Skull Of Birds Research Articles

Insight into the evolutionary assemblage of cranial kinesis from a Cretaceous bird.

The independent movements and flexibility of various parts of the skull, called cranial kinesis, are an evolutionary innovation that is found in living vertebrates only in some squamates and crown birds and is considered to be a major factor underpinning much of the enormous phenotypic and ecological diversity of living birds, the most diverse group of extant amniotes. Compared to the postcranium, our understanding of the evolutionary assemblage of the characteristic modern bird skull has been hampered by sparse fossil records of early cranial materials, with competing hypotheses regarding the evolutionary development of cranial kinesis among early members of the avialans. Here, a detailed three-dimensional reconstruction of the skull of the Early Cretaceous enantiornithine Yuanchuavis kompsosoura allows for its in-depth description, including elements that are poorly known among early-diverging avialans but are central to deciphering the mosaic assembly of features required for modern avian cranial kinesis. Our reconstruction of the skull shows evolutionary and functional conservation of the temporal and palatal regions by retaining the ancestral theropod dinosaurian configuration within the skull of this otherwise derived and volant bird. Geometric morphometric analysis of the palatine suggests that loss of the jugal process represents the first step in the structural modifications of this element leading to the kinetic crown bird condition. The mixture of plesiomorphic temporal and palatal structures together with a derived avialan rostrum and postcranial skeleton encapsulated in Yuanchuavis manifests the key role of evolutionary mosaicism and experimentation in early bird diversification.

Not like night and day: the nocturnal letter-winged kite does not differ from diurnal congeners in orbit or endocast morphology.

Nocturnal birds display diverse adaptations of the visual system to low-light conditions. The skulls of birds reflect many of these and are used increasingly to infer nocturnality in extinct species. However, it is unclear how reliable such assessments are, particularly in cases of recent evolutionary transitions to nocturnality. Here, we investigate a case of recently evolved nocturnality in the world's only nocturnal hawk, the letter-winged kite Elanus scriptus. We employed phylogenetically informed analyses of orbit, optic foramen and endocast measurements from three-dimensional reconstructions of micro-computed tomography scanned skulls of the letter-winged kite, two congeners, and 13 other accipitrid and falconid raptors. Contrary to earlier suggestions, the letter-winged kite was not unique in any of our metrics. However, all species of Elanus have significantly higher ratios of orbit versus optic foramen diameter, suggesting high visual sensitivity at the expense of acuity. In addition, visual system morphology varies greatly across accipitrid species, likely reflecting hunting styles. Overall, our results suggest that the transition to nocturnality can occur rapidly and without changes to key hard-tissue indicators of vision, but also that hard-tissue anatomy of the visual system may provide a means of inferring a range of raptor behaviours, well beyond nocturnality.

Flat or flexible? Evolutionary cranial biomechanics and the origins of modern archosaur skulls

The divergent specializations of living archosaurs, crocodilians and birds, represent two of the great transformations in vertebrate evolution. Despite hailing from common ancestor with a tall skull, braced palate and relatively unspecialized jaw muscles, these two living clades followed disparate paths in cranial biomechanics and feeding functional morphology during the Mesozoic era, resulting in the flat, rigid skulls in crocodylians and the bulbous and beaked, flexible skulls of birds. Here we use new approaches in imaging, 3D modeling, morphometrics, lever mechanics, finite element modeling and phylogenetic comparative methods of living and extinct reptiles to show how morphological and functional changes in the palate, cranial joints and jaw musculature occurred within the fossil record of these lineages that were key features in acquisition of the modern forms. We show how jaw muscles reoriented their resultants and moments about jaw joints during skull flattening, palatocranial suturing and feeding specializations in Jurassic crocodyliforms. These morphological changes resulted in a decrease in mechanical efficiency of the feeding apparatus that was compensated for by an increase in jaw muscle sizes and canalized muscle architecture. Meanwhile, along the line to living birds, the acquisition of enlarged brains in coelurosaur dinosaurs resulted in a shifting of temporal and palatal muscle positions, a canalization of vertically‐oriented of muscle forces and consequently reorientation of their moments about palatocranial joints. These changes in loading environment, along with a concatenate increase in the propulsive and bending properties of the palate, facilitated cranial kinesis in neoavian birds. Together, these examples of evolutionary innovation in reptile cranial biomechanics reveal powerful new approaches to testing biomechanical and evolutionary hypotheses in any cranial or appendicular musculoskeletal system.

