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Conserving heat
The skin of all mammals (and therefore also the skin of cetaceans), consists of 3 distinct layers:
  1. Epidermis (Outer layer)
  2. Dermis (Middle layer)
  3. Hypodermis (Inner layer), also called blubber
Whales, dolphins and porpoises spend their entire life in the water, where heat loss occurs 25 times faster than in air.
This is the reason why, at any given temperature, you feel warmer on land than you do while immersed in water.
Evolution has provided cetaceans with efficient mechanisms to conserve heat, a thick layer of insulating fat, called blubber, being one of them.
Storing energy
Some cetaceans, most notably migrating whales, alternate a breeding season with a feeding season. Following a general pattern, whales spend the summer in feeding grounds at high latitudes, where they accumulate enough blubber to survive the following winter, which they will spend at much lower latitudes in their breeding and calving grounds.
In this respect, the blubber serves the purpose of an energy reservoir, and allows whales to starve for months without incurring in catastrophic physiological conditions.
In winter, some of them lose up to 1/3 of their weight when breast-feeding their young while going without food.
Fine-tuning streamlining
The epidermis of cetaceans is very soft, hairless and has a rubbery consistence.
Along with their overall shape, these factors reduce water resistance to a minimum.
In addition to the above mentioned adaptations, the intrinsic softness of the blubber also adds to the streamlining of cetaceans, by accumulation of elastic energy in response to water pressure.
Water temperature and blubber thickness
Since it is more challenging to keep a constant, warm temperature in cold waters, cetaceans living in cold climates have a thicker blubber layer than those living in temperate or tropical waters.
The Bowhead whale (Balaena mysticetus), the only mysticete living exclusively in the Arctic, has the thickest layer of blubber of all cetaceans, reaching a stunning 70cm (28in).
In the same area, the Narwhal (Monodon monoceros) and the Beluga (Delphinapterus leucas) also have thick blubber that keeps them warm while following the advance and retreat of the ice in the Arctic.
How do cetaceans avoid overheating?
The blubber is so efficient at keeping cetaceans warm than it is actually more challenging for them to avoid overheating.
Whales, dolphins and porpoises use their dorsal fin, flukes (tail) and flippers (pectoral fins) to release excess heat, by taking advantage of the absence of blubber from these portions of their body.

The mechanism by which an organism can keep its body temperature constant is called thermoregulation.
The key to this cooling mechanism resides in their very special circulatory system.
The arteries that carry oxygen-rich blood to the extremities are closely surrounded by a network of veins carrying oxygen-depleted blood back to the body core.
When the water temperature is relatively high, the blood flow to the extremities (fins and tail) is increased, allowing efficient dispersal of excess heat through the numerous veins and capillaries along the skin surface.

On the other hand, a very efficient heat-conserving mechanism, known as counter-current heat exchange, helps keep cetaceans warm in cold waters.
In this case, when warm blood flows from the body core to the extremities, a substantial fraction of its heat is exchanged with a network of veins bringing blood back to the core in the opposite direction, thus avoiding heat loss.

The network of blood vessels involved in counter-current exchange is known as rete mirabilis.
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