The muscles of sea otters have a far greater capacity to turn food directly into heat than those of any other animal of a similar size. The findings explain how these small animals stay warm in cold waters.
Most marine mammals have large bodies insulated by a thick layer of blubber to help them retain heat. Sea otters are relatively small and usually weigh between 14 and 45 kilograms, with only the air trapped in their fur for insulation.
They have the densest fur of any mammal, but this isn’t enough to keep them warm, so they have to burn lots of energy. Their metabolic rate is around three times higher than normal for a mammal of their size, which makes it the highest of any mammal that weighs more than a kilogram.
“We knew these guys had this high metabolic rate, but we weren’t really sure where this energy was coming from,” says Traver Wright at Texas A&M University in College Station. So he and his team studied tiny pieces of living muscle tissue from both northern sea otters (Enhydra lutris kenyoni) and southern sea otters (Enhydra lutris nereis). It wasn’t necessary to kill animals to obtain the specimens – for instance, some samples came from rescued animals that were undergoing surgery to treat injuries.
Muscles generate heat when they move and many animals shiver when they get cold, but constant shivering isn’t a great strategy for a creature that lives and hunts in chilly waters. Instead, the mitochondria inside the otters’ muscle cells produce heat directly.
All mitochondria use the energy derived from “burning” food to pump out protons, creating a proton gradient. Normally, when protons flow back into mitochondria during this process, this potential energy is used to make an energy-rich molecule called adenosine triphosphate (ATP). This fuels processes that require energy, such as muscle contractions.
In sea otters, however, protons can flow back into mitochondria without producing ATP, creating heat without the muscles moving. Some other animals also use this trick to stay warm, but the researchers found that sea otters have a far greater capacity to do this than almost all other creatures.
“Their capacity to generate heat is really at higher levels than we’ve seen, except for some very small mice,” says Wright.
A similar process happens in brown fat, a form of fat that is present in some animals, including humans. But sea otters have little fat of any kind, says Wright, whereas their muscles account for nearly half of their body mass. In otters that weigh more than about 9 kilograms, their skeletal muscles alone can generate enough heat to warm the entire body.
The downside of this process is the amount of fuel required. Sea otters spend up to half their day feeding and eat up to a quarter of their body mass in food each day. “It’s metabolically costly,” says Wright. “These guys have to eat a lot of food.”
Journal reference: Science, DOI: 10.1126/science.abf4557
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