Sea otters: they bob up and down in the water, hold hands when they are sleeping, poop together at social events, stay warm by their fur and leaky mitochondria… wait, what?

Let’s rewind.

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A Cute Sea Otter Floating On Its Back

Warm-blooded marine mammals have a thick layer of fat and oils, known as blubber, as their skin layer to insulate their body. In cold waters, blubber helps retain heat and maintain homeostasis.

But what if warm-blooded marine mammals lack blubber? Sea otters are a prime example (and the only example) of a marine mammal without a layer of blubber. Instead, they have a thick coat of dense hairs, 1000x denser than human hair–the thickest on earth. This enables sea otters to trap large amounts of air within their fur coat, acting as insulation. (This is the same reason why sea otters float: the air trapped in their fur coat makes them buoyant).

But with that said, can you stay warm in a fleece jacket? Possibly. What if you were wearing it while in the ocean? That might be somewhat difficult. Similarly, fur can’t solely protect these animals from losing too much heat. These mammals are still living in water, which transfers heat 23 times as efficiently as air. Since sea otters are the smallest aquatic mammals, they have a lot of surface area relative to their volume, making it even harder for these animals to maintain homeostasis.

So how do they do it? Researchers have already understood that sea otters have an extreme metabolism, how food gets converted to energy in cells, eating about twenty-five percent of their body mass in food every day. But the pieces were still not adding up, which prompted researcher T. Wright to investigate this question on a cellular level. He and his colleagues searched for the source of heat in otters’ muscles. Playing a pivotal role in the body’s metabolism, the skeletal muscle makes up 40 to 50 percent of the sea otters’ entire body mass. His study required the collection of tissue from 21 sea otters of different ages and then measured the muscle cells’ respiratory capacity compared to that of other animals. The sea otters’ oxygen flow rate would roughly indicate the measurement of the cells’ heat production.

Mitochondria pump protons across their cell membranes to store energy in the form of ATP, like we learned in AP Biology’s diffusion unit. From this study, T. Wright concluded that the protons are diffusing back through the membrane before being used for work, resulting in excess heat. Since some of the energy is lost as heat, sea otters need to eat more food to compensate for the lost energy. This “leak in energy” is what contributes to the sea otters’ speedy metabolism.

It’s unknown if sea otters develop leaky mitochondria by living in cold water or simply inherit it. Future research into the fascinating design of sea otters may potentially reveal intriguing insight into their evolution, behavior, and maybe someday, their cuteness.