Sea otters have a difficult time keeping warm.
The animals spend the majority of their time in the water, which absorbs heat 23 times faster than air. Their habitat in the North Pacific is extremely cold, with water temperatures ranging from 0 to 15 degrees Celsius (32 to 59 degrees Fahrenheit). Sea otters are also the smallest aquatic animals, meaning they have a higher surface area per body size to lose heat through, and they lack the insulating fat present in their larger relatives.
According to Traver Wright, a biologist at Texas A&M University in College Station, “these guys have the densest fur of any animal on Earth, yet that’s [still] not quite enough to keep them warm.” “So they have a supercharged metabolism, which is assumed to be to generate heat and remain warm.”
He and his colleagues were curious as to how the creatures sustain a resting metabolic rate that is three times that of similarly sized mammals. The scientists discovered that heat seeping from the skeletal muscles could account for this astonishing capacity after examining tissue samples from wild and captive otters.
“These guys have a metabolism that is very optimized for inefficiency,” says Wright, whose findings were published in Science on July 8. “Even when it isn’t being physically active, the muscle may consume a lot of energy.”
Skeletal muscle, the most prevalent type of muscle that controls voluntary movement, is one of the most metabolically active tissues in the body. It accounts for 40 to 50% of the mass of most animals, making it the body’s biggest tissue. When muscles tense during exercise or shivering, heat is produced. They can, however, produce heat through a mechanism known as nonshivering thermogenesis, which Wright and his team felt was important for sea otters.
The study focused on how muscle cells generate energy. Carbohydrates are often broken down in the cells to generate the chemical pyruvate. Pyruvate reaches the cell’s energy-generating machinery, the mitochondria, where it is further processed. This process releases energy that is utilized to pump protons across the mitochondrial membrane, which are positively charged particles. The conversion of adenosine diphosphate (ADP) to the energy-storing molecule adenosine triphosphate is fueled by this so-called proton gradient (ATP).
“ATP can be utilized to power things in the cell—in this example, contractions and just the regular, everyday energy expenses of keeping a cell,” Wright explains.
However, the passage of charged particles that convert ADP to ATP might be disrupted at times.
“Small perforations in the inner membrane allow those protons to flow back into…the mitochondrial center,” Wright explains.
This implies the cell needs to work harder to produce the same quantity of ATP, wasting energy and generating heat that isn’t put to use. Wright and his colleagues were interested in learning more about the sea otters’ metabolic leak capacity, which refers to how much energy the sea otters’ muscles can “lose” in this way.
The researchers took tiny muscle samples from 21 otters ranging in age from infants to adults and washed out any leftover lipids or carbohydrates, as well as any persisting ADP.
After that, the researchers put the tissue samples in a sealed chamber, gave them pyruvate, and assessed how much oxygen the cells consumed to digest the molecule. Because the cells couldn’t create ATP, everything they did was focused on keeping the proton pump going. The cells became increasingly “leaky” as they worked harder.
Finally, the researchers fed ADP to the cells. This allowed them to see how the cells would function in regular circumstances.
The researchers discovered that “leaky” energy generation accounted for up to 41% of the metabolic capacity of the cells. The leak capacity of otters ranged from two to seven times that of other mammals such as Alaskan huskies, humans, horses, elephant seals, and rats.
Because the researchers only found data on a few species, Wright warns that it’s unclear how representative these other mammals are. He and his colleagues did discover, however, that the metabolic leak capacity of newborn and captive otters was comparable to that of adult and wild otters. This shows that remaining warm, rather than swimming, foraging, or other activities, is the driving force behind their very high metabolic rate.
The expense of using this heat to warm the body is high in terms of energy. Sea otters must eat for up to half of their day. They can eat up to a fifth of their body weight in a single day.
“It’s critical to understand that just because that leak capacity exists, it doesn’t imply it’s always operating at full capacity,” Wright adds. “It underlines the relevance of skeletal muscle for more than just what we generally think of as the ability to allow us to move,” he says. “It highlights the necessity of skeletal muscle for controlling the metabolism of the entire body.”
Even though people don’t have muscles that “leak” as much heat as otters, scientists are looking into whether leak metabolism can be used to control obesity, he says.
According to Wright and his colleagues, this inefficient style of metabolism may have been a crucial adaptation that allowed terrestrial ancestors of marine animals to colonize the world’s oceans.
He explains, “This ability of this muscle tissue to alter to meet the [otter’s] needs appears crucial for these creatures to thrive in what we would consider severe settings.”