It’s Cold in the Ocean but It’s Hotter Inside Every Sea Otter

Sea otters run hot. It’s not just a manner of speaking: Scientists have found that the furry mammals’ metabolisms work at a rate three times what might normally be expected from a creature their size, burning swiftly through calories.

They seem to be using much of that energy to generate heat, keeping themselves at a toasty 98.6 degrees Fahrenheit in the frigid ocean, where staying warm is a matter of life and death. But the details of their conversion of food and oxygen into vast reserves of heat have been obscure. Now researchers studying sea otters’ muscles report that the feat involves using the mitochondria in their muscle cells in an unexpected way. Their study was published Thursday in the journal Science.

Unlike whales and polar bears, sea otters don’t have a thick insulating layer of blubber, and their celebrated fur — the thickest in the world, with up to 2.6 million hairs per square inch — is not enough on its own to keep them alive in an ocean that can hover on the edge of freezing. Muscles generate heat as they contract, but scientists have known for some time there is another way that muscles can help animals keep warm, a cellular process with the delightful name of proton leak.

Inside almost all animal cells, little pill-shaped organelles called mitochondria break down sugar molecules to extract energy. (Mitochondria are often called the powerhouses of the cell.) During the final stage of this process, protons pop through a membrane. In biology textbooks, the protons helpfully trickle through tiny spinning pores, driving them like water wheels to make adenosine triphosphate, a compound that serves as the molecular battery powering cellular processes.

But reality is not always so tidy. If protons build up faster than the little water wheels can clear them, they seep across the membrane in other ways. And in skeletal muscle cells, this leakage of protons produces substantial amounts of heat. This is thought to contribute to keeping polar animals warm, said Traver Wright, a professor at Texas A&M University and an author of the new paper.

To see how much proton leak might be occurring in sea otters, Dr. Wright and his colleagues put samples of muscle cells from 21 animals into a special chamber that allowed the researchers to monitor the ins and outs of the cells’ mitochondria. They found that sea otters are capable of tremendous quantities of proton leak, suggesting substantial heat-generating capacity. And they were surprised to discover that this ability was present in both tiny otters and full-grown adults.

In general, an organism’s metabolic capacity is linked to its activity level, Dr. Wright said. But young otters, of an age when they would often be resting on their mother; adults of all sizes; and even a relatively inactive captive otter all had similarly high metabolisms and a great capacity for proton leak. In fact, they had higher rates than even Iditarod sled dogs.

“Their leak metabolic rate isn’t anywhere near as high as in sea otters,” said Dr. Wright of the dogs. For otters, he added, “that heat generation is really the driving force of their metabolic development.”

Sea otters are churning through calories even without a lot of physical activity because that energy goes straight into heat, the results suggest. Otters are among the only animals so far for whom proton leak can explain almost all of their elevated metabolism, Dr. Wright said.

The researchers are hoping to study the metabolisms of a variety of animals. They have already published related work on elephant seals, enormous creatures whose lives include both frenetic diving and eating while at sea, as well as weeks of extended lounging on the beach.

Curiously, while the seals loll on the sand for a month, their metabolic capacity does not decrease. Humans taking a similar hiatus from movement with a piña colada and beach reading in hand would not be so lucky.

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