Sea otters should die in freezing water — new discovery reveals why they don’t

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 for them to cut through the 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 chilly waters that can reach temperatures of 32 to 59 degrees Fahrenheit. Typically, mammals that live in this type of environment withstand the cold through blubber or being big.

Previous to now, scientists weren’t sure how these relatively small animals (males on average are four feet long) managed to maintain a metabolism equal to mammals three times their size. Now, a study published Thursday in the journal Science points to a quirk of their anatomy as the answer: unique skeletal muscles.

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

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

Many larger marine mammals, such as whales, use fatty deposits known as blubber to retain heat in their bodies. Lacking such blubber, sea otters retain heat in their densely packed hair —but even that mechanism isn’t enough to maintain a core body temperature of 98.6 Fahrenheit.

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

“Polar and small-bodied marine mammals are particularly vulnerable to heat loss and require increased heat production to maintain body temperature,” the study team writes.

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

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

This increased basal metabolic rate is an adaption to conserve heat in an environment with low temperatures, also known as thermogenic hypermetabolism.

Previous to this study, scientists haven’t fully understood the mechanisms underlying the otter’s thermogenic hypermetabolism and how it allows them to thrive in frigid environments.

How the discovery was made — Wright and colleagues hypothesized there might be some connection between the sea otter’s unusually high basal metabolic rate and its ability to survive in the cold North Pacific waters.

Previous research suggested sea otter’s basal metabolic rate was around “three times higher than you would predict based on their size, which indicates that their body is burning a lot of energy at rest,” Wright says.

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

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

They put the hypothesis to the test by assessing the respiratory or breathing capacity of sea otters with different body masses. The scientists used respirometry, which can test the rate of oxygen consumption of living animals.

What’s new — Sea otters warm themselves through something known as a mitochondrial leak in their skeletal muscles, the study found.

Also known as leak metabolism, previous research has shown this also helps hedgehogs and other mammals stay alive in cold temperatures. 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 measures reported for any known mammal.”

The researchers were able to connect the leak metabolism in the sea otter’s skeletal muscles to its respiratory capacity — its rate of oxygen consumption — helping explain why its basal metabolic rate is so high compared to other mammals.

“We determined that the leak metabolic capacity in muscle tissue is high enough to explain the hypermetabolism previously demonstrated in sea otters,” Wright says.

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

What’s next — Sea otters’ ability to thrive in frigid waters has puzzled scientists, but this new research finally reveals how their bodies have adapted to their cold environment. The research helps scientists better understand how small aquatic mammals stay alive in unusually harsh environments.

However, it’s difficult to say whether the sea otters’ bodies are a longstanding evolutionary adaption to lower temperatures or a natural response to cold water.

“It's hard to differentiate how much of this unusually high leak metabolism is developed through evolutionary adaptation and how much is developed as an acclimatization in response to chronic exposure to cold,” Wright says.

Ultimately, these findings do inform a better understanding of sea otters — but there’s more work to be done, the researchers say.

“This study demonstrated that sea otter skeletal muscle is well suited for thermogenesis, but was only sampled in a single muscle,” Wright says.

Further studies should look to “different muscles throughout the body” and other metabolic tissues that might be well suited for thermogenesis, he explains.

In the meantime, we’ll have to settle for appreciating how cute they are.

Abstract: Basal metabolic rate generally scales with body mass in mammals, and variation from predicted levels indicates adaptive metabolic remodeling. As a thermogenic adaptation for living in cool water, sea otters have a basal metabolic rate approximately three times that of the predicted rate; however, the tissue-level source of this hypermetabolism is unknown. Because skeletal muscle is a major determinant of whole-body metabolism, we characterized respiratory capacity and thermogenic leak in sea otter muscle. Compared with that of previously sampled mammals, thermogenic muscle leak capacity was elevated and could account for sea otter hypermetabolism. Muscle respiratory capacity was modestly elevated and reached adult levels in neonates. Premature metabolic development and high leak rate indicate that sea otter muscle metabolism is regulated by thermogenic demand and is the source of basal hypermetabolism