title: The Surrogate Arm: Functional Morphology of the Avian Cervical Column
Creators: name: Marek, Ryan
Publisher: name: University of Liverpool Repository
Year: 2020

description: The avian neck allows the head to perform a variety of tasks that would be carried out by the forelimbs in other vertebrates, as the forelimbs are primarily adapted for flight in birds. This has created a strong additional selection pressure on the cervical column and has resulted in the evolution of a vast array of neck morphologies throughout extant birds. This diversity is most evident in the large variation observed in vertebral counts of the neck, and the lack of known homology between species with differing cervical counts has hindered research into morphological variation of the avian cervical spine. The lack of quantitative assessment of this variation has stunted our understanding of how the neck became such an integral component of avian biology. Recent work has shown that Hox gene patterning is conserved within the neck across Aves and that five cervical regions exist within the avian spine, and homologous interspecific comparisons can now be made by comparing aspects of regional morphology. Iterating on previous work, this thesis uses 3D geometric morphometrics as a proxy to delineate and analyse these cervical regions within extant birds. Here I use a holistic approach to understand functional and ecological drivers in avian neck shape and length in a diverse selection of 52 specimens (46 species) of modern birds by combining three-dimensional geometric morphometrics with multivariate statistics and quantitative dissection as part of the following four objectives. The first objective sought to determine if patterns of cervical regionalisation exist within extant Aves by assessing the variation in regional vertebral counts and by using Phenotypic Trajectory Analysis to examine factors that affect morphological variation across the entire cervical spine. Results indicated that increases to cervical counts are not responsible for neck elongation in birds and that few ecological factors have a significant correlation with regional vertebral counts. Vertebral morphology across the entire cervical column only displays significant variation in birds with extreme cervical kinematics, such as carnivores. The second objective was to quantify the relationship between neck length, head mass, and body mass across extant birds. Neck length and head mass scale according to isometry. The relationship between neck length and head mass is also isometric and this indicates that neck length in birds is not as tightly constrained by head mass when compared to other vertebrates. Relative neck lengths are shorter in birds that locomote using powered flight and head mass is lower in terrestrial birds. This may indicate that powered flight is a constraint on cervical morphological variation. The third objective was to quantify factors affecting variation in the morphology and length of individual regions. Region lengths strongly correlated with neck length and this suggests that avian neck elongation is a product of increases to vertebral length rather than increases to vertebral counts, and that regions 2 and 5 are responsible for this elongation. Body mass and neck length were revealed to have the strongest correlation with morphological variation in regions across 4 out of 5 cervical regions. Only ecological factors associated with specialised cervical kinematics, i.e. adaptations to carnivory or aquatic foraging, displayed significant variation in regional vertebral morphology. The final objective was to quantify variation in avian muscle architecture and I found that muscle architecture and mass scaled predominantly according to positive allometry. Ecology has a weak correlation with muscle architecture and mass variation and may accommodate fast head strikes and adaptations to underwater foraging in certain taxa. Cranial muscles that support the head appear to be more morpho-functionally restricted, whilst caudal muscles display much more variation in architecture and mass. This thesis has shown that across multiple aspects of morphology, the avian cervical column is a generalised musculoskeletal system, only adapting to specialised patterns of cervical kinematics. The avian cervical column is often referred to as a surrogate arm due to its role in positioning the head to manipulate the environment in the stead of the forelimbs which are heavily adapted for flight and the generalised pattern of morphological variation recovered here supports the hypothesis that the avian neck is a surrogate arm. Future work is needed to be done in the wake of this thesis to ensure that more birds are sampled for their variation in muscle architecture, as well as undertaking projects that seek to quantify variation in avian cervical kinematics and patterns of intervertebral flexion.