What is a cetacean's body temperature?

A bowhead whale dives into the Arctic Ocean at -1.8 °C, the temperature at which seawater begins to freeze. A common bottlenose dolphin hunts at 28 °C in the Caribbean. A sperm whale rises from 1,200 metres down, where the temperature approaches 4 °C, to tropical surface waters at 25 °C. For these animals, regulating body temperature poses an extraordinary challenge. Water dissipates heat about 25 times faster than air, and an endothermic mammal would quickly lose its heat there without some means of insulation. The evolutionary solution is elegant, and combines two main mechanisms: massive passive insulation and a countercurrent vascular exchange system.

A remarkably constant temperature

Direct measurements of internal temperature in living cetaceans are relatively rare, since they require a degree of access to the animal that is far from trivial. But the available data, obtained mainly from animals in captivity or from carcasses shortly after death, converge: the internal body temperature of cetaceans lies around 36 to 37 °C, within a range comparable to that of land mammals and of humans.

This remarkable homeothermy is maintained despite extremely varied external conditions. A tropical common bottlenose dolphin and an Arctic narwhal share the same internal temperature. What makes such constancy possible is the evolution of a particularly efficient system of insulation and thermal regulation.

Blubber: the most efficient insulator in the animal kingdom

The first line of thermal defence is blubber, a layer of dense adipose tissue lying directly beneath the skin. Its thickness varies with species and habitat: a few centimetres in small tropical dolphins, up to 50 centimetres in the Arctic bowhead whale. Blubber is exceptional in several respects.

First, its composition: it contains an exceptionally high proportion of lipids (up to 90% by dry weight in some layers), far more than the subcutaneous fat of land mammals. This lipid density makes it a remarkable thermal insulator, since lipids are poor conductors of heat.

Next, its structure: it is not a simple fatty mass but an organised tissue, with collagen fibres arranged in a precise pattern. This fibrous structure has two important secondary functions. It gives blubber a certain elasticity, which could return elastic energy during the beats of the tail fluke (in fast cetaceans such as the killer whale or the sei whale). And it prevents excessive deformation during dives at high pressure.

Finally, blubber also serves as an energy store and as a float. Large migratory whales hold in this layer most of the energy needed for their annual journeys without feeding. Since blubber is slightly less dense than water, it also contributes to buoyancy.

The fins: a special problem

Blubber covers the whole body mass of cetaceans, except in one critical place: the fins. Flippers, dorsal fin and tail fluke need to be thin and flexible for their propulsive and control functions. Tens of centimetres of fat cannot be packed into them without compromising their hydrodynamic efficiency. The consequence is that the fins are poorly insulated. In cold water they would be a major route of heat loss without some compensating arrangement.

That arrangement is the rete mirabile: a countercurrent exchange system between arteries and veins (see our dedicated page). Warm blood descending into the fin through the arteries passes cold blood returning through the veins, and heat moves from one flow to the other by conduction. The result is that arterial blood reaches the tip of the fin already cooled, and loses little heat to the surrounding water. Venous blood reaches the trunk already rewarmed, and does not cool the core of the body.

The balance is surprisingly favourable: on current estimates, less than 5% of the heat produced by core metabolism is lost through the fins. It is exactly the same principle that allows emperor penguins to stand on the ice for hours, or herons to hold their legs in cold water.

Cooling a tropical cetacean

The thermal problem of tropical cetaceans is in fact the opposite: in warm water their metabolism produces more heat than is needed, and the excess has to be shed. The rete mirabile can then work in reverse: the vascular sphincters open wide, short-circuiting the countercurrent exchange. Warm blood goes straight to the skin to cool. The fins then become radiators.

This is why, in warm water or after intense hunting, cetaceans are sometimes seen beating their flippers repeatedly at the surface, or “resting” with the fins spread wide. These postures, long read as social or playful behaviour, are also, perhaps chiefly, strategies of active heat dissipation.

The newborn’s challenge

At birth, a calf has very little blubber. This is one of the main reasons why large whales migrate to tropical waters to give birth, see our page on migration. In those waters at 25 °C, the calf can survive without spending all its energy fighting the cold, and can quickly build a layer of blubber thanks to the extraordinarily fatty milk (30-50% lipid) that its mother passes to it, see our page on birth.

A calf born by accident in cold water, a rare but observed event, by contrast usually dies of hypothermia within a few hours. The synchronisation of migration and breeding is no coincidence: it is vital to the survival of the species.

Metabolism and endurance

A question sometimes raised: do cetaceans have a particularly high metabolism to offset their heat losses? The answer is no, on the whole. The basal metabolism of cetaceans is broadly comparable to that of land mammals of the same body mass. It is passive insulation (blubber) and heat recycling (rete mirabile) that do most of the work. That efficiency is crucial: it allows migratory baleen whales to fast for months without energetic collapse, and the bowhead whale to reach the longest lifespans in the mammal kingdom (more than 200 years documented).

Sources

• Pabst, D.A., Rommel, S.A. & McLellan, W.A. (1999). “The functional morphology of marine mammals”. In Reynolds & Rommel (eds.), Biology of Marine Mammals, Smithsonian.
• Iverson, S.J. (2009). “Blubber”. In Perrin et al. (eds.), Encyclopedia of Marine Mammals.
• Costa, D.P. & Williams, T.M. (1999). “Marine mammal energetics”. In Biology of Marine Mammals.
• Ponganis, P.J. (2015). Diving Physiology of Marine Mammals and Seabirds. Cambridge University Press.
• Kasting, N.W., Adderley, S.A.L., Safford, T. & Hewlett, K.G. (1989). “Thermoregulation in beluga (Delphinapterus leucas) and killer (Orcinus orca) whales”. Physiological Zoology, 62.

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