This winter really made me think hibernating animals had the right idea. I’d love to just sleep away the six-month cold front, and wake up as refreshed as ever. Now, as great as that sounds, there are a few minor things I’d have to worry about – you know, my heart stopping, bone loss, small things like that. I’m jealous of bears because they don’t have to worry about any of that; they have so many cool adaptations that allow them to hibernate!
Let’s start with their hearts. A recent study published in Physiological and Biochemical Zoology explains changes in Grizzly bear hearts as they hibernate, which protects them from potential complications from snoozing the next six months away. You can read the abstract here and the press release about the study here.
During hibernation, the heart rate of a Grizzly bear slows from an active 84 beats per minute to about 19 per minute. Just for comparison, the average resting human heart rate is about 70 beats per minute. Some extremely fit athletes may be able to get their rate down to the 40s, but this is pretty uncommon. So, 19 heart beats per minute… if this were to happen in a human, congestive heart failure would almost be inevitable. The study explains that such a slow beat causes blood to gather in all four chambers of the heart, and this pressure in humans would cause the chambers to stretch, making the heart weaker and less efficient. Bears can avoid this! According to the results of the study, a Grizzly bear’s left ventricle stiffens to prevent the heart chambers from stretching, and to stop the atrium from working way too hard to force blood into the stiffened left ventricle, the heart beats slower and weaker. Once bears awake from their short nap, this process stops and the heart will go back to beating at its regular pace. This process is in part due to a protein called myosin, which can either be alpha or beta. The alpha version, which researchers found was the dominant form during hibernation, creates quick but weak contractions compared to the beta, causing the heart to beat weaker and slower, and therefore protecting the atrium.
As for bone loss and skeletal damage, bears and other large hibernating mammals can also prevent this! Seriously, bears are really cool. Anyway, this study, published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, shows that while smaller hibernating mammals may lose bone, bears may have evolved in a way to allow them to maintain their bone structure and recycle calcium. You know that phrase “use it or lose it?” Well, that’s pretty applicable to bones, so how do the bears manage to not use it and keep it?
The study suggests that bears can maintain “balanced bone remodeling,” preventing bone loss from disuse. Bone resorption and formation remain balanced, meaning even though the bone is not being used and some bone minerals are being transferred from bone fluid into the blood (resorption), bone formation remains strong. In humans, there’s a pattern: while in childhood, bone formation exceeds resorption, and as aging happens, resorption exceeds formation, which is why osteoporosis is more prevalent in older adults. Researchers say cortical bone is actually more mineralized in hibernating bears than in active bears, meaning proportionately, there is less mineral being lost from bone resorption while hibernating, resulting in bones maintaining their strength. Smaller hibernating mammals have been unable to obtain this adaptation. This balance between resorption and formation may explain why bone structure and strength are not different in hibernating and active bears.