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 said.

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.

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.