Warmth From Within

Off the western coast of North America in the frigid seas of the North Pacific Ocean, the fuzzy heads of sea otters bob up and down on the surface of the water. As the smallest aquatic mammals on Earth, sea otters spend nearly their entire lives floating and swimming in the sea, rarely venturing ashore.

Scientists have long wondered how the animals manage to keep their bodies warm enough to spend so much time in the chilly waters, which can dip to 0 degrees Celsius (32 degrees Farenheit). A human could survive only several minutes in water that cold before succumbing to hypothermia—a potentially dangerous drop in body temperature. That’s because water transfers heat about 23 times faster than air of the same temperature, rapidly drawing away an animal’s body heat.

In the icy North Pacific Ocean, the fuzzy heads of sea otters bob up and down on the water’s surface. The animals live off the western coast of North America. Sea otters are the smallest aquatic mammals on Earth. For nearly their entire lives, they float and swim in the sea. They rarely come ashore. 

These waters can dip to 0°C (32°F). How do the animals’ bodies stay warm enough to spend so much time in chilly seas? Scientists have wondered that for a long time. Humans could survive only several minutes in water that cold. Then a dangerous drop in body temperature called hypothermia would overcome them. That’s because water carries away heat about 23 times faster than air of the same temperature. It quickly draws away an animal’s body heat.

Because sea otters are small and spend so much time in the cold water, they should have a difficult time maintaining their internal body temperature, says Traver Wright. He’s a physiologist—a scientist who studies how living things function—at Texas A&M University. But sea otters have a trick for staying warm in icy seas. The secret lies within each of the animal’s muscle cells—the smallest units that make up an organism.

Sea otters are small animals that spend much time in cold water. So they should have a difficult time maintaining their internal body temperature, says physiologist Traver Wright. He studies how living things function at Texas A&M University. But sea otters have a trick for staying warm in icy seas. The secret lies inside each of the animal’s muscle cells. These are the smallest units that make up a living thing.

Adult sea otters can grow to about 1.4 meters (4.5 feet) long and weigh up to 41 kilograms (90 pounds). That’s tiny when compared with much bigger marine mammals, like whales and seals, that live in the same cold waters. A sea otter’s body has a much smaller volume than the bodies of these larger animals, which means otters lose body heat faster in the cold. “It’s like if you put a small cup of water and a gallon of water in a freezer,” says Wright. “The small cup will freeze a lot faster than the gallon.”

Sea otters also lack the insulating blubber that other marine mammals have. This thick layer of fat traps warmth and helps maintain body heat. However, sea otters do have extremely thick fur that serves a similar purpose. A sea otter’s coat is the thickest of any animal on Earth. While a person’s head has an average of 100,000 hairs, a sea otter’s body can have more than 1 million hairs per square inch.

Adult sea otters can grow to about 1.4 meters (4.5 feet) long. They weigh up to 41 kilograms (90 pounds). That’s tiny, compared with marine mammals like whales and seals. These much bigger animals live in the same cold waters. A sea otter’s body has a much smaller volume than the bodies of these larger animals. That means otters lose body heat faster in the cold. “It’s like if you put a small cup of water and a gallon of water in a freezer,” says Wright. “The small cup will freeze a lot faster than the gallon.”

Other marine mammals have blubber. This thick layer of fat traps warmth and helps maintain body heat. Sea otters don’t have blubber. But they do have extremely thick fur that also traps heat. A sea otter’s coat is the thickest of any animal on Earth. A person’s head has an average of 100,000 hairs. But a sea otter’s body can have more than 1 million hairs per square inch.

Although this furry adaptation helps keep sea otters warm, says Wright, it doesn’t offer enough protection to keep sea otters from freezing in ice-cold waters. He suspected that otters were relying on another method to maintain their body temperature.

This furry adaptation helps keep sea otters warm. But it’s not enough protection to keep them from freezing in ice-cold waters, says Wright. He suspected that otters were using another method to maintain their body temperature.

When an animal eats food, its cells transform nutrients, like sugars and fats, into energy. This fuels the body’s metabolism—the many chemical processes needed to sustain life.

For some animals, like mammals and birds, energy produced by their cells during metabolism generates heat. This helps regulate their internal temperature (see Heat Regulators). These animals are endothermic, or warm-blooded. They differ from reptiles, fish, and amphibians, which are ectothermic, or coldblooded. These critters rely on the temperature of their environment to maintain their internal body temperatures.

