Saturday, October 31, 2015

Adirondack Winters: Understanding Hibernation

7-year-old mother Black Bear with cubs in a den under a fallen tree - courtesy North American Black Bear CenterOffhand I can’t think of much to say in defense of envy, greed and gluttony, but sloth is different. The lives of some creatures depend on sleeping for half the year, and I don’t mean adolescents. Survival strategies of bats, woodchucks and other animals include long periods of sloth. Ironically, sloths don’t hibernate.

If hibernation is loosely defined as a period of inactivity and lowered metabolism in warm-blooded animals (endotherms) in winter, then many of us in northern latitudes do it. Of course there’s more to it than that. Turns out that among biologists, the exact definition has been a matter of debate in the past.

It used to be a term reserved for “deep” hibernators whose core temperatures and heart rates drop to a tiny fraction of their summer values. A good example would be certain Arctic rodents that get slightly below 32F. Now it’s applied to any animal that can actively lower body temperature and metabolism. (Actively lowering one’s metabolism sounds like an oxymoron, but let’s not resort to name-calling.)

Cold-blooded animals (ectotherms) like frogs and snakes also become dormant in winter. It’s basically the same as hibernation, except that biologists call it brumation. This is because jargon makes scientists feel better, and we need to humor them so they keep up their good work.

With ectotherms, you could say hibernation happens; they don’t “do” it. Even if they don’t have to work at it like mammals do, their torpor is still impressive. Some frogs, turtles and fish can overwinter in mud essentially devoid of oxygen, and are no worse for the wear come springtime.

Most hibernators modify their schedules according to the weather – if it stays mild into November, black bears and chipmunks den up later than usual. But some critters, known as obligate hibernators, doze off according to the calendar. Even if you took a European hedgehog to Aruba for the winter, it would go narcoleptic at the same time as its mates did back in the Scottish Highlands.

Until recently, bears didn’t make the hibernator list, but now they’re lumped in with those Arctic ground squirrels that turn to popsicles in winter. Bears in the far north may not eat or drink for up to eight months, using stored fat for hydration and energy. If we were inert for that long our muscles would waste away, but they have ways to manage proteins so their muscles don’t atrophy.

In hot climates, summer is the unbearable season, and some animals sit it out by hibernating, except that’s not what it’s called. Naturally biologists coined a word for summer torpor—estivation is the proper term for hot-weather snoozing. Who does this? Some desert-dwelling frogs surround themselves with a mucus “water balloon” to wait out dry spells. African lungfish have a similar trick for when their ponds temporarily dry up.

More surprising is that at least one estivator is a primate, as we are. The fat-tailed dwarf lemur of Madagascar stays in a hollow tree for half the year until the heat’s off. If a close relative of ours can go dormant, then what about us? Science-fiction movies have depicted astronauts waking after years of travel, and this may be another instance where what’s imagined today becomes real tomorrow.

NASA announced in 2014 that they’re looking for a way to place the crews of multi-year space missions into suspended animation for three to six months at a time. Presumably this is so Mission Control won’t have to listen to incessant “Are we there yet?” whining from the back of the spaceship.
Stories of human hibernation abound, but documented cases are rare.

Occasionally someone falls through ice and is revived hours later with no evident brain damage or other long-term effects. This can happen when body temperature drops very fast, as it would if submerged in ice water, or buried in a snowbank as happened to one Canadian toddler. In 2001 she wandered outside on a sub-zero night and was found with her body near the freezing mark, and she recovered with no ill effects.

If body temperature falls slowly, hypothermia usually results, ending in death if continued. Apparently there are exceptions. One instance happened in 2006 when an injured hiker spent three frigid weeks on Mount Rokko in western Japan with no food or water. His temperature had fallen to about 72F. He also made a full recovery.

Scientists will continue to study hibernation for its medical applications. But if you’re not a winter person, don’t pretend to hibernate by being slothful, just grin and, you know, bear it.

Photo courtesy the North American Bear Center.

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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

2 Responses

  1. Evelyn Greene says:

    I just listened to the creator of Monty Python movies on NCPR. This man Paul is way funnier! And so full of the real world out the door. Heff

  2. Wally Elton says:

    Fascinating and entertaining, too. I would have liked a list of North American species that hibernate based on weather and those that go by the calendar (really day length? or some sort of internal clock?)

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