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.