Common Patterns of Skull Bone Fusion and Their Potential to Discriminate Different Ontogenetic Stages in Extant Birds

The degree of sutural closure between bones generally allows for the classification of skeleton maturity in tetrapods. In mammals, the sutural closure of skull bones was previously used as proxy to evaluate the ontogenetic stage of single individuals. However, due to temporal variation, this process can be only applied among mammalian subclades, but not for all mammals in general. In contrast, the process of sutural closures in bird skulls could be a more reliable ontogenetic proxy for this clade as adult birds commonly show a generally high degree of bone fusion. To test this, we studied the process of sutural closure in ontogenetic series of 18 extant bird species regarding the presence of an ontogenetic signal and compared the results with changes in skull size and proportions. Univariate analyses indicate that bone fusion happens faster in altricial than in precocial birds. However, the use of PCoA and multivariate regressions reveal that the skull bone fusion follows a common pattern among birds and thus can be used as proxy to identify different ontogenetic stages. In general, the process of sutural closure spreads from posterior to anterior and from ventral to dorsal. In contrast, skull measurements reflect rather interspecific allometry than ontogeny. The used of bone fusion as proxy will help to better identify and compare different stages of maturation in birds, including historical material from osteological collections.

Ecofeminism and Structural Violence

Ecofeminism and Structural Violence

Cretaceous bird with dinosaur skull sheds light on avian cranial evolution

The transformation of the bird skull from an ancestral akinetic, heavy, and toothed dinosaurian morphology to a highly derived, lightweight, edentulous, and kinetic skull is an innovation as significant as powered flight and feathers. Our understanding of evolutionary assembly of the modern form and function of avian cranium has been impeded by the rarity of early bird fossils with well-preserved skulls. Here, we describe a new enantiornithine bird from the Early Cretaceous of China that preserves a nearly complete skull including the palatal elements, exposing the components of cranial kinesis. Our three-dimensional reconstruction of the entire enantiornithine skull demonstrates that this bird has an akinetic skull indicated by the unexpected retention of the plesiomorphic dinosaurian palate and diapsid temporal configurations, capped with a derived avialan rostrum and cranial roof, highlighting the highly modular and mosaic evolution of the avialan skull.

Herd

Herd Barbara Ras (bio) Keywords Barbara, Ras, poetry, face, horse, dirt, dreams, night I felt hands cup my eyes, lower the lids, and hold themuntil they stopped shuddering, like in the story,when the girl covers a horse's eyes to calm it, releaseit from standing watch. You and I stood next to each otherin a business meeting, yes, at work, shoulder to shoulder our invisible shadows smoldering. Then I lay down on my side, stretched out my four legs,a horse able to sleep, safe with its herd watching. In my body I felt the warmth of the ground connect with the hot of the sun,a current carried the feel of your starched shirt and the gritof dirt against my cheek. Your name drifted awayinto pallid clouds, mine started to follow, and a tonguenew to me in reach reached to lick them back. Or maybe it was you who lay down like a horse givingitself to dreams. Your muscles twitched. Heat roseoff your body like remorse exploring the air for mercy. All this I knew, being your herd, and sharing your restlessnessfor tall grass, wind playing over a prairie that hides bird skull, a brass brooch, teeth. Could our colleagues smell the horses, the horse heat, the horse breath? [End Page 230] I heard "oysters," whispered in my ear as if I had asked your favorite food. No, I'd asked how many carrots a horse eats in a day. We didn't speak of what we didn't know how to hold in our hands. As always the day fell.The night caught it as always. [End Page 231] Barbara Ras barbara ras is the author of four poetry collections, including Bite Every Sorrow, which won the Walt Whitman Award and the Kate Tufts Discovery Award; One Hidden Stuff; and The Last Skin. The poems published here are from her most recent book, published in 2021, The Blues of Heaven. Ras is the editor of a collection of short fiction in translation, Costa Rica: A Traveler's Literary Companion. She has taught poetry nationally and internationally with the International Writing Program. Ras is the founding director emerita of Trinity University Press. Copyright © 2021 The Massachusetts Review, Inc.

