The secret lies not in the fat layer or fur: these animals have a more complex and subtle mechanism at the cellular level.
“Leaks” in the energy-generating regions of muscle cells contribute to maintaining high metabolic rates in otters, even when they are at rest. This process occurs three times faster than one would expect for animals of this size. This discovery sheds light on how otters keep warm and may be applicable to other marine mammals.
“This could change our understanding of the evolution of all marine mammals, not just sea otters,” notes ecophysiologist Terry Williams from the University of California, Santa Cruz, who was not involved in the study. To survive in cold oceans, mammals needed to develop efficient thermoregulation methods.
Other marine mammals also exhibit high metabolic rates; however, they often rely on their large size and subcutaneous fat. In contrast, sea otters are small and slender. These animals are the smallest mammals living in the ocean. Their fur, although considered the densest on the planet, cannot fully protect their bodies from heat loss. Water conducts heat 23 times more efficiently than air, and their small size with a low surface area contributes to faster heat loss, even with thick fur.
Scientists already knew that sea otters benefit from their remarkable metabolism, which allows them to maintain a body temperature of 37 degrees Celsius. To achieve this, the animals need to consume a significant amount of food daily—about 25% of their body weight. However, researchers did not understand the cellular mechanisms underlying the “accelerated metabolism for heat production,” says physiologist Traver Wright from Texas A&M University in College Station.
Wright and his team searched for heat sources in the muscles of otters, which make up 40% to 50% of the animals' total body mass, allowing them to significantly influence metabolism. The researchers took muscle tissue samples from 21 otters, both from captivity and the wild, including both infants and adults. Then, using a device called a respirometer, they measured the respiratory capacity of otter muscle cells in various states. The obtained data were compared to the respiratory capacity of cells from other animals, including humans, sled dogs, and elephant seals. The rate of oxygen exchange allows for an indirect assessment of heat production by the cells.
The researchers found that leaks in the mitochondria (the energy centers of cells) release additional heat and are responsible for the high metabolism of sea otters. Mitochondria pump protons across their inner membrane, and their backflow leads to heat loss. These “leaks” of protons increase the amount of energy lost as heat, causing otters to consume more food to compensate for these energy losses.
Other mammals, including very small mice, can also release heat in a similar manner. However, sea otters have achieved remarkable results in this regard: proton leaks account for about 40% of the total respiratory capacity of otter muscle cells—more than any other known mammal. This method of heat production allows the animals to feel comfortable even in northern waters, where temperatures are close to 0 degrees Celsius.
