The Circulatory System
The circulatory system of polar bears is very similar to that of many other mammals. Just like you and I, Ursus have a closed circulatory system that consists of a four-chambered heart and a network of arteries and veins that are responsible for transporting blood, oxygen and other molecules throughout the body.
Closed Circulatory System in Vertebrate Mammals
Polar bears, along with grizzlies and black bears, are unique in that they can reduce their heart and respiration rates when dormant during the winter. Similar to the physiological changes seen in diving mammals, heart rates can dramatically drop during dormant periods while oxygen consumption can fall to half the normal rate.
In a two-year study, researchers observed heart rates dropping as low as 27 bpm (Folk et al., 1972) compared to a normal resting rate of 46-58 bpm (Øritsland et al., 1977). For additional context, during exercise (walking) this increases to about 148 bpm (Øritsland et al) - meaning during dormancy, polar bear heart rate can be about five times lower than that of an active individual.
*Note how during the month of February, heart rate linearly decreased. This was the period during which observed bears were denning, and researchers believe the heart rate would have continued to fall had their radio-telemeter device not failed on the first day of March.
It's worth noting that there isn't very much research on this particular aspect of the polar bear since it can be tricky to gather relevant data. Polar bears typically den throughout the winter, making it hard to observe and track daily activity, and they don't exactly go into hibernation like other mammals. They are much more active during the winter - in fact, some of them spend the entire winter wandering for food - and this makes it difficult to gather data while the animal is in a dormant state.
In a two-year study, researchers observed heart rates dropping as low as 27 bpm (Folk et al., 1972) compared to a normal resting rate of 46-58 bpm (Øritsland et al., 1977). For additional context, during exercise (walking) this increases to about 148 bpm (Øritsland et al) - meaning during dormancy, polar bear heart rate can be about five times lower than that of an active individual.
*Note how during the month of February, heart rate linearly decreased. This was the period during which observed bears were denning, and researchers believe the heart rate would have continued to fall had their radio-telemeter device not failed on the first day of March.
It's worth noting that there isn't very much research on this particular aspect of the polar bear since it can be tricky to gather relevant data. Polar bears typically den throughout the winter, making it hard to observe and track daily activity, and they don't exactly go into hibernation like other mammals. They are much more active during the winter - in fact, some of them spend the entire winter wandering for food - and this makes it difficult to gather data while the animal is in a dormant state.