Cetacean’s Humerus bone radiodensity: variations between different families and diving behaviour

Abstract

Diagnostic imaging is often used to study bone tissue as it provides great morphological details including the possibility of radiodensity studies. Radiodensity is highly variable based on the species, the bone's function, the animal's age and the abiotic environment it is living in. The goal of this study was to determine the radiodensity of 11 cetacean species belonging to the Delphinidae, Physeteridae, Kogiidae, and Ziphiidae families and to explore if differences in bone radiodensity were better explained by the phylogenetic origin of species or the diving behaviour. The humerus body cortical and trabecular radiodensity in adult odontocetes (n=56) were measured following the methodology published by Consoli et al. in 2021. Species were grouped into shallow (<500 m depth) and deep (>500 m depth) divers. In order to discriminate between groups a Receiver Operating Characteristic (ROC) analysis was performed. The discriminant capacity of each marker was assessed as the area under ROC curves and then models for predicting the family or diving behaviour were obtained using the classification and regression trees procedure. As in terrestrial mammals, the cortex had a higher radiodensity than the trabecular bone. Following the Consoli’s et al. protocol, the humerus was divided into three sections, and it was found that the most distal section, was the most suitable for discriminating the species. Shallow diving species presented a higher cortical and trabecular bone radiodensity. However, the classification tree predicting the family had fewer nodes, and less error, with 53 out of 55 (96.4%) species classified correctly than the classification tree predicting the diving group, suggesting that the phylogeny had a higher impact on bone tissue radiodensity than the diving behaviour alone. More importantly, It has been established the normal radiodensity of the bone humerus of 11 species. Normal values are necessary to identify radiodensity anomalies (i.e., lesions).