Human activity may be impacting cheetah populations by forcing them to burn more energy -- not during their high-speed chases after prey but in the long walks they now face between hunts, researchers say.

Human-caused factors including loss of grassland habitats, fences that restrict their ranges and people killing animals cheetahs depend on as food sources are forcing the speedy felines into decline, they say.

A precipitous population decrease has seen wild cheetahs in Africa drop from 100,000 around a century ago to just 10,000.

In a study carried in the journal Science, researchers described how they tracked 19 cheetahs roaming wild at sites in Botswana and South Africa.

Injecting the cheetahs with isotope-laced water before releasing them and later analyzing their urine and feces allowed them to measure the animals' energy expenditures, they said.

A surprise finding was how little energy the fast cats expended in high-speed pursuits of prey such as impalas and gazelles, during which they can accelerate to 60 mph for short bursts, the researchers said.

"Our data suggest that they are indeed perfectly adapted to this hunting technique," biologist Johnny Wilson of North Carolina State University says. "They recover pretty fast, so the high-speed hunting doesn't really affect their quality of life."

Their real energy expenditure comes in the long journeys they must make in the search for prey, says study leader Michael Scantlebury of Queen's University Belfast.

"Cheetahs may be Ferraris but most of the time they are driving slowly," in long treks over arid terrain in in oppressive heat with little or no water, he says.

The findings echo those of another study, where researchers in California put tracking collars on four wild pumas to record their energy consumption during different behaviors including hunting, eating and resting.

The pumas, also known as cougars or mountain lions, don't use speed as a cheetah does, rather preferring an energy-saving sneak attack climaxed by a lethal pounce.

And they can precisely match the force of that pounce to the size of the intended prey, wasting very little energy, researchers at the University of California, Santa Cruz, found.

Killing prey requires only a small fraction of the animal's energy, while searching for that prey requires a lot of time and energy, they said.

"The more time that the animals could spend in low-activity level behaviors -- stalking, sit-and-waiting, slow walking -- the greater the relative benefit of the kill, basically less calories expended for obtaining prey calories," wildlife physiologist Terrie Williams said.