New clues about how the body retains a constant temperature by muscles that burn energy without contracting were found by researchers.
This new found knowledge may provide new targets for combating obesity.
Traditionally, the body's main thermostat was thought to be brown fat, which raids the white fat stores during cold conditions to burn energy and keep the body warm.
Muscles also play a key role in keeping the body warm by contracting and triggering the shiver response.
But this is only a short-term fix because prolonged shivering damages muscles.
"Our findings demonstrate for the first time that muscle, which accounts for 40 percent of body weight in humans, can generate heat independent of shivering," New Scientist quoted Muthu Periasamy of Ohio State University in Columbus as saying.
Periasamy and his colleagues proved that a protein called sarcolipin helps muscle cells keep the body warm by burning energy, almost like an idling motor car, even if the muscles do not contract by conducting experiments on mice that had their brown fat surgically removed.
All the mice had their brown fat removed, but some of them had been genetically engineered to lack sarcolipin too.
The researchers found that these rodents could not survive when held at 4 degrees centigrade and died of hypothermia within 10 hours.
By contrast, mice that could make sarcolipin were able to survive the chilly temperatures and maintained their core body temperature - despite having no brown fat.
Periasamy also showed that an inability to make sarcolipin made mice 33 percent heavier than normal when fed a high-fat diet, suggesting that idling muscles might also help combat obesity by burning off excess energy.
"The most interesting finding is that mice unable to make sarcolipin are more susceptible to obesity," Andy Whittle of the University of Cambridge, said.
"The research demonstrates that muscle is an important component even in mice, which have comparatively more brown fat than humans. In humans, burning fat in muscle is likely to be even more important for proper energy balance," Whittle added.
The study has been published in Nature Medicine.