How do we know?
↻ One of the pages with a moving picture
Tiny heart monitors went under bears’ skin.
- Bears eat a lot in summer and fall. When high-energy food runs short, they go into a den.
- Their body slows way down: about 1 breath a minute and as few as 8 heartbeats.
- Their body stays warm, above 88°F. They don’t eat, drink or pee all winter!
- They live off their fat, losing up to a third of their weight. Mothers even give birth in the den.
Scientists put tiny heart monitors under the skin of wild bears in their dens. The monitors recorded heartbeats and breathing all winter.
Other scientists watched the heartbeats and body warmth of bears sleeping in a man-made den.
Bears can lose a lot of weight. In Glacier National Park, Montana, bears may lose 20 to 40% of their weight. Most brown bears in Katmai, Alaska, den in October and November.
Wow number
Under 1 pound Newborn black bear cubs weigh under a pound and are born blind.
Myth or fact?
Bears wake up for snacks all winter. Myth. They don’t eat or drink until spring.
A sleeping bear’s body gets close to freezing. Myth. It stays above 88°F.
Think about it
Why might it be safer for cubs to be born in a den?
Where in the world?
Katmai, Alaska, USA where most brown bears den in October and November. In Glacier National Park, Montana, bears may lose 20 to 40% of their weight — 58.5547° N, 155.7937° W.
Where these facts come from
Every number on page 58, and who measured it
This page tells us when bears den and how slowly they breathe in winter.
Hibernation in Bears — The U.S. National Park Service
nps.gov/katm/blogs/hibernation-in-bears.htm
We checked it on 31 October 2026
This scientists' report tells how tiny heart monitors placed under bears' skin recorded their heartbeats and breathing in winter dens.
Monitoring the wild black bear's reaction to human and environmental stressors — A science paper from the University of Minnesota about heart monitors in hibernating bears, shared by the U.S. National Library of Medicine
pmc.ncbi.nlm.nih.gov/articles/PMC3177774
We checked it on 31 October 2026
This news story tells how scientists watched the heartbeats and body warmth of bears sleeping in a man-made den.
Hibernating Bears Slow Down More, Cool Down Less — PBS NewsHour, a U.S. public TV news show
pbs.org/newshour/science/hibernating-bears-slow-down-more-cool-down-less
We checked it on 31 October 2026
This page tells us how much weight bears lose and that cubs are born in the den.
"Hibernation-Migration-Fascination" Narrative — The U.S. National Park Service
nps.gov/glac/learn/education/hibernation-migration-fascination-narrative.htm
We checked it on 31 October 2026
This page tells us how tiny black bear cubs are when they are born.
Black Bear Species Profile, Alaska Department of Fish and Game — The Alaska Department of Fish and Game
adfg.alaska.gov/index.cfm?adfg=blackbear.main
We checked it on 31 October 2026
Has this changed?
Why bears sleep all winter is not going to change.
What changes is how much we can measure. Tiny heart monitors now record a bear’s whole winter.
Words from this page
- den
- A sheltered hole where an animal sleeps or raises its young.
- hibernation
- A long winter sleep in which an animal’s body slows way down.
Try it!
Count your breaths in a minute.
1. Sit still and relax. 2. Have a grown-up time one minute. 3. Count every breath you take in that minute.
What happens: you probably counted many breaths. A hibernating bear takes about one!
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