Hibernating bears go through hyperphagia each year to gain fat reserves before hibernation. In a recent study, heart rate measurements were used to compare the metabolic rate between two bear species: the Asian black bear (Ursus thibetanus) in Japan and the Eurasian brown bear (Ursus arctos) in Sweden.

The two species have a very different diet during hyperphagia with black bears feeding largely on fat- and carbohydrate-rich hard masts and brown bears on sugar-rich berries.

Heart rate recorded with leadless loggers
Three female Japanese black bears and four female brown bears were implanted subcutaneously with heart rate loggers. The Japanese black bears were implanted with Star-Oddi’s DST milli-HRT, heart rate and temperature loggers, set to record heart rate and temperature every 10 minutes.
Heart rate was recorded from summer into hibernation, capturing the hyperphagia phase.

Higher heart rate in black bears during hyperphagia
Before the hyperphagic period, black bears had a lower heart rate than brown bears. This changed once the bears entered the hyperphagic period when heart rate increased in black bears from an average 64bpm during summer to an average of 110bpm at beginning of October, almost double heart rate values compared to brown bears. This higher heart rate in the black bears indicates a higher metabolic rate, possibly caused by increased activity.

Plasticity may help bears deal with climate change
Despite the two species being both related and dealing with the same challenges with regards to seasonal food availability, the researchers found the physiological consequences to be different.

This physiological plasticity may contribute to how widely bears are distributed around the world and will hopefully help them deal with changes in climate in the future.

The research was carried out by scientists at Inland Norway University of Applied Sciences, Tokyo University of Agriculture and Hokkaido University. A paper describing the research was published in Biology Letters in January 2019 and can be accessed here.