Whales, dolphins, seals and other marine mammals can generate their own heat and maintain a stable body temperature despite fluctuating environmental conditions. Like people, they are endothermic homeotherms—or more colloquially, "warm-blooded."

But these animals take thermoregulation to an extreme, enduring water temperatures as low as –2 degrees Celsius (28.4 degrees Fahrenheit) and air temperatures reaching –40 degrees C (–40 degrees F).

How do they pull it off? And don't they ever feel cold? Disputatore, one of ScientificAmerican.com's fellow Twitterers recently asked, prompting us to investigate. We talked to Ann Pabst, a marine zoologist at the University of North Carolina Wilmington, who helped us understand how marine mammals survive the bitter cold.

[An edited transcript of the interview follows.]

How cold can liquid seawater get?
Polar oceanic water can get as cold as about –2 degrees C. Even seawater in temperate or tropical regions might drop as low as –1 degree C [30.2 degrees F] if you go deep enough below the surface.

[You may be wondering why water at -1 or -2 degrees C is not ice. Because seawater contains salt, it freezes at a slightly lower temperature than freshwater, which turns to ice at 0 degree C (32 degrees F). This is because the sodium and chloride in salt actually interfere with the water molecules' ability to come together and form ice crystals.—Editor's Note]

Can whales, dolphins and other marine mammals thrive in these frigid waters?
Certain species of pinnipeds—which include walruses, seals and sea lions—and some whales, dolphins and porpoises thrive in these conditions. Specific examples include humpback whales, blue whales—the largest animals on Earth—killer whales, Weddell seals and elephant seals.

Is there any evidence they can "feel" the cold like humans do?
Their skin is innervated with temperature-sensing nerve cells just as is the skin of any mammal.* They certainly have the ability to sense temperature, but how that translates to what they feel [whether they experience discomfort, for example] is a hard question to answer. But they certainly respond to temperature stimuli.

Whales and other marine mammals maintain a core body temperature similar to ours—about 37 degrees C (99 degrees F). How do they manage to do this under such extreme conditions?
They have two general types of responses: behavioral and physiological. A typical behavior response is migration. In the winter, pregnant right whales—which are endangered, partly a consequence of being entangled in fishing gear or being struck by boats—migrate from waters off Canada and New England to the coastal waters of Georgia and Florida to birth their young.

What are the physiological adaptations?
One way of minimizing heat loss is to have a relatively low surface area–to-volume ratio: a small amount of skin—across which heat is exchanged with the environment—compared to a large volume of body tissue—which generates heat. Large animals tend to have lower surface area–to-volume ratios, and most marine mammals are pretty big. Among the smallest: otters, which are roughly equal in size to large house cats.

Marine mammals also have excellent insulation in the form of fur or blubber. The sea otter has the densest hair of any mammal known: 130,000 hairs per square centimeter of skin [That's about as many hairs on an entire human head!]. Fur insulates most efficiently when it's dry because it traps air, a good insulator. Water, in contrast, is a pretty efficient heat conductor; it transfers heat away from a body some 25 times faster than air. The otter's hair is so dense it manages to trap a layer of air just above the skin even when the animal is swimming.

Animals that spend most of their time in water rely more on blubber, a layer of tissue containing fat, collagen and elastin that provides, among other things, insulation and energy storage like human fat. The amount of blubber varies from one animal to the next. Newborn harbor porpoises pack the most; some 43 percent of their total body mass is blubber.

Do marine mammals ever die as a result of being exposed to extreme temperatures?
Animals suffering from poor nutrition or health status might have trouble maintaining a healthy blubber supply, both in terms of quantity and quality—as measured by fat content—and thus could die from exposure to extreme temperatures. Or, if they find themselves displaced from their natural habitats, they might succumb to extreme temperatures. But it is unlikely this would occur among healthy wild animals.

*Note (5/14/09): This sentence was edited after publication to correct a spelling error.