Bears hardly lose any muscle when they hibernate, say zoologists at the University of Wyoming. Their finding could lead to new treatments for muscle wasting in humans, or ways to conserve muscle tissue during space flight.
Henry Harlow and his colleagues made the discovery after analysing muscle biopsies from sedated bears at the start and end of hibernation.
They believe the bears have evolved a way of conserving muscle so that if disturbed by predators, such as wolves or mountain lions, they could instantly fight or flee.
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Harlow鈥檚 team has been studying black bears (Ursus americanus) in the Rockies for the past four years and the latest findings came from studies on about 10 bears per year.
Deep sleep
By analysing the muscle samples, he found that over the five to seven months of hibernation, the bears lose just 22 per cent of their muscle strength and 10 to 15 per cent of their protein.
Over a similar period, bed-bound humans would lose 85 per cent of their strength and 90 per cent of their protein.
Harlow believes that through evolution, the bears have developed tricks that conserve muscle. 鈥淭hey don鈥檛 eat, drink or pee for 130 days,鈥� he says. 鈥淭he bear reabsorbs urea through the bladder and uses the nitrogen to conserve protein.鈥�
If humans tried the same trick, he says, they would suffer from a severe form of poisoning called uremia.
Hot spot
Readings from small thermometers implanted on the bears show that they probably 鈥渆xercise鈥� by shivering periodically during sleep. Although core body temperature stayed constant, temperatures in the neck spiked around four times a day.
鈥淚t means the bear is showing very vigorous muscle contractions,鈥� says Harlow. He thinks it is a type of shivering similar to repeatedly tensing one鈥檚 muscles.
Harlow and his colleagues track and trace hibernating bears with the help of radiotransmitters previously attached to the bears鈥� collars. He creeps into the caves himself, armed only with a torch and a sedative-containing syringe on a pole.
He says that the black bears are nowhere near as aggressive as grizzlies, and generally try to escape rather then attack. 鈥淣one of my crew have been damaged by a bear, and we鈥檝e probably been in around 300 bear dens,鈥� he says.
Careful aim
The difficult bit, he says, is to aim the syringe into the shoulder. Once the jab has been administered, the bear falls asleep in 15 minutes and can be hauled outside on a blanket.
The researchers take a small biopsy the size of a pea. They also measure muscle strength with a device that is temporarily clamped onto the leg below the knee. They measure muscle strength by stimulating a reflex muscle in the ankle. Then they return the bear to the cave.
Harlow says that the research might reveal ways to prevent muscle wastage in bed-bound people. They might learn, for example, how to periodically stimulate nerves in muscles so that they 鈥渆xercise鈥� by shivering.
More at: Nature (vol 409, p997)
