Gulper, ripper and scrapper: anatomy of the neck in three species of vultures

The Role of Vulture (Accipitriformes) Cutaneous Microbiota in Infectious Disease Protection

Vultures (Accipitriformes), as obligate scavengers, are regularly exposed to a diverse array of pathogens present in decomposing carcasses. Nevertheless, they exhibit a remarkable ability to resist infections, suggesting a crucial role of skin microbiota in host defense. The microbial communities residing on necrophagic birds' skin create a protective barrier through competitive interactions, antimicrobial compound production, and immunity priming. Additionally, vultures contribute to ecosystem balance by reducing the spread of infectious agents. However, they may also serve as vectors for antimicrobial resistance (AMR) due to their exposure to contaminated food sources. Understanding the dynamics of their microbiota can provide valuable insights into host-microbe interactions, wildlife conservation, and public health. This review examines the composition and functional significance of vulture cutaneous microbiota. Specifically, it explores the role of necrophagic birds' skin microbiota in pathogen exclusion, immune system modulation, and environmental adaptation, with the aim of suggesting further research routes, besides clarifying the ecological implications of such birds.

Flesh and bone: The musculature and cervical movements of pterosaurs

The osteological variations present in the cervical vertebrae of pterosaurs represent changes in the soft tissues of the neck and reflect their function. Here, we infer the presence, volume, and capacity of the cervical musculature of pterosaurs. We performed our analyses on three-dimensionally preserved cervical series of Anhanguera sp. (AMNH 22555), Anhanguera piscator (NSM-PV 19892), Azhdarcho lancicollis (ZIN PH and CCMGE, several specimens), and Rhamphorhynchus muensteri (MGUH 1891.738), the last three of which were digitally modeled for muscle reconstruction. We identified osteological correlates from structures observed in extant archosaur vertebrae and skulls and supported by Extant Phylogenetic Bracket (EPB) criteria. We estimated the muscular capacity using the "Maximal Force Production" formula. According to our analyses, at least thirteen muscles were present in the neck of pterosaurs, only one of which does not correspond to an EPB level I inference. The muscles that performed skull and neck pitching were more robust and stronger to execute the movements. Muscles that showed extremely low potential had a more cervical stabilization function. Specializations we identified in the muscles are compatible with the foraging habits previously inferred for these pterosaurs, namely surface fishing by Rhamphorhynchus and Anhanguera and capture of small terrestrial prey by Azhdarcho.

Research on motion characterization of goose neck in narrow space

Introduction

Inspired by the obstacle avoidance mechanism of goose neck, a theoretical design method of bionic robotic arm was proposed to solve the contradiction between high flexibility and strong bearing capacity in narrow space.

Methods

Taking the goose neck as the test object, a narrow space test environment with a width of 10 cm was built, and a 6 × 4 obstacle matrix was set up, to analyze the maximum value of joint angle, motion rate and trajectory in different target areas.

Results

The test results showed that the goose neck movement has continuity and transmissibility. The overall posture of the goose neck was adjusted through the synergistic movement of the anterior, middle and posterior segments to move toward the target position. The regulating effect of the anterior segment was significantly stronger than that of the middle and posterior segments. Specifically, the anterior segment of goose neck exhibited mostly transverse movement, with significant horizontal regulation; the middle segment of the goose neck was coupled with longitudinal movement, with similar movement ability in all directions, the posterior segment of the goose neck has mostly longitudinal movement, with significant height regulation.

Conclusion

In addition, the YOLOv7-pose recognition network was used to recognize goose neck motion pose, which provides a new method for animal behavior research.

