Ask Sam: Why Do Bears Hibernate When They Could Eat Trash All Winter?
Every other Friday on Morning Edition, Outside/In host Sam Evans-Brown tackles a question from a listener.
Jahleah asks: I have two questions for you. Number one: How do bears hibernate in New Hampshire? Number two: do they even need to hibernate since there’s trash everywhere?
Oh man, there are so many fantastic hibernating bear facts that I am simply not going to be able to get to them all. I learned them all by talking to Heiko Jansen, a professor with the Washington State University Bear Center.
Now to start out we need to settle a debate: do bears even hibernate?
This is a matter of great contestation between bear and rodent biologists. So called “true hibernators” — creatures which sleep so deeply they cannot be woken — include groundhogs and bats, but not bears. However, bears do in fact go into a den and spend all winter there and don’t eat and sometimes also don’t drink.
Technically they are in a state called torpor. “They’re not in this state of almost unconsciousness,” explains Jansen, “They’re very alert, and people have found this out the hard way. They’ve stepped into a den and been attacked by a bear.”
However, he maintains this is a question of semantics. If you spend all winter in a tiny den not eating and mostly sleeping, what’s that if not hibernating?
To get ready for the winter bears undergo hyperphagia, which as far as I can tell is latin for hungry all the time. Grizzly bears can gain up to ten pounds a day during this period. (Shout out to Holly, our #fatbearweek champion this year.)
On the flip side, they lose a quarter of their body weight or more over the course of the winter. In polar bears, this can be more than a pound a day.
OK, But Here Comes the Crazy Stuff
It is still something of a mystery how exactly, biologically, bears survive their long fast.
“They don’t have any bone loss. They don’t have any loss of muscle mass to speak of,” explains Jansen. “So all these things that would happen to you if you were bed ridden for any length of time, or if you were in space as an astronaut. Now, the bears are somehow able to handle all of that without coming out the other end unable to walk, or breaking bones as they try.”
Insanely enough, we know one key ingredient in this mystery is urine. Bears hardly pee all winter, which to us would be toxic, but bears are actually able to recycle their urea and turn it back into amino acids, which their body uses to make proteins.
Not yet impressed with bear biology?
How about this: they essentially become type II diabetics all winter —Jansen says it might be in order to preserve all the glucose floating in their blood for use by the brain — but in the spring they switch back to a normal metabolism. That would be a neat trick to be able to reproduce in diabetic humans, right?
Bear Facts Lightning Round
Ok, all this bear awesomeness is running long, we've got to pick up the pace:
- Bears mate in the spring, but the fertilized eggs don’t develop until the fall, and the amount of fat the females are able to put on determines how many embryos grow into a fetus. The others are simply reabsorbed into her body.
- Mama bears actually give birth while they hibernate, produce milk and nurse young ALL WITHOUT EATING, which is why female bears lose more weight over the winter than males.
- Bears’ blood pressure drops so much when they hibernate that sometimes they just stop using one of the chambers of their heart… kinda like a Pinto running on 3 cylinders.
All of these biological changes kinda look like super powers. “I just simply say, you know… bears are from Mars,” jokes Jansen.
Now, to answer the listeners adorable final question, would a bear ever not hibernate because they have so much access to trash?
Hypothetically, Heiko says, the answer to that question is yes. In fact, there are black bears in the South that don't hibernate, because they’ve got enough food year round.
But it’s unlikely.
“I think they consider it lucky to find a resource like a garbage can,” he says, “But I think they’re ‘smart’ enough to know this may not last and is probably not going to be something that can sustain them through the winter.”
In other words, bears in northern climes have hundreds of thousands of years of evolution whispering in their ears that they should fatten up and den down, because the food may soon disappear. It will take more than a couple juicy garbage cans to convince them to change their behavior.
Sam Evans-Brown, is host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to firstname.lastname@example.org, OR call the hotline, 1-844-GO-OTTER, OR submit it here.