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°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 (Tw) at and above their normal core temperature (Tc). Tw (2 sites), rectal temperature (Tre; 3 depths), and skin temperature (Tsk; 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, Treconsistently 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 Tre occurred even while heat loss to the environment was prevented by continued controlled warming of the water that kept Tw slightly above Tsk and while metabolic heat production alone should have added 1.6-2°C/h to the Tc. This reduction in Tc could only be due to a massive redistribution of heat from the core to the blubber layer.