How dolphins use their blubber to avoid heat stress during encounters with warm water

Dolphins have been observed swimming in inshore tropical waters as warm as 36-38 degrees 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 similar to 0.43 degrees C/min until temperatures of 34-36 degrees 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 degrees 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 degrees 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.