Activation of the rodent synaptic nervous system by cold exposure or β3 agonist administration can induce UCP1 positive adipocytes within WAT in a process called “browning”. In humans, cold-activated brown adipocytes were observed primarily in the cervical, supraclavicular and paravertebral regions using integrated positron emission tomography-computed tomography and 18F-labelled glucose analogue, fluorodeoxyglucose, as a tracer. Although these “brown adipocytes,” referred to as inducible brown fat cells, “brown in white” adipocytes, or beige cells, also exhibit UCP1 positive activity, their lineage is distinct from that of classical brown adipocytes, and their origins are different : brown adipocytes are differentiated from Myf5 positive cells, precursors to skeletal muscle, whereas beige cells are derived from Myf5 negative cells. It is thought that beige cells could be transdifferentiated from white adipocytes. Aquatic mammals such as cetaceans, pinnipeds and sirenians have blubber, a thick subcutaneous fatty deposit that contributes to the storage of metabolic energy, hydrodynamics, positive buoyancy and thermal insulation. However, despite fat being a poor thermal conductor, blubber alone is unlikely to adequately insulate a cetacean against heat loss in cold conditions. Accordingly, alternate means of thermoregulation might be required or advantageous for aquatic mammals. We hypothesize the existence of brown or brite/beige adipocytes in cetacean blubber, and propose these cells enable cetaceans to adapt to cold environments. This hypothesis was tested by investigating the expression of adipocytes and UCP1 in blubber samples collected from four delphinoid taxa occurring in the Pacific Ocean: the Pacific white-sided and bottlenose dolphins, Lagenorhynchus obliquidens and Tursiops truncatus, and Dall’s and harbour porpoises, Phocoenoides dalli and Phocoena phocoena. Our study reveals that BAT occurs within a layer of blubber and connective tissue that extends almost the entire length of the delphinoid body, possibly enabling these GDC-0941 animals to withstand temperatures below those blubber as an insulator alone might otherwise allow, or enabling animals to maintain body temperature in cool waters during periods of physical inactivity. It has been assumed that cetaceans swim continuously, even during their sleep, to maintain body temperature. However, long periods of inactivity have been reported for captive bottlenose dolphins, the body temperature of which correlated negatively with water temperature and positively with serum noradrenalin and adrenalin concentration. How these animals regulated their body temperature in cool waters during periods of prolonged inactivity was a mystery. Our thesis was that thermogenesis by BAT occurred among these dolphins during these inactive periods, though until now BAT was unknown from cetaceans. We detected UCP1 expression in the blubber of two species each of dolphin and porpoise using a combination of techniques, and showed it to be distributed the length of the body, with the exception of the rostrum.