Thermoregulation in Cetaceans

Humpback whale breaching

The challenge of keeping a constant temperature

As all mammals, cetaceans are endotherms (warm-blooded) – i.e. they have mechanisms to keep a constant body temperature which is generally higher than the temperature of the environment. Being endothermic presents both advantages and disadvantages. Since most metabolic processes need a high enough temperature to work properly, a warm-blooded animal has the advantage of being always “ready to go”.
On the other hand, keeping warm in a cold environment is very costly in terms of energy and endotherms need to invest a substantial fraction of their energy budget on it.
Since water absorbs heat 25 times faster than air, cetaceans have developed much specialised mechanisms to avoid massive heat loss.

  1. Increased insulation, via a thick layer of fat, called blubber
  2. Counter-current heat exchange systems
  3. Low surface-to-volume ratios
  4. High metabolic heat production

Blubber and rete mirabilis

The blubber acts as a very good insulator. It is so efficient at keeping cetaceans warm than it is actually more challenging for them to avoid overheating.
When a whale, dolphin or porpoise finds itself in warm waters or needs to cool down because of intensive exercise, warm blood simply flows from the body core to the extremities (flukes, flippers, dorsal fin). These parts of the body completely lack of blubber and are highly vascularised (rich of blood vessels) so that heat can be exchanged between the warm blood and the cold environment.
Is the blubber enough to allow whales and dolphins to thrive in cold, sometimes freezing waters? Since the extremities of a cetacean completely lack of blubber, they are very inefficient at keeping a constant body temperature and an extra mechanism is therefore needed in order to prevent a catastrophic heat loss from the tail (flukes), flippers (pectoral fins) and dorsal fin.
Evolution has provided cetaceans with a “miraculous” solution: the arteries that carry oxygen-rich blood to the extremities are closely surrounded by a network of veins that bring oxygen-depleted blood back to the body core. This network of veins, known as rete mirabilis (the Latin for “miraculous network”), plays a crucial role in the so-called counter-current heat exchange mechanism. In cold conditions, when warm blood flows through the arteries from the body core to the extremities, a substantial fraction of its heat is taken by the cold blood in the “rete mirabilis” rather than being dispersed in the environment, thus preventing heat loss.

Surface-to-volume ratio

Consider a warm, spherical object immersed in cold water. Given enough time, the sphere and the surrounding water will attain thermal equilibrium – i.e. they will reach a common, intermediate temperature. Heat exchange has occurred at the surface of the object, at a rate that depends on how large the object surface is. If you now double the diameter of your sphere, you will find that a longer time is needed to achieve thermal equilibrium. Even more important, you will find that the time needed to attain equilibrium has more than doubled. The explanation is easy: the amount of energy (heat) the sphere can transfer to the water depends on its volume (i.e. how large the inside is), while the rate at which heat can be transferred depends on its surface (the only part in contact with the water).
Since surface is a two-dimensional physical property and volume is a three-dimensional physical property, you will find different SURFACE/VOLUME values as you vary the linear dimensions of the object.
If you now run the same experiment on an object having the same volume of one of your spheres but a different shape, you will find that it loses heat faster. Indeed it can be mathematically demonstrated that a spherical shape is the one that minimises surface-to-volume ratios.

Two important points can now be made: For any given shape, a bigger object loses heat at a slower rate than a smaller object. For any given volume, the more an object approaches a spherical shape, the slower the rate at which heat loss occurs. It is now easy to understand why, generally speaking, cetaceans are large animals, with some of them being the biggest ever to live on Earth (e.g. the Blue whale and the Fin whale). It can also been observed that species that live in cold waters are bigger than those living in temperate/tropical environments. One could argue that their shape is not spherical, but they actually get closer to a spherical shape than most other animals. If they were spherical, it would be impossible for them to swim efficiently, since hydrodynamics require a different shaping to heat conservation. The shape of cetaceans is therefore a compromise between heat conservation requirements and streamlining.

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