hibernation is cool

Feb 27 2011 Published by under research blogging

Well, that sucked! Last week was totes crazy. There was much writing and knashing of teeth, but very little sleep. But everything worked out, considering. I have promised myself that I will never again cut deadlines so close. (Alas, it is not the first time I have made such a pronouncement).

Unfortunately, the weather did not cooperate, so I don't have a surprise to announce this week. You all will have to wait until I get another try. It is sort of like a space shuttle launch, except not.

Anyway, when I was digging out from underneath the disaster which is my desk, I ran across this paper:
Tøien Ø, Blake J, Edgar DM, Grahn DA, Heller HC, & Barnes BM (2011). Hibernation in black bears: independence of metabolic suppression from body temperature. Science (New York, N.Y.), 331 (6019), 906-9 PMID: 21330544

Since I thought it was so cool when I read it the first time I figured I'd blog it. Bear hibernation isn't really my field, and this is my first try at the Research Blogging. So let's see how this goes!

Ever since we learned that animals could hibernate, people have wanted to know about how that works. When a mammal is hibernating, its metabolism is really slow (some small mammals can have metabolic rates 90% lower than normal!). And they get cold. Really, really cold. But the cold doesn't kill them. In fact, hibernating mammals are resistant to a whole host of injuries and infections. In hibernation, some small mammals can breath less than 1 time/min, and heart rate can drop to 1-2 beats/min. But these animals don't have massive tissue damage from ischemia and no sign of reperfusion injury? How do they do it? If you could make a person hibernate "on demand" could we keep people from dying when they had a heart attack (or other ischemic damage)?

holy crap! (says the 13-lined ground squirrel)

But, back to the bear paper. Bears don't hibernate exactly the same as their small mammalian cousins. For instance, it has been known that core body temp of hibernating bears doesn't ever drop so low as it does in little ground squirrels. It is pretty hard to study the bears, though. For some reason, it seems graduate students are not lining up to volunteer to go measure the core body temperature of a bear when it is hiding in a small cave.

you want to do what?

The authors of this paper didn't try to go out and find bears when they were hibernating. Instead, they had the bears come to their place for the winter. The black bears were relocated from places where they were a "nuisance" and moved to the Institute for Arctic Biology. When they were moved, the scientists were able to implant little devices that would allow them to remotely measure the core body temperature, muscle activity and heart beat. then they let the bears out into the yard where they had set up several hibernacula-boxes that a bear would find cozy for hibernation and they could measure oxygen consumption, ambient temperature, etc. The hibernacula were also equipped with a web-cam, so the scientists could check in on the animals.

Sure enough, the bears curled up in the cozy hibernacula and settled in for the winter. There were 4 bears, but one was pregnant, which makes it hard to compare to the others. The scientists saw a lot of what you would expect: hibernating bears curled up and didn't move very much. The core body temperature and metabolism dropped. One interesting observation is that the animals didn't let core body temp fall below about 30C (6C or so below summer-time core body temps). Instead, when core temps dropped to around 31 the bear would shiver and breathing and heart rate would increase momentarily.Β  So, even though the bear wasn't regulating temperature "normally" there was still obviously some monitoring and temperature regulation going on.

The most exciting result from this work, though, is that metabolic rate changes independently of core body temperature. It is generally believed, based on studies from those small critters, that metabolic depression during hibernation results mostly from just getting cold. It's called the Q10 effect, which has been described for (in vitro) enzyme activities: for every 10C decrease in temperature, reaction rate slows down 2x. The idea is that if core body temperature drops 10C, then metabolic rate will decrease 2x. The measurements on bears suggest, however, that changes in core body temperature are not driving decreased metabolic rate. Here are the data:

Fig 3: the data money shot

See where I drew that dark black verticle line? That is when you can start to see (approximately) an increase in the core body temperature of the bears (bottom graph). But, at this time the metabolic rate is still low (top graph). By the time the bears emerge - the dashed verticle line drawn by the authors- the core body temp is almost up to normal. But the metabolic rate still has two weeks to ramp back up to normal.

Now, I'm not sure that you can't argue that there are SOME changes in metabolic rate that are happening in between those lines that could be driving the change in body temperature. I, for one, have no idea how much energy is actually required to maintain core temp in a bear. And it would have been super if they measured CO2 output, so that we could have a sense of the respiratory quotient. But the data do show that core body temp makes it back to normal before the metabolism. And that is cool!

some selected references
Metabolic rate depression: the biochemistry of mammalian hibernation
Regulation of body temperature and energy requirements of hibernating alpine marmots (Marmota marmota)
Metabolic Rate and Body Temperature Reduction During Hibernation and Daily Torpor

15 responses so far

  • TheGrinch says:

    I am curious: how much does the core body temperature drop by while bears are sleeping in winter time? I am no biologist; so I am assuming there is something like normal sleep during winter time.

