Thermoregulation

37^oC underwater

For a mammal that spends most of its life in water–an element that conducts heat 25 times faster than air–maintaining an average body temperature of 37^oC, like any other self-respecting mammal, is a big challenge. To accomplish this, marine mammals have tricks of their own. The most obvious is located just beneath the skin: a thick layer of fat that insulates internal organs from the often-cold surrounding environment. This layer can be as much as 50 cm thick in the larger whales. As well as the fat layer, pinnipeds (seals and sea lions) and sea otters can also count on their thick fur to help them conserve heat. Yet, the most extraordinary marine mammal tricks for conserving body heat are without a doubt their vascular adaptations. These adaptations control heat loss by controlling blood flow.

An admirable network for heat recovery

The rete mirabile –Latin for "admirable network"– is a countercurrent heat exchange system. This network is located essentially in the poorly insulated portions of the body, such as the pectoral, dorsal and caudal fins of cetaceans, the feet of pinnipeds and the caudal fins of sirenians. These structures are composed of arteries surrounded by several veins. They function something like the domestic heat exchange systems that can be found in many recently constructed houses. Heat from the blood flowing through the arteries, which travels from the internal organs towards the extremities, is recovered by the blood in the veins, which is travelling in the opposite direction from the extremities towards the internal organs. Blood that reaches the capillaries near the body’s surface has already cooled down, thus limiting the loss of body heat to the surrounding environment. Conversely, blood returning from the capillaries near the body’s surface is warmed up before it reaches the organs, thus lessening the risk of lowering the body’s internal temperature. These admirable networks can also work to rid the body of excess heat. As it dilates, the central artery crushes the veins that surround it, limiting blood flow in these veins, consequently limiting the exchange of heat. Arterial blood that attains the capillaries is therefore still warm and loses its heat to the surrounding environment. As for the blood returning to the organs, it flows through veins that are not part of the admirable networks; therefore it gains less heat before reaching the organs.

The rete mirabile not only control heat loss through the skin; they also regulate the internal temperature of certain organs. For example, the admirable networks situated in the dorsal and caudal fins serve to maintain marine mammal testicles at a temperature that is 1^oC below body temperature, thus avoiding the production of malformed or non-functional spermatozoids. Similarly, admirable networks situated in the abdomen will serve to evacuate excess heat produced by the foetus of a gestating female, which is very sensitive to high temperatures.

Bridges beneath the skin to reroute blood

Arterio-veinous anastomoses are "bridges" that connect an artery to a vein. Situated in the fat layer, they allow blood to flow directly from the artery to the vein, avoiding the capillaries situated beneath the surface of the skin. Blood flow to this skin is reduced as it bypasses the capillaries through these bridges. In this manner the insulating function of the fat layer is optimized and heat loss is minimal. On the other hand, to evacuate heat during periods of intense physical activity, marine mammals can bypass the bridges and increase blood flow to the capillaries beneath the skin. This explains the pinkish tinge of the normally white belly of a minke whale when it is surface feeding: blood is rushed to the surface of the skin in order to avoid overheating!