Evolutionary and ontogenetic changes of the anatomical organization and modularity in the skull of archosaurs

Comparative anatomy studies of the skull of archosaurs provide insights on the mechanisms of evolution for the morphologically and functionally diverse species of crocodiles and birds. One of the key attributes of skull evolution is the anatomical changes associated with the physical arrangement of cranial bones. Here, we compare the changes in anatomical organization and modularity of the skull of extinct and extant archosaurs using an Anatomical Network Analysis approach. We show that the number of bones, their topological arrangement, and modular organization can discriminate birds from non-avian dinosaurs, and crurotarsans. We could also discriminate extant taxa from extinct species when adult birds were included. By comparing within the same framework, juveniles and adults for crown birds and alligator (Alligator mississippiensis), we find that adult and juvenile alligator skulls are topologically similar, whereas juvenile bird skulls have a morphological complexity and anisomerism more similar to those of non-avian dinosaurs and crurotarsans than of their own adult forms. Clade-specific ontogenetic differences in skull organization, such as extensive postnatal fusion of cranial bones in crown birds, can explain this pattern. The fact that juvenile and adult skulls in birds do share a similar anatomical integration suggests the presence of a specific constraint to their ontogenetic growth.

Data for 3D printing enlarged museum specimens for the visually impaired.

Museums are embracing new technologies and one of these is the use of 3D printing. 3D printing allows for creating physical replicas of items which may, due to great value or significance, not be handled by the public, or which are too small or fragile to be handled or even seen with the naked eye. One such application of new technologies has been welcomed by the National Museum in Bloemfontein, Free State, South Africa. Here, blown-up (enlarged) Museum specimens were 3D printed for various interactive exhibits that are aimed at increasing the accessibility of their permanent displays for visually impaired visitors who rely greatly on touch as a source of observation. A selection of scorpions, pseudoscorpions, mites and archetypal bird skulls were scanned, processed and 3D printed to produce enlarged, highly functional nylon models. This data paper provides the raw micro Computed Tomography (micro-CT) scan data and print ready STL files processed from this data. The STL files may be used in their current format and details of the printing are provided.

3D Muscle Fiber Architecture of Avian Jaw Muscles and their Significance for Avian cranial functional morphology evolution

Birds evolved a diversity of head shapes, feeding behaviors and jaw muscles which drive the feeding apparatus. However, capturing the anatomical and functional diversity of these muscles remains challenging and we know little about how jaw muscles develop, function and evolve among lineages of the clade and this hampers our ability to understand skeletal tissue biology, joint function, and patterns of evolution in bird skulls. Here we use contrast imaging, 3D muscle fiber tracking and new data visualization methods to explore the morphology, architecture and functional significance of the jaw muscles of a sample of birds to highlight the potential of these approaches. First, we show differences in functional morphology of jaw muscles between a hatchling and adult Mallard duck (Anas platyrhynchos). Ducks show a marked change in adductor chamber shape during ontogeny including an elongated postorbital process, quadrate orbital process and retroarticular process along with a broad temporal fossa which likely track with changes in muscle function and cranial kinesis. Indeed, we found m. depressor mandibulae in baby and adult ducks shifts from a rostrocadual orientation to a more dorsoventral orientation that offers additional mechanical advantage to jaw opening and propalinal mandible movement as well as concomitant changes in the 3D resultant and pennation of other jaw muscle bellies. Second, we show differences between a long‐ and short‐faced, dabbling birds (mallard, green‐wing teal), long faced orthally‐biting bird (belted kingfisher) and a short‐faced, hard‐biting bird, the grey parrot (Psittacus erithacus). We expected position, architecture and biomechanics of homologous muscles to differ substantially between these avian species given their different behaviors and overall cranial morphology. Although some muscles showed marked differences in resultant and pennation, others, including protractor pterygoideus, the primary actuator of kinesis, were remarkably similar in morphology suggesting a mosaic of changes happening during avian evolution. Together, these new, 3D high fidelity anatomical data on jaw muscle functional morphology in birds will not only better illuminate ecomorphological evolution but also illustrate how other clades of birds and vertebrates adapt their muscle architecture to meet functional demands.

