Sea otters are arguably the Michael Phelps of the marine mammal world. They are excellent swimmers, complete with muscular bodies and webbed feet that allow them to cross water with ease.

They are also like Phelps in another way: they are extraordinary in their abilities; able to succeed where other mammals of their size wouldn’t stand a chance.

Sea otters live in cold waters that can reach temperatures of 32 to 59 degrees Fahrenheit. As a rule, mammals that live in this type of environment are resistant to cold due to their fat or size.

Until now, scientists have not known how these relatively small animals (males are on average four feet long) manage to maintain a metabolism equal to that of mammals three times their size. Now, a study published Thursday in the newspaper Science points to a quirk of their anatomy as an answer: unique skeletal muscles.

“This study demonstrates how the skeletal muscle of sea otters is well suited to generate heat, which is essential for these small marine mammals to survive in cold water,” said the lead author. Traver Wright, assistant research professor at Texas A&M University, says Reverse.

What you need to know first – Otters are the smallest marine mammals in the world, and their small size puts them at a disadvantage when it comes to surviving in the cold waters of the North Pacific.

Many large marine mammals, such as whales, use fatty deposits called fat to retain heat in their bodies. In the absence of such fat, sea otters retain heat in their dense hairs, but even this mechanism is not sufficient to maintain a core body temperature of 98.6 degrees Fahrenheit.

If the sea otter’s body temperature drops below this core temperature for too long, they will die, just like humans.

How can the tiny bodies of sea otters survive the cold waters of the North Pacific? Scientists now have an answer.Getty

“Small and polar marine mammals are particularly vulnerable to heat loss and require increased heat production to maintain body temperature,” the study team wrote.

Sea otters, in turn, have adapted in another way: their bodies generate a basal metabolic rate that is three times the rate predicted by mammals of similar size.

“Basal metabolic rate is essentially the minimum metabolism required for an animal to maintain basic bodily functions without moving, eating and digesting,” says Wright.

This increased basal metabolic rate is an adaptation to conserve heat in a low temperature environment, also known as thermogenic hypermetabolism.

Prior to this study, scientists had not fully understood the mechanisms underlying otter thermogenic hypermetabolism and how it allows it to thrive in freezing environments.

Researchers explain how the hypermetabolic body of the sea otter adapts to cold climates. Wright et al

How the discovery was made – Wright and his colleagues speculated that there may be a link between the sea otter’s unusually high basal metabolic rate and its ability to survive in the cold waters of the North Pacific.

Previous research suggested that the sea otter’s basal metabolic rate was about “three times higher than what you might expect based on its size, indicating that its body burns a lot of energy while at rest,” says Wright. .

Considering this, the team speculated that sea otter hypermetabolism was linked to thermogenesis – a function that allows mammals to stay warm by generating heat through certain tissues and skeletal muscles.

“Given how difficult it is for these animals to stay warm, this hypermetabolism is thought to work for thermogenesis,” says Wright.

They put the hypothesis to the test by evaluating the respiratory or respiratory capacity of sea otters of different body masses. Scientists used respirometry, which tests the rate of oxygen consumption in living animals.

What’s up – Sea otters warm up through what’s called a mitochondrial leak in their skeletal muscles, according to the study.

Also known as Leak Metabolism, previous research has shown it also helps hedgehogs and other mammals stay alive in cold weather. Mitochondria are the tiny organelles in our cells that produce energy that keeps us warm and helps us stay alive.

“These values ​​are also higher than comparable measurements reported for all known mammal.”

Researchers were able to link the leaked metabolism in the sea otter’s skeletal muscles to its respiratory capacity – its rate of oxygen uptake – helping to explain why its basal metabolic rate is so high compared to other mammals. .

“We have determined that the metabolic leakage capacity in muscle tissue is high enough to explain the hypermetabolism previously shown in sea otters,” explains Wright.

The sea otter’s escape metabolic capacity is so high that it exceeds the same capacity even in energetic Alaskan huskies and Iditarod sled dogs. “These values ​​are also higher than comparable measurements reported for any known mammal,” the study team wrote.

And after – The ability of sea otters to thrive in freezing waters has intrigued scientists, but this new research is finally revealing how their bodies have adapted to their cold environment. The research is helping scientists better understand how small aquatic mammals stay alive in exceptionally harsh environments.

However, it is difficult to say whether sea otters’ bodies are a long-standing evolutionary adaptation to lower temperatures or a natural response to cold water.

“It is difficult to differentiate the part of this unusually high leakage metabolism that develops through evolutionary adaptation and the part that develops as acclimatization in response to chronic cold exposure,” says Wright.

Ultimately, these results provide a better understanding of sea otters, but there is still work to be done, the researchers say.

“This study demonstrated that the skeletal muscle of the sea otter is well adapted to thermogenesis, but was only sampled from a single muscle,” explains Wright.

Further studies should focus on “the different muscles of the body” and other metabolic tissues that may be well suited to thermogenesis, he explains.

In the meantime, we will have to be content to appreciate how cute they are.

Abstract: Basal metabolic rate generally varies with body mass in mammals, and variation from predicted levels indicates adaptive metabolic remodeling. As a thermogenic adaptation to live in cold water, sea otters have a basal metabolic rate about three times the expected rate; however, the tissue source of this hypermetabolism is unknown. Because skeletal muscle is a major determinant of whole body metabolism, we characterized respiratory capacity and thermogenic leakage in sea otter muscle. Compared to that of previously sampled mammals, thermogenic muscle leakage capacity was high. and could explain the hypermetabolism of the sea otter. Muscle respiratory capacity was modestly elevated and reached adult levels in newborns. Premature metabolic development and high leakage rate indicate that sea otter muscle metabolism is regulated by thermogenic demand and is responsible for basal hypermetabolism.

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