How animals hibernate

We traditionally regard us mammals as “warm blooded”, but some among our numbers have adapted to allow their body temperatures to periodically plunge, dramatically cutting their metabolic rate and thus making energy reserves last much longer. This, of course, is the basis of hibernation. But, as Christopher Lewis, working at the time at the University of Copenhagen, has found, it’s more complicated than just dialling down the temperature. Speaking with Chris Smith, he’s found that myosin in muscles can make heat just in response to becoming cold. But when very small animals - like the dormice he studied - want to hibernate, they suppress this process by putting their myosin into an altered configuration that doesn’t get hot when it’s cold. Bigger animals like bears, on the other hand, with far more energy reserves, can afford not to do this and they stay warm all the time…

Christopher - Hibernation is a unique phenomenon which certain mammals take part in every year. And it's where they will shut their body down for six months to eight months of the year in order to survive, to survive during harsh winter periods where there's little food. And really that's quite an incredible feat to be able to effectively sleep for six months. Not eat, not go to the toilet, not drink any water and actually remain healthy! The amount of energy that your body uses really shuts down.

Chris - Do all animals hibernate equally? As in if I compare a tiny mammal with say a big brown bear, is hibernation the same or are we lumping together different behaviours that look sort of similar, all under the same umbrella, but there are differences?

Christopher - Yes, there absolutely are differences. And this is something that we, we look into with the paper and, and what you find is with larger animals such as bears - in this manuscript we looked at two types of bears, brown bears and black bears - they hibernate at quite normal temperatures. Their body temperature doesn't really drop very much, maybe by a few degrees. You then have smaller mammals and they will hibernate at very, very different temperatures. Actually, their core body temperature goes from 37 down to about four degrees. And this we think is very important in actually how these animals hibernate. So this is why in this paper we looked at large hibernators but also small hibernators.

Chris - And when you say you looked at them, what did you actually study?

Christopher - We are really interested in skeletal muscle. And the reason we're interested in that is because skeletal muscle makes up a large amount of our body mass. It's a very large organ or set of organs, tissues within all mammals. And therefore it uses a lot of energy. So it's particularly important in hibernation when you're trying to reduce how much energy that you use. So what we did was we took small biopsies from each of these animals whilst they were both hibernating and active. And we were able to dissect each single muscle fibre and we were able to observe how much energy each fibre was using whilst either they were active or they were hibernating.

Chris - How do you know how much energy they use?

Christopher - We actually put them under a microscope, insulate the fibre with a fluorescent energy or ATP molecule. Because it's fluorescent, it'll make this muscle fibre glow. And therefore what we can do is we can effectively video this muscle fibre over the course of five minutes and we can watch how quickly it uses the energy by how quickly the fluorescence of the muscle fibre decreases. The quicker that the glow disappears, the more energy the muscle fibre is using.

Chris - And what's actually doing the energy consumption? Where's the energy going?

Christopher - Yeah. So what we were really interested in in this study is something called myosin. It is the motor unit which allows our muscle to contract. It effectively uses energy directly and it moves in order to allow the muscle to move. However, what has been recently discovered is that even when you're resting, so when you're sleeping for example, or hibernating, your muscle still uses energy in the same manner. However, this is very variable and the myosin conformation is able to very much dictate how much energy is being used by your muscle when you're resting. So what we were interested in, of course, was monitoring this in hibernation to see if there is a change in myosin in animals which are hibernating.

Chris - And also, can I preempt one possible other question that must have occurred to you? Yeah. Which is when you've got these big animals and small animals and the big ones stay warmer and the small ones allow themselves to get a lot colder, do they have a difference in how much energy their myosin is is burning off at the time in order to contribute to that?

Christopher - Yeah, exactly. So that was a key question for us in this, in this study, what we did is we did these experiments at different temperatures to try and mimic the kind of environment that these animals would be experiencing during hibernation. And what we found was that when we cooled down the temperature of the muscle fibre, the amount of energy that it used actually went up. And the reason why we think this is, is because it's kind of similar to shivering. When you're cold, your muscles start to shiver. And this actually uses energy to produce heat. So what we think we found in this paper is a new way which your muscle uses energy to produce heat. And what was particularly interesting in these small mammals which are very cold, whilst they hibernate, when we cooled down the hibernating muscle, it didn't increase the energy which it was using, which is almost certainly very important for its survival to allow it to maintain a cold body temperature.

Chris - And that's the key distinction, I suppose, isn't it? That means these animals that are smaller do allow themselves to get much colder. Because, what, do they stop their myosin doing this, or do they have a different form of myosin that means they're not able to produce heat in this way?

Christopher - Yeah. So they stop their myosin from doing this. The myosin undergoes changes to its structure and this stops it from using energy. And this only happens in the animals, which were cold and hibernating. So it's, yeah, as I said, it's definitely a kind of new novel way of we've, I think, discovered of how animals can produce heat via using energy in their resting muscle.

Chris - Now 1,001 other questions exist though, don't they? Because upstream of this is still whatever the trigger is that says to the animal, turn this process on or change your myosin in this way. Is it just temperature in its local sensation of the muscle or is there some sort of central trigger that the animal releases to make this happen?

Christopher - I think it's a really good question because there are so many triggers which occur both on the onset of hibernation in these animals, but also before hibernation. So for example, before they go into hibernation, they start to eat a lot of high fat foods to effectively store energy that actually can affect the signaling pathways within the muscle and the fat. And this could actually potentially be one of the triggers which is telling the myosin to start to behave differently in preparation for hibernation. Another way it might be via the brain appetite is controlled for the brain. So it is possible there's a crosstalk between the, the kind of central nervous system and the muscle for the onset of hibernation to make myosin enter this kind of a different state during hibernation. But that is what we don't know and what we would very much like to know because we think it'd be very important.