Perhaps more surprising, though, cheetahs spend only about as much energy in a day as a person does, the scientists found.
The studies are the first to calculate the actual, minute-by-minute costs of being a big, wild carnivore—information sorely needed by scientists working to save big cats, says ecologist Terrie Williams of the University of California, Santa Cruz, who led the mountain lion study. (Read "Cheetahs on the Edge" in National Geographic magazine.)
As people alter wild habitats, they may throw this delicate balance off-kilter and make it harder for big cats to make a living, scientists caution.
Cheetahs keep their energy use low, the researchers found, even after accounting for their explosive bursts of speed when taking down prey. (Watch an exclusive National Geographic video of the world's fastest animal.)
In fact, a cheetah's energy use is "remarkably similar" to that of a person who burns an average of 2,000 calories a day, or about 9,000 kilojoules.
"I guess both humans and cheetahs rest a lot to offset high-energy activities," said cheetah-study co-author Johnny Wilson, a biologist at North Carolina State University in Raleigh.
In California pumas, specially designed radio collars—think Fitbits for big cats—show that the ambush predators exert a huge effort to take down prey, but then make up for that the rest of the time by hiding and waiting.
The studies, published Thursday in the journal Science, show that "these cats, in their own individual ways, figured out a way to make hunting cost-effective," Williams says.
"Biggest Bang for Your Energetic Buck"
Sporting more speed than brawn, cheetahs often lose their hard-won meals to more powerful rivals, like African lions. So researchers wanted to learn whether food thieves might be forcing cheetahs to spend energy chasing down more food.
Scientists captured and tranquilized 19 wild cheetahs from wildlife reserves in the Karongwe and Kalahari regions of South Africa. The team injected the animals with water containing harmless isotopes, or chemical varieties, of hydrogen and oxygen. When the cheetahs work hard, their bodies use the isotopes at different rates than when relaxing, so scientists get a measure of their energy use from the water in the cats' feces.
The scientists were surprised to find that cheetahs spend about 9,000 kilojoules a day, less than the 12,000 kilojoules predicted as an upper end, says Wilson.
In comparison, an African wild dog, which is smaller than a cheetah, burns about 15,000 kilojoules a day.
The big cats spent the most energy walking long distances to find food. Habitat development has led to fewer prey animals for cheetahs in South Africa. (Learn more about National Geographic's Big Cats Initiative.)
"If they're going to spend more energy finding food than [they're] getting out of their food, that puts them in an energy deficit," Wilson explained.
When prey is not scarce, cheetahs are able to rest most of the day because they eat high-calorie meals—impalas, mostly.
Wilson and colleagues' finding fits perfectly into an ecological theory called optimal foraging, which says an animal will spend the least amount of energy needed to get the most energy-rich food, says Luke Dollar, a conservation scientist and head of National Geographic's Big Cats Initiative.
In other words, Dollar says, "you want the biggest bang for your energetic buck."
Williams and colleagues used specially designed radio collars to track the American predators' movement and speed. To calibrate the collars, they put captive mountain lions on a treadmill and measured their oxygen consumption, then converted that to energy use.
"People just didn't believe you could get a mountain lion on a treadmill, and it took me three years to find a facility that was willing to try," Williams said in a statement.
The team trapped four wild mountain lions in the Santa Cruz Mountains and found they spent about 10 to 20 percent of their total daily energy taking down prey, which can be up to four times bigger than themselves. (See more pictures of the secretive mountain lion.)
Williams and team have not yet calculated the average kilojoules expended by a mountain lion each day.
Mountain lions will tailor the intensity of their pounce to the size of their prey, the data revealed. A bigger, more energetic pounce will take down a big buck, while a smaller pounce is all that's needed to kill a fawn.
The data are especially valuable because mountain lion attacks are rarely witnessed, Williams says. To get an idea of what the attacks are like, though, Williams recommends the viral video of "Hero Cat," a domestic cat that body-blocked a dog attacking a boy in May.
"Anyone who's seen that video," she says, "will know how a mountain lion will take down a deer."