Sea otters are equipped with “muscle heating” | Polarjournal
Unlike other marine mammals, sea otters do not have a blubber layer. Instead, they have the densest fur in the entire animal kingdom, which helps them stay warm and dry. However, to survive in the cold waters of the North Pacific, thick fur alone is not enough. Photo: Wikipedia/MikeBaird

Sea otters live mainly in the cold coastal waters of the North Pacific Ocean from Washington State to the Aleutian Islands. The most important thing for them is to stay warm, which is not at all easy as the smallest marine mammal without a thick layer of blubber. Their extraordinarily dense fur insulates them very effectively, but is not sufficient to retain body heat. Their high metabolic rate is responsible for the extra warmth they need to survive, as has long been known. But only now have researchers at Texas A&M University discovered how otters produce the heat they need.

Sea otters spend virtually their entire lives in water without the need to go ashore. As the smallest marine mammals, they are particularly vulnerable to heat loss and require increased heat production to maintain their body temperature of 37°C in water that is 0° to 15°C cold. And for this they need a lot of energy. That’s why they eat up to 25 percent of their body weight in sea urchins, mussels, crabs and other invertebrates every day to have enough energy available for heat production.

In their recent study, published in the journal Science, scientists found that the muscles of sea otters(Enhydra lutris) do not only generate heat when they move. Physiologist Traver Wright, assistant professor in the College of Education & Human Development at Texas A&M University and lead author of the study, explains that in sea otters, a process called “leaky breathing” ensures that energy is converted into heat even without muscle activity. Leaky breathing is a kind of metabolic short-circuit by which muscles generate heat without doing any work. This also provides the reason for the otters’ high metabolic rate, which is about three times higher than in other mammals.

Newborn sea otters stay with their mothers for about six to eight months until they are strong enough to dive for food themselves. Even their muscles show a high metabolic rate. Photo: Wikipedia/MikeBaird

To decipher the mechanism of heat production, the research team collected skeletal muscle samples from southern sea otters living in California and from northern sea otters in Alaska. In the laboratory, they determined the respiratory capacity of the muscle cells, i.e. the rate at which the muscle can consume oxygen, and found out that the energy produced by the muscle is not only used for movement.

A mechanism better known for heat generation by muscles is shivering. Wright describes that involuntary movement activates the muscles to generate heat. In leaky breath, on the other hand, the activation of the muscles occurs without them contracting and beginning to shiver.

Even the muscles of newborn sea otters had metabolic rates as high as those of adults, which Wright said surprised the researchers greatly. “This really underscores how heat production seems to be the driving factor in determining the metabolic capacity of muscle in these animals,” Wright said.

“The regulation of tissue metabolism is also an active area of research in the fight against obesity. These animals can give us clues about how to manipulate metabolism in healthy people and those with diseases that affect muscle metabolism.”

Traver Wright, Assistant Professor in the College of Education & Human Development at Texas A&M University.

This study highlights that muscles therefore not only play a role in activities such as foraging, avoiding predators and finding mates, but are also essential for animal survival and ecology, as Wright points out.

Nevertheless, there is still a lot for the researchers to decipher. For example, in the current study they could not yet find out how the animals regulate heat production when the demand is higher. It is also not yet known whether all the different muscle types produce heat in the same way.

Julia Hager, PolarJournal

Link to the study: Traver Wright, Randall W. Davis, Heidi C. Pearson, Michael Murray, Melinda Sheffield-Moore. Skeletal muscle thermogenesis enables aquatic life in the smallest marine mammal. Science, 2021; 373 (6551): 223 DOI: 10.1126/science.abf4557

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