Can metric data be an effective tool for galliform skull identification in archaeological contexts?

Galliforms, or game birds, are commonly found in zooarchaeological assemblages but several taxa within the order (e.g. chicken, pheasant and grouse) are difficult to distinguish from one another morphologically. Osteometrics is a tool for understanding skeletal variation in animal populations that has been shown to be useful in distinguishing between closely related genera in galliform post-cranial elements. In this paper, we test whether metrics are also effective in identifying galliform skulls (crania and mandibles) to the genus level. Using osteometric data collected from nine North American gallinaceous genera, we found that size was the greatest source of variation within the order. Several dimensions (greatest height, smallest breadth between orbits and length of incisivum) on the cranium were successful in distinguishing Domestic chicken (Gallus gallus domesticus) from Ring-necked pheasant (Phasianus colchicus), while measurements on the mandible were not very effective in separating closely related taxa. The application of osteometrics to fragmentary bird skulls from an historic era site assemblage in New York was not effective in identifying birds beyond general size-classes even with the aid of morphology and comparative collections. This suggests that previous work identifying fragmentary skull remains to the species level is not accurate, and that more advanced methods need to be developed to identify morphological and metric variation between taxa in these fragmentary elements.

Complete Ichthyornis skull illuminates mosaic assembly of the avian head.

The skull of living birds is greatly modified from the condition found in their dinosaurian antecedents. Bird skulls have an enlarged, toothless premaxillary beak and an intricate kinetic system that includes a mobile palate and jaw suspensorium. The expanded avian neurocranium protects an enlarged brain and is flanked by reduced jaw adductor muscles. However, the order of appearance of these features and the nature of their earliest manifestations remain unknown. The Late Cretaceous toothed bird Ichthyornis dispar sits in a pivotal phylogenetic position outside living groups: it is close to the extant avian radiation but retains numerous ancestral characters1-3. Although its evolutionary importance continues to be affirmed3-8, no substantial new cranial material of I. dispar has been described beyond incomplete remains recovered in the 1870s. Jurassic and Cretaceous Lagerstätten have yielded important avialan fossils, but their skulls are typically crushed and distorted 9 . Here we report four three-dimensionally preserved specimens of I. dispar-including an unusually complete skull-as well as two previously overlooked elements from the Yale Peabody Museum holotype, YPM 1450. We used these specimens to generate a nearly complete three-dimensional reconstruction of the I. dispar skull using high-resolution computed tomography. Our study reveals that I. dispar had a transitional beak-small, lacking a palatal shelf and restricted to the tips of the jaws-coupled with a kinetic system similar to that of living birds. The feeding apparatus of extant birds therefore evolved earlier than previously thought and its components were functionally and developmentally coordinated. The brain was relatively modern, but the temporal region was unexpectedly dinosaurian: it retained a large adductor chamber bounded dorsally by substantial bony remnants of the ancestral reptilian upper temporal fenestra. This combination of features documents that important attributes of the avian brain and palate evolved before the reduction of jaw musculature and the full transformation of the beak.