Covariation in the Craniocervical Junction of Carnivora

The craniocervical junction is the transition between the skull and the vertebral column that provides mobility while maintaining sufficient stability (i.e., protection of the brainstem and the spinal cord). The key elements involved are the occiput, the first cervical vertebra (CV1, atlas) and the second cervical vertebra (CV2, axis). The two vertebrae forming the atlas-axis complex are distinct in their morphology and differences in form have been linked to differences in ecological function in mammals. Here, we quantified the morphological diversity of the cranium, CV1 and CV2 in a sample of Carnivora using 3D geometric morphometrics to reveal phylogenetic and ecological patterns. Our results indicate that the observed variation in CV2 is related to the taxonomic diversity (i.e., strong phylogenetic signal), whereas variation in CV1 appears to be decoupled from species diversity in Carnivora and, thus, is likely to reflect a functional signal. The phylogenetically informed correlation analyses showed an association between the CV1 morphology and diet. Taxa that primarily feed on large prey tend to have larger transverse processes on CV1 which provides larger muscle attachment areas and may correlate with stronger muscles. The latter needs to be verified by future quantitative covariation analyses between bone and muscle data. Morphological peculiarities within Pinnipedia and Mustelidae could be explained by differences in terrestrial locomotion between Phocidae and Otariidae and the exceptional defensive behavior (i.e., handstanding) in Mephitidae. Despite differences in the degree of morphological diversity, covariation between cranium, CV1 and CV2 morphology is consistently high (≥ 0.82) highlighting that overall, the craniocervical junction is an integrated structure, but there are traits that are not constrained.

Cervical anatomy and its relation to foraging habits in aquatic birds (Aves: Neornithes: Neoaves)

Birds have extremely flexible necks, which help in their search for food. However, studies on the variation in bird cervical anatomy and its relationship with foraging are rare, despite the different habits presented between species. Here, we analyze the anatomy of the neck of aquatic birds and relate it to their foraging strategies. We dissected specimens representing four species of Charadriiformes, 11 species of Phaethoquornithes, and two specimens belonging to the outgroup Telluraves. We chose to emphasize Charadriiformes and Phaethoquornithes because they present several strategies that require cervical mobility and stability. We note that vertebral anatomy and dimensions vary, which affects the shape and size of the soft tissues attached throughout the neck. The synovial cartilage present in the articulatio intercorporalis represents an additional length in the neck, however, this is not longer than that observed in animals with intervertebral discs. Our analysis indicates that birds have a prevalence of dorsoventral movements in the middle of the neck and lateral and rotational movements near the base of the neck, while the region near the head presents a wide range of movement in all directions. Cervical ligaments and muscles throughout the neck provide stability in all segments, although the robustness of the soft tissues indicates that the most caudal portion of the neck is the most stable. The vertebral and soft tissue anatomy is consistent with the extensive mobility in pitching, yaw, and roll movements performed mainly by the head and first segment of the neck during the different foraging of the analyzed birds. Furthermore, the muscles closer to the skull are robust and allow the execution of a variety of habits to capture food in different species. The subsequent cervical segments present differences that explain their reduction in mobility, but they are equally stable.

A comparative study on grasping-related muscles in five Accipitriformes species

Accipitriformes are diverse in their prey preferences and use their grasping feet for hunting. Little is known about the architectural design of muscles related to grasping among species of different sizes, diets, and foraging behaviors. In the present study, we report quantitative data and analysis of the pelvic musculature of the Japanese sparrowhawk (Accipiter gularis), Eurasian sparrowhawk (Accipiter nisus), common buzzard (Buteo buteo), northern goshawk (Accipiter gentilis), and cinereous vulture (Aegypius monachus). As expected, mass and architecture of the considered muscles were very different between the cinereous vulture and the four other species. The cinereous vulture allocates more mass and physiological cross-sectional area (PCSA) to the proximally inserted flexor muscles, which indicates the rudimentary grasping ability of the foot and is a myological reflection of its carrion preference. Furthermore, in the cinereous vulture, muscles were built with the lowest architectural index (AI) compared with the other species, and the intrinsic foot muscles were short-fibered, which is disadvantageous for rapid manipulation and foot dexterity. The other four species, as a whole, featured large flexor hallucis longus (FHL) muscles and better development of distally inserted flexors, reflecting their predatory lifestyle. Some differences were also found between the species that consumed birds and those that consumed mammals. The two avivorous species were superior in AI and fiber length of the intrinsic foot muscles which are suitable for good hunting speed and digit flexibility, the prerequisition for hunting agile prey.

