Archive for the 'research blogging' category

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.  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

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