Thermoregulation

1. Heat Retention
1. Surface Area
2. Metabolic Rate
3. Blubber
4. Counter-current Heat Exchange Mechanism
2. Homeothermy

Like all mammals, dolphins are warm-blooded. They need to maintain a steady body temperature (homeothermy) of around 36-37 degree celsius, similar to the body temperature of human. But one may ask, "How does a dolphin manage to endure the low temperature underwater and still maintain a temperature of 37 degree celsius?"

Indeed, to retain heat in the dolphin's body is particularly difficult in water as water, being an excellent conductor of heat and having a high heat capacity, can remove heat from the body at a rate twenty-five times faster than air. Body heat is also lost in breathing when warm air is exhaled, and also when faeces and urine are excreted.

Dolphins are not able to conserve body heat like how other terrestrial mammals do, by curling up and huddling together, building habitats and seeking shelter or migrating to warmer regions. Dolphins also do not have hair or fur like other mammals which can protect them from the cold. However, they have developed their own ways in which heat loss can be kept to a minimum.

Heat loss through the body's surface is the greatest in dolphins. To reduce such losses, dolphins have a low surface area to volume ratio. This low surface area will reduce the rate of heat loss to the environment. The low surface area is achieved by streamlining, when the size of forelimbs are reduced and the hind limbs are absent.

Dolphins have a higher metabolic rate than terrestrial mammals of similar size, meaning that more energy is being produced at a faster rate. A big portion of this energy is converted to heat energy so as to sustain the body temperature. This high metabolic rate helps to keep them warm in cold water.

The absence of fur in dolphins is compensated by a thick layer of fat in the deepest layer of the skin, referred to as blubber. Blubber is an excellent heat insulator. It has been found that the heat conductivity of blubber is very low and heat transfer occurs at a very slow rate. Only 10 percent of all heat loss of the body takes place here.

In the blubber, the arteries running to the skin are surrounded by spirals of veins. The blubber must also receive some blood to supply their tissues with oxygen, but blood flowing through them at body temperature would cause significant loss of body heat to the skin. Therefore, these spirals of veins, known as periarterial venous plexuses, act as counter-current heat-exchangers.

This basic mechanism is present in many terrestrial mammals, including humans. The counter-current system is an modification of the organisation of the blood vessels that minimises the loss of heat incurred when blood travels to the different parts of the body.

The system is best understood by studying a heat-generator which is within an insulating barrier. Pipes are carrying a heated liquid from the heat-generator to the colder region of the barrier and then back again in a closed circuit. Heat will be lost outside the insulation and thus, the returning liquid will be cold. Hence, to prevent unnecessary heat loss, the flow pipes and the return are in close contact and heat will be exchanged between the pipes. Less heat will be lost this way.

The system works the same way in dolphins. The veins of the dolphin, which resemble the return pipes, will carry the cold blood back to the heart. On its way, the veins will come into close contact with the superficial arteries (the return pipes) of the skin of the flukes and flippers. The blood gets warmed up when heat is transferred from the blood of the arteries.

In the dolphin, the arrangement has been modified so that the arteries are completely enclosed in a complex bundle (plexus) of veins, increasing the efficiency of the countercurrent heat exchange.

Fluctuations of temperature take place sometimes and dolphins have to adapt itself quickly to any new temperature. Dolphins may sometimes find themselves facing the potential danger of getting overheated. This may happen when the water around them suddenly gets hotter than the usual temperature. Or another possibility can be when the dolphin engages in bursts of activity. Under these two circumstances, the dolphin has gained too much heat and cannot lose their heat fast enough to get rid of excess body heat. Overheating may result from this.

Hence, homeothermy plays a major role in thermoregulation. It depends almost entirely on the tail, the flippers and the dorsal fin because they have a generous supply of blood vessels, and very little blubber insulation.

If the internal temperature of the dolphin's body rises, the large arteries running up through the blubber to the skin dilate and large volumes of blood are allowed to pass through these vessels. The skin will be warmed by the blood and heat is quickly dissipated to the environment quickly.

If the internal temperature of the dolphin's body falls instead, the arteries running up to the skin will constrict, restricting the flow of blood to the surface of the skin and in turn reducing heat loss.

It seems likely that the diameter of the arteries and the flow of blood are under the control of the autonomic nervous system, which, among other functions, controls the body temperature.

Bibliography

Bonner, Nigel. Whales of the World. New York: Facts on File, Inc., 1989.

Cousteau, Jacques. Whales. New York: Harry N. Abrams, Inc, Publishers, 1988.

Harrison, Sir Richard, et. al. Whales, Dolphins and Porpoises. New York: Facts on File, Inc., 1994.