ORIGINAL PAPER D. P. Noren Æ T. M. Williams Æ P. Berry Æ E. Butler Thermoregulation during swimming and diving in bottlenose dolphins, Tursiops truncatus Accepted: 4 January 1999 Abstract Heat transfer from the periphery is an important thermoregulatory response in exercising mammals. However, when marine mammals submerge, peripheral vasoconstriction associated with the dive re- sponse may preclude heat dissipation at depth. To determine the eects of exercise and diving on thermo- regulation in cetaceans, we measured heat flow and skin temperatures of bottlenose dolphins (Tursiops truncatus) trained to follow a boat and to dive to 15 m. The results demonstrated that skin temperatures usually remained within 1 °C of the water after all exercise levels. Heat flow from peripheral sites (dorsal fin and flukes) in- creased over resting values immediately after exercise at the water surface and remained elevated for up to 20 min. However, post-exercise values for heat flow from the flukes and dorsal fin decreased by 30–67% when dolphins stationed at 15 m below the surface. The pattern in heat flow was reversed during ascent. For example, mean heat flow from the flukes measured at 5 m depth, 40.10  2.47 W Æ m )2 , increased by 103.2% upon ascent. There is some flexibility in the balance between thermal and diving responses of dolphins. During high heat loads, heat transfer may momentarily increase during submergence. However, the majority of excess heat in dolphins appears to be dissipated upon resurfacing, thereby preserving the oxygen-conserving benefits of the dive response. Key words Thermoregulation Æ Diving Æ Exercise Æ Dolphin Æ Heat flow Abbreviations T a ambient air temperature Æ T s skin temperature Æ T w ambient water temperature Æ RPM revolutions per minute Introduction The high thermal conductivity and heat capacity of water poses a unique thermoregulatory challenge for active marine mammals that diers from that of terres- trial mammals. During exercise, terrestrial mammals increase both heat production and muscle blood flow (Mitchell 1977). Excess heat generated by the active muscles during exercise may be dissipated by increased blood flow through dilated vessels of the skin (Berger 1982; Franklin et al. 1993; Kellogg et al. 1993). Ulti- mately, the heat is transferred to the surrounding envi- ronment by four dierent pathways, conduction, convection, radiation, and/or evaporation (Mitchell 1977; Berger 1982; Brooks and Fahey 1984). In com- parison, the primary thermal adaptation in phocid seals and cetaceans is a thick blubber layer that insulates against heat loss to the water (Kanwisher and Sundnes 1966; Irving 1969; Ryg et al. 1993). Poorly insulated peripheral areas are utilized as thermal windows which permits the transfer of excess heat via conduction and convection during exercise or when ambient water is warm (McGinnis et al. 1972; Kanwisher and Ridgway 1983). To limit heat transfer at the peripheral sites, cetace- ans maintain a counter-current arrangement of vessels in their fins and flukes (Scholander and Schevill 1955; Hampton and Whittow 1976). During periods of ele- vated activity, however, the counter-current heat ex- changer is bypassed and blood flow through superficial veins near the skin increases to provide maximum cooling (Scholander and Schevill 1955). Cetaceans take advantage of cooled blood from these peripheral sites to J Comp Physiol B (1999) 169: 93–99 Ó Springer-Verlag 1999 D.P. Noren (&) Departments of Marine Sciences and Biology, Earth and Marine Sciences Building, Room A316, University of California, Santa Cruz, California 95064, USA e-mail: dnoren@biology.ucsc.edu, Tel.: +1 831-459-4133, Fax: +1 831-459-5353 T.M. Williams Department of Biology, University of California, Santa Cruz, California, USA P. Berry Æ E. Butler The Dolphin Experience, Port Lucaya, Grand Bahama Island, The Bahamas