Arthrological reconstructions of the pterosaur neck and their implications for the cervical position at rest

The lack of any pterosaur living descendants creates gaps in the knowledge of the biology of this group, including its cervical biomechanics, which makes it difficult to understand their posture and life habits. To mitigate part of this issue, we reconstructed the cervical osteology and arthrology of three pterosaurs, allowing us to make inferences about the position of the neck of these animals at rest. We used scans of three-dimensionally preserved cervical series of Anhanguera piscator, Azhdarcho lancicollis and Rhamphorhynchus muensteri for the reconstructions, thus representing different lineages. For the recognition of ligaments, joint cartilages, and levels of overlapping of the zygapophyses, we applied the Extant Phylogenetic Bracket method, based on various extant birds and on Caiman latirostris. We inferred that pterosaur intervertebral joints were probably covered by a thin layer of synovial cartilage whose thickness varied along the neck, being thicker in the posterior region. Ignoring this cartilage can affect reconstructions. According to the vertebral angulation, their neck was slightly sinuous when in rest position. Our analyses also indicate that pterosaurs had segmented and supra-segmented articular cervical ligaments, which could confer stabilization, execute passive forces on the neck and store elastic energy.

Are owls technically capable of making a full head turn?

The three-dimensional configuration of the neck that produces extreme head turn in owls was studied using the Joint Coordinate System. The limits of planar axial rotation (AR), lateral, and sagittal bending in each vertebral joint were measured. They are not extraordinary among birds, except probably for the extended ability for AR. The vertebral joint angles involved in the 360° head turn do not generally exceed the limits of planar mobility. Rotation in one plane does not expand the range of motion in the other, with one probable exception being extended dorsal bending in the middle of the neck. Therefore, the extreme 360° head turn can be presented as a simple combination of the three planar motions in the neck joints. Surprisingly, certain joints are always laterally bent or axially rotated to the opposite side than the head was turned. This allows keeping the anterior part of the neck parallel to the thoracic spine, which probably helps preserve the ability for peering head motions throughout the full head turn. The potential ability of one-joint muscles of the owl neck, the mm. intertransversarii, to ensure the 360° head turn was addressed. It was shown that the 360° head turn does not require these muscles to shorten beyond the known contraction limit of striated vertebrate muscles. Shortening by 50% or less is enough for the mm. intertransversarii in the middle neck region for the 360° head turn. This study has broad implications for further research on vertebral mobility and function in a variety of tetrapods, providing a new method for CT scan-based measurement of intervertebral angles.

Comparative digital reconstruction of Pica pica and Struthio camelus and their cranial suture ontogenies

To date, several studies describe post-hatching ontogenetic variation in birds; however, none of these studies document and compare ontogenetic variation of the entire skull in multiple avian species. Therefore, we studied ontogenetic skull variation of two bird species with very different ecologies, Pica pica, and Struthio camelus, using μCT based 3D reconstructions. For each specimen, we performed bone-by-bone segmentation in order to visualize and describe the morphological variation of each bone during ontogeny and estimated the average sutural closure of the skulls to identify different ontogenetic stages. Although bone fusion of P. pica occurs more rapidly than that of S. camelus the general sequence of bone fusion follows a similar trend from posterior to anterior, but a more detailed analysis reveals some interspecific variation in the fusion patterns. Although growth persists over a longer period in S. camelus than in P. pica and adults of the former species are significantly larger, the skull of the most mature S. camelus is still less fused than that of P. pica. Different growth and fusion patterns of the two species indicate that the interspecific ontogenetic variation could be related to heterochronic developments. Nevertheless, this hypothesis needs to be tested in a broader phylogenetic framework in order to detect the evolutionary direction of the potential heterochronic transformations.

