Every other Friday, the Outside/In team answers a listener question about the natural world. In this week’s question from Alyssa via Instagram, we consider adaptations different animals make to survive winter:
How slow can an animal's heartbeat get without it dying? So how many beats per minute are needed to keep an animal alive? And how does this change with hibernation and other behaviors?
The size of an animal is generally what determines how often its heart beats, or its beats per minute (BPM). Small animals have a faster breathing and heart rate because they lose heat through their skin faster than big animals do. They need to produce energy and heat at a faster pace and then redistribute it repeatedly through their body as it disperses. For example, the hummingbird has a heart rate of 1,000 beats per minute.
Both large and small animals adapt their heart rates
The largest mammal, the blue whale, has a heart the size of a sofa, and their heart beats have been recorded as low as two per minute. That's the slowest heartbeat of any warm blooded mammal.
Some animals employ an involuntary survival strategy called torpor to survive winter and the lack of food. Not only does the heart rate drop, but so does body temperature, breathing rate and metabolism. In hibernating black bears, torpor causes the heart rate to drop from an average of 55 down to about 14 beats per minute.
Hummingbirds can experience torpor to conserve energy every night, even a cold summer night. They can vary the level from shallow to deep torpor. Their heart rates drop from that super-rapid 1,000 beats per minute to as slow as 180 to 50 beats per minute on a cold winter’s night.
How cold-blooded animals adapt to winter
And then there are animals who can put their heartbeat and respiration on pause all winter without dying. It's called brumation, and it's essentially hibernation for cold-blooded animals.
That's how some frogs, like the wood frog and spring peepers, survive winter. They overwinter by burrowing into mud or leaf litter and stay there, seemingly dead, no breathing or heartbeat. They are able to do this by flooding their cells with glucose which acts like antifreeze in their bodies, preventing the water in their cells from freezing and expanding. While they appear solidly frozen, they are only about 70% frozen.
Come spring, they defrost in about an hour, and hop on their way with a normal heart rate of 40 to 50 beats per minute.
Turtles use this adaptation as well. A turtle's heart beats about 40 beats per minute when they're basking in the sun in the summer. But during the winter, when they're buried in the mud at the bottom of the lake or in the woods, their heartbeat drops to about one beat every 10 minutes.
Did you ever think about snakes in winter?
Snakes are also able to slow down their heart rates to almost nothing. Some snakes use “communal brumation” in a den beneath the frost line. They gather in a clump of hundreds (or even thousands) of snakes, and go dormant, all bundled up together to conserve energy and protect themselves from the cold.
Submit your question about the natural world to the Outside/In team. You can record it as a voice memo on your smartphone and send it to [email protected] or call the hotline, 1-844-GO-OTTER.
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