The morpho-functional evolution of bird beaks and skulls

Lead Research Organisation: University of Hull
Department Name: Hull York Medical School

Abstract

It is widely described how certain bird beaks and skulls are modified and adapted for feeding on distinct and different foodstuffs. Darwin's finches are such a classic example that to cite them in this context is almost a cliché, yet they illustrate the current level of research in this area quite effectively. Despite decades of detailed ecological and more recently developmental studies, it was only earlier this year that a study finally began to quantify how different shaped finch skulls and beaks generated different bite forces and resistance to fracture - features which dictate which foodstuffs and environments these birds are capable of exploiting. This is probably a good example of how much we think we know about form, function and diversity of birds and their skulls, compared to how much quantitative analysis has actually been performed in this area. We do not know a huge deal about how the shapes of bird beaks and skulls influence their function and feeding ecology, beyond just linking different shapes to different environments. This is despite the fact that cranial and beak morphology contributes to a significant amount of avian diversity. For example, do all birds that feed on carrion, or large seeds, have skulls and beaks that are shaped the same, and function in the same way? Do they bend and stress in a similar manner, and generate equivalent amounts of bite force? And to function in the same manner, do they indeed need to be the same size and shape? Furthermore, because most members of the group can fly, birds are often assumed to be a classic example of animals that are constrained in the number of different body forms they can adopt by the need to reduce weight (mass). Is this indeed the case? Are there unrealised shapes of skulls and beaks that could do the job better? And why do we not see beaks and skulls shaped in this manner in living animals? This study sets out to address these questions, and what this means for musculoskeletal evolutin and diversity in birds. Our team has the expertise to employ a range of methodologies to quantify variation in form and biomechanical function in skulls, and link this to new tools that we have pioneered to interrogate the function of hypothetical or ancestral morphologies. Between us we have the technical ability, experience and knowledge of the animals concerned to quantitatively determine the patterns of skull and beak shape variation in selected groups of birds that actively process their food. We will then use our strong background in biomechanical and engineering analysis of skulls to determine functional variation in feeding biomechanics, and whether birds that share similar feeding ecologies are have similar shaped beaks and skulls, and function in the same manner. We would predict this to be the case if there were a tight link between form and function and convergent or parallel evolution were at play. We will also test what role shared ancestry plays in determining the range of different shapes and functions we see. Using our recently developed software tools we will find out if hypothetical or ancestral skull and beak shapes were better at resisting feeding loads than those possessed by our living bird groups. Finally, we will determine to what extent changes in the shape of the beak are contingent on changes to the shape of the braincase, and whether a loose relationship between beak and braincase shape can help birds develop more diverse skull morphologies. Researchers from a variety of disciplines will benefit from this work - comparative anatomists, biomechanists, evolutionary and developmental biologists, palaeontologists, and biomedical engineers. There will be benefits to the UK science base through multidiscplinary training of young scientists and overseas EU collaboration. The visual aspect of this work, and the focus on birds, is likely to appeal to the general public, offering public engagement opportunities and media interest.

Technical Summary

Decoding patterns of morphological variation and evolution of convergent form are essential to understanding diversity in past and present ecosystems. Bird skulls and beaks are seen as a classic example of morphological convergence towards similar feeding ecologies, yet surprisingly little work has attempted to quantify the nature and extent of their craniofacial homoplasy. This is especially surprising given the potential for birds to inform on evolutionary changes within an aerial environment supposed to impose severe constraints on craniofacial form. How facial complexity translates to functional complexity is also little understood, as is the degree of integration between braincase and rostra. Whether the morphologies presented in extant taxa are close to the optimal construction or 'design' for a particular function, is also largely unknown (and a moot point in itself), and could be contingent on historical or ahistorical constraints. This proposal seeks to address these issues, and seeks to quantify the nature of morphological and functional diversity in avian taxa that have previously been cited as exemplars of convergent morphological evolution. We will use a combination of geometric morphometrics, biomechanical and structural engineering analysis (e.g. finite element analysis), underpinned by phylogenetic comparative methods and statistical analysis, and coupled with newly developed state-of-the-art software that permits the functional interrogation of occupied and unoccipued regions of morphospace. The results of this study will represent a major advance in our understanding of how morphological diversity translates and equates to functional and ecological diversity. It will uncover how supposed constraints imposed by an aerial existence manifest in morphofunctional variation and homoplasy, and how modularity and integration may influence morphological and functional, disparity.

