion, in contrast to the inferred primitive condition of non-avian theropods: more upright posture and limb movement powered by femur retraction. Such functional differences, which are associated with a gradual, anterior shift of the centre of mass in theropods along the bird line, make the use of extant birds to study non-avian theropod locomotion problematic. Here we show that, by experimentally manipulating the location of the centre of mass in living birds, it is possible to recreate limb posture and kinematics inferred for extinct bipedal dinosaurs. Chickens raised wearing artificial tails, and consequently with more posteriorly located centre of mass, showed a more vertical orientation of the femur during standing and increased femoral displacement during locomotion. Our results support the hypothesis that gradual changes in the location of the centre of mass resulted in more crouched hindlimb postures and a shift from hip-driven to knee-driven limb movements through theropod evolution. This study suggests that, through careful experimental manipulations during the growth phase of ontogeny, extant birds can potentially be used to gain important insights into previously unexplored aspects of bipedal non-avian theropod locomotion.

Ontogenetic changes in limb bone structural proportions in mountain gorillas (Gorilla beringei beringei)

Behavioral studies indicate that adult mountain gorillas (Gorilla beringei) are the most terrestrial of all nonhuman hominoids, but that infant mountain gorillas are much more arboreal. Here we examine ontogenetic changes in diaphyseal strength and length of the femur, tibia, humerus, radius, and ulna in 30 Virunga mountain gorillas, including 18 immature specimens and 12 adults. Comparisons are also made with 14 adult western lowland gorillas (Gorilla gorilla gorilla), which are known to be more arboreal than adult mountain gorillas. Infant mountain gorillas have significantly stronger forelimbs relative to hind limbs than older juveniles and adults, but are nonsignificantly different from western lowland gorilla adults. The change in inter-limb strength proportions is abrupt at about two years of age, corresponding to the documented transition to committed terrestrial quadrupedalism in mountain gorillas. The one exception is the ulna, which shows a gradual increase in strength relative to the radius and other long bones during development, possibly corresponding to the gradual adoption of stereotypical fully pronated knuckle-walking in older juvenile gorillas. Inter-limb bone length proportions show a contrasting developmental pattern, with hind limb/forelimb length declining rapidly from birth to five months of age, and then showing no consistent change through adulthood. The very early change in length proportions, prior to significant independent locomotion, may be related to the need for relatively long forelimbs for climbing in a large-bodied hominoid. Virunga mountain gorilla older juveniles and adults have equal or longer forelimb relative to hind limb bones than western lowland adults. These findings indicate that both ontogenetically and among closely related species of Gorilla, long bone strength proportions better reflect actual locomotor behavior than bone length proportions.

Finite element modelling versus classic beam theory: comparing methods for stress estimation in a morphologically diverse sample of vertebrate long bones

Classic beam theory is frequently used in biomechanics to model the stress behaviour of vertebrate long bones, particularly when creating intraspecific scaling models. Although methodologically straightforward, classic beam theory requires complex irregular bones to be approximated as slender beams, and the errors associated with simplifying complex organic structures to such an extent are unknown. Alternative approaches, such as finite element analysis (FEA), while much more time-consuming to perform, require no such assumptions. This study compares the results obtained using classic beam theory with those from FEA to quantify the beam theory errors and to provide recommendations about when a full FEA is essential for reasonable biomechanical predictions. High-resolution computed tomographic scans of eight vertebrate long bones were used to calculate diaphyseal stress owing to various loading regimes. Under compression, FEA values of minimum principal stress (σ(min)) were on average 142 per cent (±28% s.e.) larger than those predicted by beam theory, with deviation between the two models correlated to shaft curvature (two-tailed p = 0.03, r(2) = 0.56). Under bending, FEA values of maximum principal stress (σ(max)) and beam theory values differed on average by 12 per cent (±4% s.e.), with deviation between the models significantly correlated to cross-sectional asymmetry at midshaft (two-tailed p = 0.02, r(2) = 0.62). In torsion, assuming maximum stress values occurred at the location of minimum cortical thickness brought beam theory and FEA values closest in line, and in this case FEA values of τ(torsion) were on average 14 per cent (±5% s.e.) higher than beam theory. Therefore, FEA is the preferred modelling solution when estimates of absolute diaphyseal stress are required, although values calculated by beam theory for bending may be acceptable in some situations.