Beware the black bear – even if it looks asleep

Hibernating bears are more alert than they appear, according to a study showing that their heart rates increase in the presence of a silent intruder even though they appear to be fast asleep.

Scientists have for the first time monitored the heart rates of wild American black bears, Ursus americanus, throughout the year in an effort to understand the extraordinary physiological changes that take place during the winter months.

One of the most intriguing discoveries was that hibernating bears appeared to be aware of humans approaching their winter dens even when every effort was made to ensure that the sleeping bears were not disturbed by any loud noises. Their heart rates increased in preparation for a sudden attack.

"When we retrieved our data, even though we tried to be as quiet as possible, the bears' heart rates increased before we reached the entrance to their winter den and remained elevated for a number of days," said Timothy Laske of Medtronic, an American medical technology company based in Minneapolis.

"This confirms that despite apparent deep sleep, bears are always alert to danger and ready to act... Black bears often make their way into suburban areas which can be dangerous an stressful for both bears and humans. Understanding the silent effect of humans and the environment on bears will also allow better bear management," said Dr Laske, the lead author of the study published in the online journal BMC Physiology.

American black bears are one of the largest animals to engage in hibernation. They spend half the year in this state of prolonged dormancy – without food or water, and without urinating and defecating.

The scientists fitted insertable heart monitors into 15 wild bears and recorded daily cardiac rhythms through the year. During the summer months, when bears are active for up to 18 hours a day, heart rates increased to more than 200 beats per minute – reaching 250 beats a minute when they were near human hunters.

During winter, meanwhile, heart rates fell dramatically from a typical 55 beats a minute to about 14 beats a minute. The bears also showed variations in heart rate with breathing. As the hibernating animal breathed in, heart rate also increased, but fell again between breaths.

When the bears were hibernating, respiration rates rates fells as low as two breaths per minute, and gaps of more than 14 seconds were recorded without a single heartbeat. However, the bears lost relatively little muscle bulk during winter.

The core body temperature of hibernating bears does not fall as low as for smaller hibernating mammals, declining by just a few degrees below the normal level of 36C rather than plummeting to the near-freezing temperatures seen in smaller hibernating species.

However, previous research has shown that hibernating bears experience a 75 per cent decline in basal metabolic rate – the amount of oxygen consumed by their body's tissues – indicating that they experience true hibernation rather than some kind of extended sleep.

Despite the fall in metabolic rates, black bears can rouse themselves quickly and defend themselves within seconds if their den is disturbed, the scientists said. This is necessary because such large animals cannot easily hide from potential predators by burrowing below ground.