Scientists have long known that aquatic mammals have revved-up metabolisms that help maintain their internal temperatures in frigid waters. In fact, a sea otters’ metabolism is three times greater than would be expected of an animal their size. But no one knew how that mechanism worked or whether it was the reason sea otters could sustain their temperatures. That led Wright to take a deeper look at what was going on inside sea otters’ cells.

Food contains nutrients, like sugars and fats. When an animal eats, its cells turn these nutrients into energy. This fuels many chemical processes that sustain life. These processes are known as the body’s metabolism.

During metabolism, the cells of some animals produce energy that creates heat. This helps control their internal temperature (see Heat Regulators). These animals include mammals and birds. They’re endothermic, or warm-blooded. They differ from reptiles, fish, and amphibians. Those animals are exothermic, or cold-blooded. They rely on the temperature around them to maintain their internal body temperatures.

Aquatic mammals have faster metabolisms. That helps maintain their internal temperatures in icy waters. Scientists have known that for a long time. In fact, a sea otters’ metabolism is three times greater than that of other animals their size. But how did the process work? Was it the reason sea otters could maintain their temperatures? No one knew. So Wright took a deeper look at what was happening inside sea otters’ cells.

Within animal cells are structures called mitochondria (see Inside a Cell). These structures are the powerhouses of cells where metabolism occurs. To measure the amount of heat energy produced by sea otters’ mitochondria, the scientists took tiny samples of muscle tissue from 21 different sea otters. They put the samples into a machine called a respirometer to determine how much energy was lost as heat from the mitochondria during metabolism.

Wright and his colleagues found that most of sea otters’ body heat was produced by their muscle cells. The team also discovered a surprising measurement: A whopping 40 percent of the sea otter’s energy leaks out of the cells’ mitochondria as heat that warms the animals’ bodies. To keep up with their extreme metabolic needs, the tiny sea otters have an enormous appetite. They need to eat the equivalent of 25 percent of their body mass each day.

Structures called mitochondria are inside animal cells (see Inside a Cell). These structures are the powerhouses of cells. Metabolism occurs within them. The scientists wanted to know how much heat energy is produced by sea otters’ mitochondria. To find out, they took tiny samples of muscle tissue from 21 different sea otters. They put the samples into a machine called a respirometer. It would show how much energy was lost as heat from the mitochondria during metabolism.

Wright and his team looked at the measurements. They found that sea otters’ muscle cells produced most of their body heat. The team also found something surprising about sea otters’ energy. Forty percent of it leaks from the cells’ mitochondria as heat that warms the animals’ bodies. To fuel their extreme metabolism, the tiny sea otters eat a huge amount of food. The amount equals 25 percent of their body mass each day.

“It’s like a hydroelectric dam with a leak,” says Wright. “The water stored in the dam is used to create electricity, but it’s less efficient if you have large leaks. This means you need to keep adding more water to make the same amount of energy,” he explains.

It’s unclear whether sea otters’ adaptation is inherited or whether the animals’ bodies acclimate, or change because of the environmental conditions in which they live. The scientists also tested baby sea otters, which can weigh as little as 1.4 kg (3 lbs) at birth. They found that the babies’ cells behaved almost identically to the adults’. This suggests that the animals are born with the ability to stay warm in cold waters. Understanding sea otters’ ability to create such large amounts of heat from within their cells could also tell scientists more about how other aquatic animals evolved to survive in extreme environments, says Wright.

“It’s like a hydroelectric dam with a leak,” says Wright. “The water stored in the dam is used to create electricity, but it’s less efficient if you have large leaks. This means you need to keep adding more water to make the same amount of energy,” he explains.

Do sea otters get this adaptation from their parents? Or do the animals’ bodies acclimate, or change because of conditions around them? The answer isn’t clear. The scientists also tested baby sea otters. At birth, the animals can weigh as little as 1.4 kg (3 lbs). The scientists found that the babies’ cells behaved almost the same as the adults’. This suggests that they can stay warm in cold waters from birth. Scientists want to understand how sea otters can create such large amounts of heat from within their cells. Wright says that could also reveal more about how other aquatic animals evolved to survive in extreme environments.