Sea otters are skinny and small compared to most other marine mammals, which mostly rely on a thick layer of blubber to keep their bodies warm in cold seas. The supposed explanation for the sea otter’s svelte figure was that their amazingly dense fur traps air bubbles and creates an insulative barrier between these backstroking fuzz-balls and their chilly home waters.
But new research reveals that the super-fluffy fur that nearly led the sea otter to be hunted into extinction isn’t the whole story. A paper published last week in the journal Science finds these shellfish munchers also have revved-up metabolisms to keep them toasty in water between 32 and 59 degrees Fahrenheit, reports Kate Baggaley for Popular Science.
Sea otters have metabolisms that burn through calories three times faster than researchers would typically expect for an animal with their body size, according to the study. All those extra calories are mostly burned by the otter’s muscles.
“You mostly think of muscle as doing work to move the body,” says study author Tray Wright, a physiologist at Texas A&M University, in a statement. “When muscles are active, the energy they use for movement also generates heat. Muscles can also generate heat without doing work to move by using a metabolic short circuit known as leak respiration.”
In humans, shivering is one of the body’s ways of activating muscles to produce heat when temperatures drop. But shivering involves actual contractions of the muscles, and sea otters are up to something a bit different. Instead of rapid-fire muscular contractions, the sea otters have leaks in the energy-producing cellular machinery that powers their muscles.
Normally, these cellular energy factories—oblong organelles called mitochondria—break down sugars to pump protons across their inner membrane and then use the protons that flow back across that membrane to create a molecule called adenosine triphosphate (ATP) that stores energy that can be used to power work such as muscular contractions, reports Michael Le Page for New Scientist. But in sea otters, some of those protons leak back across the membrane without being used to make ATP, causing their energy to be lost in the form of heat.
“These guys have got a metabolism that is really kind of tailored for being inefficient,” Wright tells Popular Science. “The muscle can burn a lot of energy even when it’s not being physically active.”
The team figured this out by placing muscle tissue from 21 captive and wild sea otters inside a device called a respirometer that researchers used to measure how much oxygen the muscle cells were using. Jaime Chambers explains in Science News that the muscle cells’ oxygen usage provided researchers with an indirect measurement of how “leaky” they are.
These tests revealed significant proton leakage, with up to 41 percent of the cells’ energy usage going toward producing heat, according to the paper. That’s between two and seven times higher than other mammals, including Alaskan huskies, humans, horses, elephant seals, and rats, according to Popular Science.
This all means that even a relatively inactive sea otter needs to consume a lot of calories just to stay warm. According to New Scientist, sea otters must spend up to half of each day wolfing down up to a quarter of their body weight in food. “It’s metabolically costly,” Wright tells New Scientist. “These guys have to eat a lot of food.”
The discovery may even expand scientists’ understanding of the metabolisms of other marine mammals.
“This could be a game changer in terms of how we think about the evolution of all marine mammals, not just sea otters,” Terrie Williams, an ecophysiologist at the University of California, Santa Cruz who was not involved in the study, tells Science News. Most ocean waters are significantly cooler than a mammal’s internal body temperature, which means that marine mammals would have needed to solve the problem of maintaining a stable internal body temperature early in their evolution. Williams tells Science News that “this is probably one of the clearest pieces of evidence saying, ‘Here’s how they did it.’”