Dolphins have been observed swimming in inshore tropical waters as warm as 36-38°C. A simple protocol that mimicked the thermal conditions encountered by a dolphin moving from cool pelagic to warm inshore water was used to determine how dolphins avoid hyperthermia in water temperatures (T_w) at and above their normal core temperature (T_c). T_w (2 sites), rectal temperature (T_re; 3 depths), and skin temperature (T_sk; 7 sites) and rate of heat flow (4-5 sites) between the skin and the environment were measured while the dolphin rested in a chamber during a 30-min baseline and 40-60 min while water was warmed at ~0.43°C/min until temperatures of 34-36°C were attained. Instead of the expected increase, T_re consistently showed declines during the warming ramp, sometimes by amounts that were remarkable both in their magnitude (1.35°C) and rapidity (8-15 min). The reduction in T_re occurred even while heat loss to the environment was prevented by continued controlled warming of the water that kept T_w slightly above T_sk and while metabolic heat production alone should have added 1.6-2°C/h to the T_c. This reduction in T_c could only be due to a massive redistribution of heat from the core to the blubber layer.

Tursiops truncatus; bottlenose dolphin; core temperature; heat flow; heat storage