The neck as a keystone structure in avian macroevolution and mosaicism

BACKGROUND

The origin of birds from non-avian theropod dinosaur ancestors required a comprehensive restructuring of the body plan to enable the evolution of powered flight. One of the proposed key mechanisms that allowed birds to acquire flight and modify the associated anatomical structures into diverse forms is mosaic evolution, which describes the parcelization of phenotypic traits into separate modules that evolve with heterogeneous tempo and mode. Avian mosaicism has been investigated with a focus on the cranial and appendicular skeleton, and as such, we do not understand the role of the axial column in avian macroevolution. The long, flexible neck of extant birds lies between the cranial and pectoral modules and represents an opportunity to study the contribution of the axial skeleton to avian mosaicism.

RESULTS

Here, we use 3D geometric morphometrics in tandem with phylogenetic comparative methods to provide, to our knowledge, the first integrative analysis of avian neck evolution in context with the head and wing and to interrogate how the interactions between these anatomical systems have influenced macroevolutionary trends across a broad sample of extant birds. We find that the neck is integrated with both the head and the forelimb. These patterns of integration are variable across clades, and only specific ecological groups exhibit either head-neck or neck-forelimb integration. Finally, we find that ecological groups that display head-neck and neck-forelimb integration tend to display significant shifts in the rate of neck morphological evolution.

CONCLUSIONS

Combined, these results suggest that the interaction between trophic ecology and head-neck-forelimb mosaicism influences the evolutionary variance of the avian neck. By linking together the biomechanical functions of these distinct anatomical systems, the cervical vertebral column serves as a keystone structure in avian mosaicism and macroevolution.

Eurasian griffon vulture (Gyps fulvus) as a bone modifying agent and its implications for archaeology

Neo-taphonomic studies have allowed us to detect bone damage patterns linked to carnivore preferences and behavioral traits as well as to improve our understanding of the origin of different alterations on vertebrate fossil faunas. However, taphonomically speaking vultures are among the least studied of all common, obligate scavengers. The research reported here contributes to characterise Eurasian griffon vulture (Gyps fulvus) behavior from a taphonomic perspective describing bone damage on 12 small-sized ungulate carcasses. The combination of observational data from photo/video-trap together with taphonomic analyses allowed us to manage factors like feeding behavior or time of consumption, as well as to accurately record bone modified items. Some bone-modifying effects are described here for the first time as vulture-made bone-damage distinctiveness. Still, some others may pose equifinality problems especially regarding small carnivores. This taphonomic conundrum leaves an interpretation problem particularly in archaeological sites in which those agents are present and consequently, an individualization dilemma about the taphonomic actors involved in bone modified assemblages.

A surrogate forelimb: Evolution, function and development of the avian cervical spine

The neck is a critical portion of the avian spine, one that works in tandem with the beak to act as a surrogate forelimb and allows birds to manipulate their surroundings despite the lack of a grasping capable hand. Birds display an incredible amount of diversity in neck morphology across multiple anatomical scales-from varying cervical counts down to intricate adaptations of individual vertebrae. Despite this morphofunctional disparity, little is known about the drivers of this enormous variation, nor how neck evolution has shaped avian macroevolution. To promote interest in this system, I review the development, function and evolution of the avian cervical spine. The musculoskeletal anatomy, basic kinematics and development of the avian neck are all documented, but focus primarily upon commercially available taxa. In addition, recent work has quantified the drivers of extant morphological variation across the avian neck, as well as patterns of integration between the neck and other skeletal elements. However, the evolutionary history of the avian cervical spine, and its contribution to the diversification and success of modern birds is currently unknown. Future work should aim to broaden our understanding of the cervical anatomy, development and kinematics to include a more diverse selection of extant birds, while also considering the macroevolutionary drivers and consequences of this important section of the avian spine.

