Whales, like all other cetaceans, do not sweat because they lack the necessary sweat glands. Managing a core body temperature of around 37°C while surrounded by water is complex, given that water removes heat from the body approximately 25 times faster than air. This difference means that the cooling strategies used by land mammals would be ineffective and dangerous for a whale.
Absence of Sweat Glands in Whales
Whales do not sweat because the mechanism of evaporative cooling, which relies on sweat turning into vapor, is pointless in water. Terrestrial mammals possess eccrine and apocrine glands, but these structures are vestigial or absent in whales. Even if sweat were produced, the surrounding water would immediately wash it away, preventing the necessary evaporation.
The high thermal conductivity of water makes evaporative cooling a liability. Attempting to lower skin temperature through sweating would cause rapid, uncontrolled heat loss, risking hypothermia. The lack of sweat glands is an evolutionary adaptation to prevent catastrophic heat dissipation in a habitat that constantly draws warmth away from the body. Their skin surface is instead cooled to nearly the same temperature as the surrounding water, which reduces the thermal gradient and minimizes heat loss through conduction.
Blubber and Heat Retention
To counteract the constant heat drain of the ocean, the primary defense is blubber, a thick layer of specialized adipose tissue beneath the skin. This layer consists of lipid-rich cells interlaced with collagen and elastic fibers, serving as a highly effective thermal insulator. Unlike the fur of other marine mammals, blubber retains its insulating properties even during deep dives because it does not rely on trapping air.
Blubber thickness varies significantly by species and season, acting as both insulation and an energy reserve. Large baleen whales, such as the bowhead and right whale, can develop blubber up to 50 centimeters thick after feeding in polar waters. This fat tissue has a low thermal conductivity (0.10 to 0.25 W·m⁻¹·K⁻¹ in some species), efficiently minimizing heat transfer from the core to the environment. The blubber layer also contributes to buoyancy, aiding in movement through the water column.
Specialized Cooling Mechanisms
While blubber retains heat, whales must shed excess warmth generated by intense activity, such as chasing prey or migrating through warmer waters. Active cooling is accomplished through specialized areas called “thermal windows.” These areas are highly vascularized and lack the thick insulating layer of blubber. They are primarily located in the appendages: the flukes, pectoral flippers, and dorsal fin.
When a whale needs to dissipate heat, vasodilation occurs. Blood vessels near the surface of these appendages widen, allowing warm blood from the core to flow close to the skin. Heat is then transferred directly to the cooler surrounding water. The appendages are thin and possess a large surface area compared to the main body, making them ideal radiators for thermal regulation.
This cooling process works in concert with a sophisticated system called countercurrent heat exchange (CCH), concentrated in the vascular networks of the appendages and sometimes the tongue. Within the flukes and flippers, arteries carrying warm blood from the core are situated adjacent to the veins returning cooler blood from the periphery. Heat transfers from the arterial blood to the venous blood before it reaches the extremity’s surface, which pre-warms the returning blood and conserves core heat.
The CCH system is dynamic and can be modulated to dump heat when necessary. By shunting blood directly to veins near the skin’s surface, or by increasing overall blood flow through vasodilation, the whale temporarily bypasses the heat-conserving function of the CCH. This allows the whale to actively expel surplus heat into the water, performing the equivalent function of sweating without the risk of hypothermia. This precise control over blood flow represents the whale’s refined physiological strategy for maintaining a stable body temperature in a challenging aquatic environment.