The Origin of the Avian Head as told by Transformations in the Fossil Record and Embryonic Development

The head of birds is immediately and profoundly distinguishable from those of other major vertebrate clades, owing to a bewildering number of derived features including an expanded brain and eyes, a shortening of the maxillary face, and an expansion of the premaxillary region at the tip of the snout into the characteristic avian beak. Considered separately, multitudes of individual transformations are required to bridge the gap between an ancestral reptilian cranium and a bird skull. However, in our work, we have found that large‐scale cranial evolution operates in such a way that multiple transformations have a single underlying developmental cause, resulting in character nonindependence and a simpler and more elegant evolutionary story than has previously been assumed. Foremost, we find that the avialan skull is fundamentally a juvenilized or paedomorphic version of the ancestral dinosaurian skull, and that many if not most of the iconic features of birds can be explained using a simple growth or scaling mechanism. In addition, the beak and skull roof — the entire modern avian feeding system, which features a great deal of mobility or kinesis — is patterned by one or a few early developmental influences. Finally, the highly modified avian skull roof is primarily influenced by the transformed brain, and indeed evidence suggests that a tight linkage between these structures is ancestral for Osteichthyes. We also found indications that the expanding brain restricted the size of the jaw‐closing musculature. Remarkably, the fossil record supports many of these hypotheses, showing clear signals of juvenilization and correlated shifts in brain and skull, and also showing simultaneous transformation of the rostral beak and the palate as suggested by our developmental work. We find particular support for our hypotheses using new material of the most important fossil taxa demonstrating the “modernization” of the avian skull, the Cretaceous toothed stem‐birds Ichthyornis and Hesperornis. However, this material simultaneously demonstrates that the avian adductor chamber was, in at least some taxa, remarkably plesiomorphic even after the expansion of the brain, falsifying our hypotheses of a strict causal relationship between brain expansion and adductor diminution. Thus, despite the importance of developmental data in demonstrating the ways in which character transformations are interrelated during major evolutionary transitions, the fossil record is always the final arbiter of the merit of any large‐scale evolutionary hypothesis.Support or Funding InformationFunding provided by Yale University, the Yale Peabody Museum of Natural History, the National Science Foundation, and the Human Frontier Science Foundation.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Developmental origins of mosaic evolution in the avian cranium

Mosaic evolution, which results from multiple influences shaping morphological traits and can lead to the presence of a mixture of ancestral and derived characteristics, has been frequently invoked in describing evolutionary patterns in birds. Mosaicism implies the hierarchical organization of organismal traits into semiautonomous subsets, or modules, which reflect differential genetic and developmental origins. Here, we analyze mosaic evolution in the avian skull using high-dimensional 3D surface morphometric data across a broad phylogenetic sample encompassing nearly all extant families. We find that the avian cranium is highly modular, consisting of seven independently evolving anatomical regions. The face and cranial vault evolve faster than other regions, showing several bursts of rapid evolution. Other modules evolve more slowly following an early burst. Both the evolutionary rate and disparity of skull modules are associated with their developmental origin, with regions derived from the anterior mandibular-stream cranial neural crest or from multiple embryonic cell populations evolving most quickly and into a greater variety of forms. Strong integration of traits is also associated with low evolutionary rate and low disparity. Individual clades are characterized by disparate evolutionary rates among cranial regions. For example, Psittaciformes (parrots) exhibit high evolutionary rates throughout the skull, but their close relatives, Falconiformes, exhibit rapid evolution in only the rostrum. Our dense sampling of cranial shape variation demonstrates that the bird skull has evolved in a mosaic fashion reflecting the developmental origins of cranial regions, with a semi-independent tempo and mode of evolution across phenotypic modules facilitating this hyperdiverse evolutionary radiation.

Linkage mechanisms in the vertebrate skull: Structure and function of three-dimensional, parallel transmission systems.

