Grizzly bears can hibernate, and I'm jealous. Grizzlies eat to excess and fatten themselves up without health problems and, for all their gluttony, they're rewarded with half a year of rest. I'd love to use hibernation as an excuse for my overeating. While I barter with my landlord to keep my apartment warm, grizzly bears bask in the cold: they reduce their metabolism and temperature so they can save energy throughout the winter. They've turned being lazy into a superpower, so of course I want to know how they do it. Luckily, a recent study by Hannah Hapner Hogan and colleagues at Washington State University, USA, sought to answer this very question. When grizzly bears hibernate, how do their cells know to slow down and avoid disease? Is their metabolism slowed because of something in the blood?

Hogan and colleagues started by taking fat cells from grizzly bears (Ursus arctos horribilis) – some hibernating, some awake – and growing them in dishes. Then, they sampled blood from bears in and out of hibernation. This allowed the researchers to mix and match the fat cells with different blood serum samples from awake or hibernating bears to grow ‘awake’ or ‘hibernating’ cells. Was serum from hibernating animals enough to make awake cells burn less energy, like the cells of hibernating animals? It turns out that it is. When fat cells that had been collected from active bears were grown in serum from hibernating animals, they had lower metabolisms; they consumed less glucose, they respired less oxygen and they produced less energy (ATP). Serum from active animals had the opposite effect: it kick-started the metabolism of hibernators, increasing glucose and oxygen consumption to make more energy. It turns out that blood serum had a strong effect on cellular metabolism.

Blood serum also influences where hibernators get their energy from. Cells have two favourite foods: glucose and fatty acids. Hogan and her team found that active cells favoured glucose much more than hibernating cells; hibernating cells preferred fat. This makes sense; bears fatten themselves up before hibernating, so it follows they would use this fat.

But the researchers also found that hibernating cells were picky eaters. They gave cells insulin, a molecule which tells cells to absorb and use glucose. Active cells readily responded to insulin by eating more glucose. But when given insulin, hibernating cells stubbornly continued prioritizing fats. Moreover, serum from the opposite season could flip these results: hibernating cells responded more to insulin when they were kept in awake serum, and awake cells resisted insulin while in hibernating serum. The Washington State team noted just how fascinatingly different bears are from humans. Bears fatten themselves up and ignore insulin without issue. But if we humans gained so much fat within a few months and developed insulin resistance, we would suffer diabetes.

The team's work tells us that the profound drop in metabolism during hibernation is due at least in part to the blood instead of some intrinsic mechanism inside cells. As a next step, Hogan and colleagues are analyzing blood proteins to learn exactly what causes the drop in metabolism and insulin sensitivity. Imagine what secrets bear blood could contain! I can picture an EpiPen-like autoinjector – a SleepiPen – capable of inducing hibernation on command, whether to buy time during medical crises or to extend the shelf life of organs for transplant. But for now, I'm left to wonder what magic bear blood contains, and to dream of a cozy, grizzly future when science will help me rest through winter.

H. R. H.
B. D. E.
C. T.
H. T.
Changing lanes: seasonal differences in cellular metabolism of adipocytes in grizzly bears (Ursus arctos horribilis)
J. Comp. Physiol. B.