Otters use 'leaky' metabolism to stay warm

Sea otters are the smallest marine mammals, and so they’ve had to adapt to their cold and wet environment. They have the thickest fur of any animal, and on top of that, an incredible ability to generate body heat. It’s that second feature that recently got some American scientists interested: how are otters so good at turning the energy from their food into heat? It turns out that inside their mitochondria - the parts of their cells that generate energy - the normally-sound membranes are a little bit leaky, which lets energy get released as heat rather than, say, used to contract a muscle. Phil Sansom heard about the process - and the remarkable otters themselves - from Texas A&M University’s Tray Wright...

Tray - They're really interesting animals because they've got this really high metabolic rate. It's about three times the metabolic rate that you would expect for an animal of their size. And we think that this high metabolic rate is really because they're trying to generate heat. Being a small mammal living in cold waters, it's really challenging for them to maintain their body temperature.

Phil - When you say metabolic rate, what exactly do you mean?

Tray - Well, metabolism is really just how these animals break down fats and sugars to use them and make energy in a form that they can use. And what our research looked at is the muscle itself. Muscle makes up a big chunk of the body mass of these animals; it uses a lot of energy. So it made sense for us to look at how these animals are consuming energy in the muscle tissue.

Phil - How do you do that? Do you get an actual otter and you put it in your lab somewhere, and you feed it a bunch of stuff - or something different?

Tray - No, what we do is we actually had small pieces of muscle, and so we can test specifically how that muscle metabolises this energy. Here we found that the sea otter muscle was very good at being inefficient. It had a capacity to make a lot of heat, but if your main goal is to stay warm, that's really helpful.

Phil - I don't understand. How is inefficiency good in this situation?

Tray - Well, think of a light bulb. The old incandescent bulbs were really inefficient. They could make the same amount of light as an LED bulb, but they generated a lot of heat and used up a lot of energy to do it. That's a bad thing when it comes to your energy bill, but it's a good thing if you actually want to heat up a room. So that's what these otters are doing: they're metabolising this energy, but they're doing it inefficiently; as a by-product they're heating their body.

Phil - Do you have any idea what specific part of the metabolic process is being inefficient?

Tray - Most of that energy is derived from inside the mitochondria, and the mitochondria pump protons across a membrane. Instead of using that proton gradient to power work, they get protons leaking across the membrane.

Phil - So you're saying it's an inefficiency in this proton pumping mechanism.

Tray - Yes. That proton gradient isn't able to be used to make functional work.

Phil - And does this actually explain their high metabolic rate in the first place, or is this just how they get their heat from their metabolic rate that they already have?

Tray - Yes, this leak metabolism is able to account for this really revved up high metabolism of these animals. And one of the really interesting things for our study was that this metabolic capacity and this leak metabolism were just as high in the neonates as they were in the adults. Typically you don't think of the metabolism as developing until the muscle is really mature, but these guys seem to have a really high metabolic capacity even when they're newborns.

Phil - The other consequence, of course, is the otters must be pretty hungry little folks...

Tray - They are! Just like we talked about earlier with the light bulb, you may have a really high energy bill if you're using this inefficient means of converting energy, and these guys do. They spend about 20-50% of their day feeding and can consume up to 25% of their body mass a day.