    • gerty-z says:

      Core body temp drops (in bears) by about 6C. This is actually quite a dramatic temperature drop. During non-hibernating times, body temp cycles over the course of the day, but usually only changes by 0.5-1 C. Hibernation has other features that distinguish it from normal sleep. But, there is no consensus about what sleep "is for" or how it evolved-which makes it hard to understand the relationship to hibernation (if there is one).

      • wavebunny says:

        During hibernation animals don't really sleep. Siberian hamsters 'wake up' from their hibernation from time to time to eat and catch up on their sleep. For them, hibernation has some effects similar to sleep deprivation. I have no idea about bears though.
        And gerty-z is right, the function(s?) of sleep are still pretty unclear, so that makes things a bit more complicated (and also really fun imo)

  • Dr. O says:

    So I thought the hibernation title referred to getting some sleep - foiled again!

    But gladly - this was pretty cool! Though I wouldn't want to be the grad student that implanted the little monitoring device in those bears. πŸ˜‰

  • Okay, so I get that the main finding here is that body temp and metabolism are de-coupled. But ...what does that mean? What are the implications?

    • gerty-z says:

      My understanding is that this means that there is a way to control metabolic rate in mammals that is not just cooling down the animal. How animals might suppress metabolism is pretty cool. If you could learn how to do this in non-hibernators then you could, potentially, use it to help people survive during ischemic events-the EMT would "hibernate" the person having a heart attack or stroke, preventing cell death and damage until a doctor could correct the problem. Just cooling can do some (beneficial hypothermia), but cold is damaging, too.

      HOWEVER, the don't do anything that could explain HOW this happens. And it has been known for a long time that metabolic rate drops before temp as animals enter hibernation.

      This study also raises interesting questions about how animals to regulate body temp. How did the bear change from keeping its core at 36 to 30? How did it warm up again without an increase in metabolism?

      • This article is a bit more interesting than I initially thought based on the title...for that, thank you! Since the reason it likely ended up in Science is because of the implications you discuss for applications to humans (otherwise it wouldn't be of broad interest I imagine), I wonder how applicable this may actually be. Given that we do not hibernate, is it reasonable to assume that such mechanisms for metabolic regulation would even exist in humans? I suppose I would be more convinced of the utility of this kind of work if we could induce hibernation/slowed metabolic rate in a mammalian species that does not typically hibernate, like ye' olde laboratory rat/mouse, or even a non-human primate model.

        I suppose this might be difficult to determine without understanding at least some of the molecular or physiological basis for the decrease in metabolism. I don't think I would want to be the one to try and figure this out using a bear model! Perhaps such a high profile finding will spur some research into examining these mechanisms in other more tractable animal models (or perhaps it's already under way?! Like you, this isn't really my bag). Either way, thanks for bringing some attention to an interesting paper!

        • gerty-z says:

          I think that, at this point, the idea of inducing hibernation in humans is still pretty far-out. That being said, there are many examples of people that have survived for a long time with very low metabolism. These are generally people that have accidentally ended up stranded in really cold places (fallen through frozen ice, trapped outside in the winter) and have survived for a long time with no heart beat. This happens frequently enough that in the ER you are not dead until you are warm AND dead. So maybe there is some way for humans to enter into extreme hypometabolic states. But you are right, I think we will need to learn more about hibernation to know how/if it might be applicable.

          As for more tractable models, if you google "induce hibernation mouse" you get this article in the Seattle PI:

          which is referring to this paper in Science:

          Maybe there is hope for us in the (apparently stinky) future. This fellow Roth also gave a TED talk, for your enjoyment:

          • Paul says:

            If induced hibernation can be made to work there are plenty of applications waiting for it in hospitals. If there is one thing that physicians and surgeons can always use it is more time, and induced hibernation may be able to give them that.

            As has been mentioned hypothermia is regularly used to slow metabolism during cardiac surgery, and is now being applied in other settings such as preventing brain damage in cased of oxygen starvation http://speakingofresearch.com/2009/10/09/cool-heads-required/

  • becca says:

    "For some reason, it seems graduate students are not lining up to volunteer to go measure the core body temperature of a bear when it is hiding in a small cave."
    Actually, they are lining up for that. Why do you think attrition rates are so high?

  • brooksPhD says:

    Excellent review! bravo on yur first Research bloggin'


  • Willa Baker says:

    I am curious, I'm looking for three differences between bears and other hibernating small mammals. I already know that core body temperature is a major difference as bears tend to keep their temperatures a lot higher than small mammals, yet there must be more differences right? Is arousal a difference? If so how does it affect core temperature and oxygen differences between the bears and small mammals? Does shivering come into affect in this as well?

    I would love some answers!!!
    Thank you!
    Curious bear lover.

    • gerty-z says:

      You should start at the wikipedia page for "hibernation". There are a lot of refs that seem good - but this is not my area, so I can't answer your question. Have fun learning more about bears and hibernation!

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