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Compared and functional morphology of the hoatzin (Opisthocomus hoazin)

Abstract : The hoatzin (Opisthocomus hoazin, Muller, 1776) is the unique extant species of the Opisthocomiformes. It is unquestionably among the most bizarre and enigmatic bird species in terms of its appearance, life history, morphological specializations and physiology. The hoatzin juveniles have exceptional swimming and climbing abilities thanks to fully functional claws on the wing. It is moreover the only folivorous bird with a pre-gastric fermentation, as found in mammals, with a hypertrophied crop. This dietary specialization has an important impact on its bone shape, such as a reduced carina and a sternum fused to the furcula and to the coracoids. Several authors linked these morphological changes to functional implications such as reduced flight abilities. Since the first partial descriptions in the early 20th, the whole skeleton of the hoatzin has not been described to date. Its anatomy remains consequently only partially known. Taking advantage of the recent 3D techniques allowing us to get access to part of the anatomy that can be difficult to describe based on classic dissections, a complete monography on the skeletal anatomy was realized. After the description of the whole skeleton and functional inferences, highlighting the unique characters of the hoatzin compared to other birds became possible by means of previously published comparative data on skeletal anatomy in birds. It appears that the scapular girdle of the hoatzin has a lot of morphological particularities and thus needs further comparative analyses. Using 3D techniques, geometric morphometric methods and collection specimens, we quantitatively investigated evolutionary changes in the morphology of the scapular girdle of birds. Nevertheless, before working on a large comparative dataset of the scapular bones, the possible impact of preparation effects on the shape of bones from osteological collection should be quantified. Indeed, several authors have described the effect of preparation processes on bones. This study allowed us to assess the impact of bones preparation process on the shape of each bone of the scapula girdle using both intra- and inter-specific datasets of birds. Our results have shown that preparation effects can be well estimated using the appearance of the bone and this proxy was used to collect data for further comparative analyses on the scapular girdle of birds.Literature of the hoatzin refers to it as a poor flier with non-agile flight because of its sternum shape modification providing only a small area for insertion of the flight muscles. But this hypothesis of a direct link between modified sternum shape and flight capacity has never been tested. Using a comparative dataset composed of fifty-eight species for which the locomotor behavior is well known, shape differences for each bone of the scapular girdle depending on flight type were tested. Sternum, coracoids, scapulae and mainly the humeri have significantly different shapes depending on flight type. These results were used to infer the flight type of the hoatzin based on the shape of its scapular bones. Assignation tests performed on scapular bone shapes of the hoatzin showed that nearly all its bones are classified among gliding species, except for the scapula. It appears that the unique sternum shape of the hoatzin is not the sole reason for its poor flying abilities. Moreover, these comparative studies were supplemented with an anatomical and comparative description of a developmental series of hoatzin specimens. This developmental analysis allowed us to determine that sternum shape is determined in the early development whereas the complete fusion of the sternum complex happens in later in juveniles. The use of a comparative dataset and 3D geometric morphometric methods allowed us to generate quantitative data on the morphological specificities of the hoatzin. This work provides a preliminary answer to the role of its sternum shape and reduced carina in its flight abilities.
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Fanny Pagès. Compared and functional morphology of the hoatzin (Opisthocomus hoazin). Tissues and Organs [q-bio.TO]. Museum national d'histoire naturelle - MNHN PARIS, 2019. English. ⟨NNT : 2019MNHN0013⟩. ⟨tel-02929631⟩

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