Abstract. 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 (δ18Op) 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 δ18Op variations and temperature
heterogeneities recorded by classical methods. Our δ18Op
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 δ18Op, paving the way to
direct assessment of thermophysiological specificities of both living and
extinct vertebrates. From a palaeoenvironmental perspective, the significant
observed δ18Op variability questions the use of some taxa
or random skeletal elements for the reconstruction of palaeoceanographic
parameters such as seawater temperature and δ18O.