Cetaceans Humerus Radiodensity by CT: A Useful Technique Differentiating between Species, Ecophysiology, and Age

SIMPLE SUMMARY: Cetaceans are marine mammals whose tissues have undergone an evolutionary process to adapt to life in the aquatic medium. In addition, depending on the species, the adaptations will have developed differently. However, some features of the bones of these animals are still poorly studied, and a good technique for analysing this tissue is computed tomography. With this, it is possible to measure bone density, compare bone density between distinct species, and then relate it with the ecophysiological aspects of the animal. For this reason, this study aimed to create a protocol to measure the bone radiodensity of the humerus of cetaceans through computer tomography. The purpose was to be able to make comparisons between different species to advance the understanding of this cetacean tissue. This project considered two species with different characteristics and behaviours: the Atlantic spotted dolphin (Stenella frontalis) and the pygmy sperm whale (Kogia breviceps). The main results observed were significant differences in bone density between different types of bone (trabecular and cortical bone) and various portions of the humerus within a given species. Such differences were also observed between species with varying habits of diving (shallow divers and deep divers) and distinct swimming habits (swimming style with low-medium energy expenditure and swimming style that requires medium-high energy consumption rates). These results allow us to consider the unique adaptations of this tissue to adapt to varying lifestyles of these animals in the aquatic ecosystem. Therefore, this technique will allow for comparisons to distinguish different characteristics related to species, age, diving behaviour, and swimming style. ABSTRACT: Cetaceans are mammals that underwent a series of evolutionary adaptations to live in the aquatic environment, including morphological modifications of various anatomical structures of the skeleton and their bone mineral density (BMD); there are few studies on the latter. BMD is related to the radiodensity measured through computed tomography (CT) in Hounsfield units (HU). This work aimed to test and validate the usefulness of studying humeral bone radiodensity by CT of two cetacean species (the Atlantic spotted dolphin and the pygmy sperm whale) with different swimming and diving habits. The radiodensity was analysed at certain levels following a new protocol based on a review of previous studies. Humeral radiodensity values were related to four aspects: species, diving behaviour, swimming activity level, and age. We observed that the consistent differences in the radiodensity of the cortical bone of the distal epiphysis between animals of different life-history categories suggest that this bone portion could be particularly useful for future ontogenetic studies. Hence, this technique may be helpful in studying and comparing species with different ecophysiologies, particularly distinguishing between swimming and diving habits.