Short days and fat bears

What do these animals have in common during the fall months?

A Black Bear scrounging for goodies on the forest floor in the Grand Tetons, WY, in September 2016. Black bears consume prodigious quantities of food before retreating to their dens for their winter “sleep”.
One of my many chipmunk friends that live in the backyard. This one had its cheeks stuffed full of sunflower seeds and peanuts from the bird feeder — probably taking them down its burrow to add to the food stores for the winter.
A female (or juvenile) Yellow-rumped Warbler was chowing down on Juniper berries in late September. The berries are consumed whole, but the bird’s digestive tract will separate seeds from pulp and excrete the seeds, while cleaving off and digesting the waxy coating of the fruit for a few extra calories. High sugar and fat diets help these migrants put on body fat quickly before they migrate.
Not all Robins migrate — some stay here all winter. But those that do fly south seek out the high carb fruits like crabapples and service berries that allow them to build their fat stores up quickly.

The answer to the question above is that all of these (and many more bird and mammal species) exhibit excessive consumption of food in the fall, technically becoming hyperphagic.

There really are only a few viable solutions to surviving the long, cold winters of the far north: 1) get out of town — migrate! 2) build fat stores to last you several months and sleep as much as possible, and 3) stay active to search for what little food remains, tolerate the cold, but enter a starvation state by metabolizing a lot of of your muscle (when you run out of fat).

The temporary condition of hyperphagia is brought on by decreasing photoperiod — i.e., the continually declining number of daylight hours in the transition from late summer to fall — that triggers the change in an animal’s eating habits. Fortunately, this also happens to be when food is most abundant with the ripening of seeds, fruits, excess numbers of young, naive juvenile animals roaming the countryside, etc. So food is easy to come by and fattening is easily accomplished by overeating.

To take Black bears as a good example of this strategy, consider the following comparisons of its diet and caloric consumption from summer to fall.

In the summer Black bears consume about 5,000-8,000 kilocalories per day. If food and water are restricted at this time, they break down their muscles for energy, may accumulate too much nitrogenous waste in their blood, and may die. They cannot “hibernate” at this time.

In the fall, Black bears become voracious, begin consuming 15,000-20,000 kilocalories and drink gallons of water per day, excreting 1-2 gallons of urine as they metabolize all those calories into fat stores. Then, they stop eating and enter a lethargic, hypo-metabolic state of winter sleep, in which their resting heart rate of 80-100 beats per minute falls to less than 22 per minute and their breathing slows down to 2 or 3 times per minute. For the duration of their winter “sleep” they don’t eat, don’t drink, don’t break down any muscle, and females give birth to their cubs. It’s an amazing physiological transformation.

This is Alaskan brown bear #901 from Katmai National Park, a winner of the Fat Bear annual contest for most immense Fall season body mass. Photo from the New York Times, Oct. 2022.

But have you ever wondered if we humans exhibit a similar response to the waning photoperiod and colder days of fall weather? It’s true that humans cannot hibernate the way small rodents do, but could they increase their consumption of carbohydrates and fatten up in the fall and then decrease their daily activity and sleep more in the winter to conserve energy — like bears do? [Side note: carb craving in the fall is a real thing for me — how about you?]

Well, here is the answer, in an article from the New York Times written more than a hundred years ago, back in November 1906. (Click on the image to enlarge it to be readable.)

Apparently, it has been common practice in some cultures (in the past?) that face temporary periods of starvation in winter to prepare multiple loaves of substantially nutritious bread in the fall, prior to beginning a routine of reduced activity and increased bouts of sleeping during long winters. Sleeping with farm animals for warmth was encouraged, I guess.

Drawing from the British Medical Journal May 3, 2000.

4 thoughts on “Short days and fat bears

Please Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.