The relative contributions of multiarticular snake muscles to movement in different planes

Muscles spanning multiple joints play important functional roles in a wide range of systems across tetrapods; however, their fundamental mechanics are poorly understood, particularly the consequences of anatomical position on mechanical advantage. Snakes provide an excellent study system for advancing this topic. They rely on the axial muscles for many activities, including striking, constriction, defensive displays, and locomotion. Moreover, those muscles span from one or a few vertebrae to over 30, and anatomy varies among muscles and among species. We characterized the anatomy of major epaxial muscles in a size series of corn snakes (Pantherophis guttatus) using diceCT scans, and then took several approaches to calculating contributions of each muscle to force and motion generated during body bending, starting from a highly simplistic model and moving to increasingly complex and realistic models. Only the most realistic model yielded equations that included the consequence of muscle span on torque-displacement trade-offs, as well as resolving ambiguities that arose from simpler models. We also tested whether muscle cross-sectional areas or lever arms (total magnitude or pitch/yaw/roll components) were related to snake mass, longitudinal body region (anterior, middle, posterior), and/or muscle group (semispinalis-spinalis, multifidus, longissimus dorsi, iliocostalis, and levator costae). Muscle cross-sectional areas generally scaled with positive allometry, and most lever arms did not depart significantly from geometric similarity (isometry). The levator costae had lower cross-sectional area than the four epaxial muscles, which did not differ significantly from each other in cross-sectional area. Lever arm total magnitudes and components differed among muscles. We found some evidence for regional variation, indicating that functional regionalization merits further investigation. Our results contribute to knowledge of snake muscles specifically and multiarticular muscle systems generally, providing a foundation for future comparisons across species and bioinspired multiarticular systems.

New Zealand's extinct giant raptor (Hieraaetus moorei) killed like an eagle, ate like a condor

The extinct Haast's eagle or harpagornis (Hieraaetus moorei) is the largest known eagle. Historically, it was first considered a predator, then a scavenger, but most recent authors have favoured an active hunting ecology. However, the veracity of proposed similarities to carrion feeders has not been thoroughly tested. To infer feeding capability and behaviour in harpagornis, we used geometric morphometric and finite-element analyses to assess the shape and biomechanical strength of its neurocranium, beak and talons in comparison to five extant scavenging and predatory birds. The neurocranium of harpagornis is vulture-like in shape whereas its beak is eagle-like. The mechanical performance of harpagornis is closer to extant eagles under biting loads but is closest to the Andean condor (Vultur gryphus) under extrinsic loads simulating prey capture and killing. The talons, however, are eagle-like and even for a bird of its size, able to withstand extremely high loads. Results are consistent with the proposition that, unlike living eagles, harpagornis habitually killed prey larger than itself, then applied feeding methods typical of vultures to feed on the large carcasses. Decoupling of the relationship between neurocranium and beak shape may have been linked to rapid evolution.

Modeling intervertebral articulation: The rotule à doigt mechanical joint (RAD) in birds and mammals

The vertebrate skeleton is composed of articulated bones. Most of the articulations are classically described using mechanical joints, except the intervertebral joint. The aim of this study was to identify a joint model with the same mechanical features as the cervical joints. On the neck vertebrae, six articular surfaces participate in the joint: the cranial part of the centrum and the facets of the two prezygapophyses of a vertebra articulate on the caudal part of the centrum and the two articular facets of the postzygapophyses of the previous vertebra. We used the intervertebral joints of the birds neck to identify the mechanical joint representing intervertebral linkage. This link was described in the literature as a joint allowing two or three rotations and no translation. These features correspond to the rotule à doigt (RAD) joint, a ball and socket joint with a pin. We compared the RAD joint to the postaxial intervertebral joints of the avian neck and found it a suitable model to determine the geometrical features involved in the joint mobility. The difference in the angles of virtual axes linking the geometrical center of the centrum to the zygapophysis surfaces determines the mean dorsoventral flexion of the joint. It also helps to limit longitudinal rotation. The orientation of the zygapophysis surfaces determines the range of motion in both dorsoventral and lateral flexion. The overall system prevents dislocation. The model was validated on 13 joints of a vulture neck and 11 joints of a swallow neck and on one joint (C6-C7) in each of three mammal species: the wolf (Canis lupus), mole (Talpa europaea), and human (Homo sapiens). The RAD mechanical joint was found in all vertebral articulations. This validation of the model on different species shows that the RAD intervertebral joint model makes it possible to extract the parameters that guide and limit the mobility of the cervical spine from the complex shape of the vertebrae and to compare them in interspecific studies.