Many musculoskeletal systems, including the skulls of birds, fishes, and some lizards consist of interconnected chains of mobile skeletal elements, analogous to linkage mechanisms used in engineering. Biomechanical studies have applied linkage models to a diversity of musculoskeletal systems, with previous applications primarily focusing on two-dimensional linkage geometries, bilaterally symmetrical pairs of planar linkages, or single four-bar linkages. Here, we present new, three-dimensional (3D), parallel linkage models of the skulls of birds and fishes and use these models (available as free kinematic simulation software), to investigate structure-function relationships in these systems. This new computational framework provides an accessible and integrated workflow for exploring the evolution of structure and function in complex musculoskeletal systems. Linkage simulations show that kinematic transmission, although a suitable functional metric for linkages with single rotating input and output links, can give misleading results when applied to linkages with substantial translational components or multiple output links. To take into account both linear and rotational displacement we define force mechanical advantage for a linkage (analogous to lever mechanical advantage) and apply this metric to measure transmission efficiency in the bird cranial mechanism. For linkages with multiple, expanding output points we propose a new functional metric, expansion advantage, to measure expansion amplification and apply this metric to the buccal expansion mechanism in fishes. Using the bird cranial linkage model, we quantify the inaccuracies that result from simplifying a 3D geometry into two dimensions. We also show that by combining single-chain linkages into parallel linkages, more links can be simulated while decreasing or maintaining the same number of input parameters. This generalized framework for linkage simulation and analysis can accommodate linkages of differing geometries and configurations, enabling novel interpretations of the mechanics of force transmission across a diversity of vertebrate feeding mechanisms and enhancing our understanding of musculoskeletal function and evolution. J. Morphol. 277:1570-1583, 2016. © 2016 Wiley Periodicals, Inc.

Looking Glass Worlds: The Queen and the Mirror

Looking Glass Worlds: The Queen and the Mirror

How to Make a Bird Skull: Major Transitions in the Evolution of the Avian Cranium, Paedomorphosis, and the Beak as a Surrogate Hand.

The avian skull is distinctive in its construction and in its function. Much of bird anatomical variety is expressed in the beak; but the beak itself, largely formed of the premaxillary bone, is set upon a shortened face and a bulbous, enlarged braincase. Here, we use original anatomical observations and reconstructions to describe the overall form of the avian skull in a larger context and to provide a general account of the evolutionary transformation from the early dinosaur skull-the skull of an archosaurian macropredator-to that of modern birds. Facial shortening, the enlargement of the braincase around an enlarged brain (with consequential reduction of circumorbital elements and the adductor chamber), and general thinning and looser articulation of bones are trends. Many of these owe to juvenilization or paedomorphosis, something that is abundantly evident from comparison of a juvenile early theropod (Coelophysis) to early avialans like Archaeopteryx Near the avian crown, the premaxilla becomes dramatically enlarged and integrated into the characteristic mobile kinetic system of birds. We posit that this addition of a large element onto the skull may be biomechanically feasible only because of the paedomorphic shortening of the face; and kinesis of the beak only because of the paedomorphic thinning of the bones and loosening of articulations, as played out in reverse during the maturation of Coelophysis Finally, the beak itself becomes elaborated as the hands are integrated into the wing. There are structural, kinematic, and neurological similarities between avian pecking and primate grasping. The ability to precision-select high-quality food against a complex but depauperate background may have permitted crown birds to survive the end-Cretaceous cataclysm by feeding on insects, seeds, and other detritus after the collapse of higher trophic levels in the food web.

Birds have paedomorphic dinosaur skulls

The interplay of evolution and development has been at the heart of evolutionary theory for more than a century. Heterochrony—change in the timing or rate of developmental events—has been implicated in the evolution of major vertebrate lineages such as mammals, including humans. Birds are the most speciose land vertebrates, with more than 10,000 living species representing a bewildering array of ecologies. Their anatomy is radically different from that of other vertebrates. The unique bird skull houses two highly specialized systems: the sophisticated visual and neuromuscular coordination system allows flight coordination and exploitation of diverse visual landscapes, and the astonishing variations of the beak enable a wide range of avian lifestyles. Here we use a geometric morphometric approach integrating developmental, neontological and palaeontological data to show that the heterochronic process of paedomorphosis, by which descendants resemble the juveniles of their ancestors, is responsible for several major evolutionary transitions in the origin of birds. We analysed the variability of a series of landmarks on all known theropod dinosaur skull ontogenies as well as outgroups and birds. The first dimension of variability captured ontogeny, indicating a conserved ontogenetic trajectory. The second dimension accounted for phylogenetic change towards more bird-like dinosaurs. Basally branching eumaniraptorans and avialans clustered with embryos of other archosaurs, indicating paedomorphosis. Our results reveal at least four paedomorphic episodes in the history of birds combined with localized peramorphosis (development beyond the adult state of ancestors) in the beak. Paedomorphic enlargement of the eyes and associated brain regions parallels the enlargement of the nasal cavity and olfactory brain in mammals. This study can be a model for investigations of heterochrony in evolutionary transitions, illuminating the origin of adaptive features and inspiring studies of developmental mechanisms.

