Intra-skeletal variability in phosphate oxygen isotope composition reveals regional heterothermies in marine vertebrates

Strategies used by marine vertebrates to regulate their body temperature can result in local variations, and the knowledge of these regional heterothermies is crucial for better understanding the thermophysiologies of extant and extinct organisms. In order to investigate regional heterothermies in vertebrates, we analysed the oxygen isotope composition of phosphatic skeletal elements (delta O-18(p)) of two endothermic fishes (Thunnus thynnus and Xiphias gladius) and three dolphins (two Delphinus delphis delphis and one Cephalorhynchus commersonii kerguelensis). We observed a consistent link between delta O-18(p) variations and temperature heterogeneities recorded by classical methods. Our delta O-18(p) data indicate that: (i) bone hydroxylapatite of the axial skeleton of dolphins mineralise at a warmer temperature than that of the appendicular one, (ii) the skull is the warmest body region in X. gladius, and (iii) T. thynnus possesses high body temperature in the skull and visceral mass region. These results demonstrate the possibility of tracking regional heterothermies in extant marine vertebrates using the delta O-18(p), paving the way to direct assessment of thermophysiological specificities of both living and extinct vertebrates. From a palaeoenvironmental perspective, the significant observed delta O-18(p) variability questions the use of some taxa or random skeletal elements for the reconstruction of palaeoceanographic parameters such as seawater temperature and delta O-18.