Somite development and regionalisation of the vertebral axial skeleton

A critical stage in the development of all vertebrate embryos is the generation of the body plan and its subsequent patterning and regionalisation along the main anterior-posterior axis. This includes the formation of the vertebral axial skeleton. Its organisation begins during early embryonic development with the periodic formation of paired blocks of mesoderm tissue called somites. Here, we review axial patterning of somites, with a focus on studies using amniote model systems - avian and mouse. We summarise the molecular and cellular mechanisms that generate paraxial mesoderm and review how the different anatomical regions of the vertebral column acquire their specific identity and thus shape the body plan. We also discuss the generation of organoids and embryo-like structures from embryonic stem cells, which provide insights regarding axis formation and promise to be useful for disease modelling.

Estimating motion between avian vertebrae by contact modeling of joint surfaces

Estimating the motion between two bones is crucial for understanding their biomechanical function. The vertebral column is particularly challenging because the vertebrae articulate at more than one surface. This paper proposes a method to estimate 3D motion between two avian vertebrae, by bones surface reconstruction and contact modeling. The neck of birds was selected as a case study because it is a functionally highly versatile structure combining dexterity and strength. As such, it has great potential to serve as a source for bioinspired design, for robotic manipulators for instance. First, 3D models of the vertebrae are obtained by computed tomography (CT). Next, joint surfaces of contact are approximated with polynomial surfaces, and a system of equations derived from contact modeling between surfaces is established. A constrained optimization problem is defined in order to find the best position of the vertebrae for a set of given orientations in space. As a result, the possible intervertebral range of motion is estimated.

A revision of vulture feeding classification

Pioneering fieldwork identified the existence of three feeding groups in vultures: gulpers, rippers and scrappers. Gulpers engulf soft tissue from carcasses and rippers tear off pieces of tough tissue (skin, tendons, muscle), whereas scrappers peck on small pieces of meat they find on and around carcasses. It has been shown that these feeding preferences are reflected in the anatomy of the skull and neck. Here, we demonstrate that these three feeding groups also emerge when body core and limb bones are added to the analysis. However, the resulting classification differs from that which is based on skull morphology for three species, namely Gypaetus barbatus (Linnaeus, 1758), Gypohierax angolensis (Gmelin, 1788) and Gyps indicus (Scopoli, 1786). The proposed classification would improve the interrelationship between form and feeding habits in vultures. Moreover, the results of this study reinforce the value of the categorisation system introduced by Kruuk (1967), and expanded by König (1974, 1983), Houston (1988) and Hertel (1994), as it would affect not only the skull morphology but the whole-body architecture.

Evolutionary versatility of the avian neck

Bird necks display unparalleled levels of morphological diversity compared to other vertebrates, yet it is unclear what factors have structured this variation. Using three-dimensional geometric morphometrics and multivariate statistics, we show that the avian cervical column is a hierarchical morpho-functional appendage, with varying magnitudes of ecologically driven osteological variation at different scales of organization. Contrary to expectations given the widely varying ecological functions of necks in different species, we find that regional modularity of the avian neck is highly conserved, with an overall structural blueprint that is significantly altered only by the most mechanically demanding ecological functions. Nevertheless, the morphologies of vertebrae within subregions of the neck show more prominent signals of adaptation to ecological pressures. We also find that both neck length allometry and the nature of neck elongation in birds are different from other vertebrates. In contrast with mammals, neck length scales isometrically with head mass and, contrary to previous work, we show that neck elongation in birds is achieved predominantly by increasing vertebral lengths rather than counts. Birds therefore possess a cervical spine that may be unique in its versatility among extant vertebrates, one that, since the origin of flight, has adapted to function as a surrogate forelimb in varied ecological niches.