Implementation of spin‐echo blood oxygen level‐dependent (BOLD) functional MRI in birds

The advent of high-field MRI systems has allowed the implementation of blood oxygen level-dependent functional MRI (BOLD fMRI) on small animals. An increased magnetic field improves the signal-to-noise ratio and thus allows an improvement in the spatial resolution. However, it also increases susceptibility artefacts in the commonly acquired gradient-echo images. This problem is particularly prominent in songbird MRI because of the presence of numerous air cavities in the skull of birds. These T(2)*-related image artefacts can be circumvented using spin-echo BOLD fMRI. In this article, we describe the implementation of spin-echo BOLD fMRI in zebra finches, a small songbird of 15-25 g, extensively studied in the behavioural neurosciences of birdsong. Because the main topics in this research domain are song perception and song learning, the protocol implemented used auditory stimuli. Despite the auditory nature of the stimuli and the weak contrast-to-noise ratio of spin-echo BOLD fMRI compared with gradient-echo BOLD fMRI, we succeeded in detecting statistically significant differences in BOLD responses triggered by different stimuli. This study shows that spin-echo BOLD fMRI is a viable approach for the investigation of auditory processing in the whole brain of small songbirds. It can also be applied to study auditory processing in other small animals, as well as other sensory modalities.

WEIGHING IN ON BIRD BONES

Although the list of things I know about bird biology is short, one thing I felt confident about was that bird skeletons are lightweight. I've seen for myself the thin-walled, hollow bones found in many bird species, and this always made intuitive sense: flying is certainly easier if you're lighter. Betsy Dumont at the University of Massachusetts recently decided to look more deeply into this apparent truism by asking just how lightweight bird bones are.Using museum specimens, Dumont measured bone density across a wide variety of perching birds, rodents and bats. She chose to focus on the cranium, humerus and femur, which represent the largest components of the feeding and locomotor systems of these animals. Rodents were chosen for comparison as they are of a similar size to birds and also maintain relatively high metabolic rates. Bats were studied because outside birds they are the only vertebrates capable of generating powered flight, and bats too are purported to have lightweight skeletons. To measure bone density, Dumont weighed the bones to find their mass and measured the bone volume using helium displacement, which is like using water displacement, except that helium molecules are considerably smaller than water molecules and so can infiltrate much smaller spaces in bone, leading to better density estimates.It turns out that humeral density is not distinguishable among the groups, averaging just above 2.0 g cm–3 in birds, rodents and bats. Femoral density is lowest in birds, near 1.8 g cm–3; however, cranium density is highest in birds, averaging nearly 2.2 g cm–3 (for comparison, rodent crania have densities averaging below 1.8 g cm–3). In short, different bones within and across major vertebrate clades vary with respect to density, a fact that may not raise many eyebrows. What is surprising though is that average bone density, which accounts for femora, humeri and crania and is weighted to reflect the relative contribution each bone makes to the total mass of its respective skeleton, is highest in birds and lowest in rodents. Flying animals appear to have denser bones than terrestrial counterparts of similar size.In bones, differences in density generally mean differences in mineral content. Higher mineral contents are linked to greater stiffness and strength. Dumont thinks that such material properties may be central to understanding why bone density is relatively high in birds and bats. Along with flight, birds and bats have also evolved small body size, and one way to design a small skeleton while preserving its strength is to make its bones more dense. Investigating links between the material properties of bone and the evolution of flight and small body size in birds and bats is going to require much more work – if only we could get bone density measurements from transitional forms in the fossil record like Archaeopteryx. In any case, the next time you happen to handle a bird skull, don't worry so much about it breaking. What may appear to be a fragile, lightweight structure, is likely both heavier and stronger than you think!