Planned Impact

Who will benefit and how will they benefit from this research? ACADEMIC COMMUNITY. The primary beneficiaries of this research will be our immediate academic community in functional morphology, feeding biomechanics and palaeontology. This research will also be of benefit to other closely allied academic groups who have interests in: integrating developmental biology and ecology; vertebrate feeding ecology; and evolutionary biology and ecology. WIDER COMMUNITY. More generically the underpinning methodologies used and furthered in this project are of benefit to a wider academic circle. The computational modelling approaches developed and used by the investigators in this project fit well with the RCUK 3Rs strategy (replacement, refinement and reduction) for animal experimentation. These modelling techniques provide an approach for modelling feeding behaviour in rare and endangered animals for which invasive experimental techniques are undesirable or inappropriate. The dissemination of these simulation methods and the integration of geometric morphometrics statistical tools to simulations, as employed in this project, will be of interest to groups working on mechanical simulation in a wide variety of systems and taxa which have a wide variety of impacts including health and animal welfare. GENERAL PUBLIC. This project will also generate interest from and impact upon the general public. Members of the bird clade are one of the most keenly studied groups by the general public; the RSPB alone has over 1 million members in the UK. Public interest centres around bird biodiversity and ecology especially in relation to the depletion of natural environments and climatic change, both of these are key areas which this project will impact. The avian beak has provided a classic and accessible example of adaptation to a very wide audience, from the specialist and public alike, in textbooks, popular science literature and television documentaries. Yet the actual research underpinning understanding of such a widely disseminated evolutionary example has been surprisingly limited. With regard to cultural impact, therefore, this project will raise interest and continue to inform the public about evolution; providing an engaging, and accessible example with which to understanding an organism's place in nature and its relationship to the environment. Measures taken to ensure that beneficiaries have the opportunity to benefit from this research DISSEMINATION OF ACADEMIC RESULTS. We will continue to publish in high impact journals (Nature, Proceedings of the Royal Society, Biology Letters) and in high impact but specialised journals (Journal of Biomechanics, Journal of Anatomy, Paleobiology, Evolution and Development) to ensure that our research targets those who will benefit the most and the widest possible audience. We have a good track record at promoting our research through conference presentations, including keynote lectures and organised symposia. We outline in our impact plan how we will run a further workshop in the mould of the very successful on Craniofacial Biomechanics, run by Cobb and part sponsored by the Anatomical Society in 2009. By contributing to website development we will ensure that the visual nature of our research is promoted but also accessible online. DISSEMINATION TO GENERAL PUBLIC. We will continue the strong outreach programmes in operation in Bristol and Hull-York. Recently our research has been promoted through widening participation programmes with local schools and continuing education access courses (Bristol). The Palaeobiology and Biodiversity Research Group (Bristol) has recently employed a dedicated outreach officer for three years through Heritage Lottery Funded support, who facilitates, amongst other things, our continuing participation in community events such as the Bristol Festival of Nature. Rayfield is about to take on the role of Publicity Officer for the Palaeontological Association.

Publications

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Bright JA (2016) The shapes of bird beaks are highly controlled by nondietary factors. in Proceedings of the National Academy of Sciences of the United States of America

 
Description So far, we have collected 3D landmark data from skulls of birds of prey and parrots. Geometric morphometric analysis of raptor data shows that carrion feeders (the Old and New World vultures) tend to cluster together in morphospace regardless of phylogeny, indicating strong morphological as well as dietary convergence. However, despite obvious dietary convergences, Falconids plot separately to all other Accipitrids. Thus it seems that although dietary niche may be predicted based on cranial morphology in some families, ecology alone is insufficient to explain the variety of forms seen in the diurnal birds of prey. This may reflect the fact that many raptors hunt and kill with
the talons not the beak, meaning that talon morphology may additionally predict dietary ecology. Further functional analysis of the range of talon and beak forms will aim to test this.
Further to this we have discovered that allometry (size) plays a key role in determining skull form and feeding ecology. Raptors and parrots simply get big in order to eat different things. We also found that the beak and braincase are tightly integrated modules, meaning that a change in beak morphology is accompanied by a predictable change in braincase morphology. This is completed counter to initial predictions and the text book story that birds can modify their beaks to adapt to different feeding behaviours without impacting the remainder of the skull.
Exploitation Route The research will be beneficial to academic working in this field and beyond as we have discovered some fundamental information about the construction and evolution of the skull of birds. Further work will be undertaken by a PhD student, funded by the University of Bristol, who will explore whether phenomena found in raptors and parrots are applicable to other avian clades and how deep into the evolutionary history of birds does this extend.
Sectors Environment