If brain size dictates self-control, what does that say about America, the second most obese nation in the world? Unfortunately, scientists didn't broach that topic. But they did undertake a series of experiments that suggests if squirrels had easy access to Big Macs and extra-large fries, they'd probably be even fatter than we are.

That's because the smaller an animal's brain is -- in total volume, not volume relative to body size -- the less self-restraint that animal possesses. A team of researchers from Duke University, UC Berkeley, Stanford and Yale were able prove this by putting a range of animals through a series of tests that measured intelligence and impulse-control.

First, animals were taught different tricks for earning a hidden food reward. Next, the test subject was presented with a farther away and potentially less accessible -- but visible -- food reward. Animals that went for the obvious -- but not necessarily attainable -- treat, instead of relying on their newly acquired intelligence, were deemed to have less self-control.

Each test, for each animal, was slightly different, but the basic idea (or question) was the same: does the animal have the necessary self-control to allow intelligence to trump impulse under changing circumstances? For fox squirrels trying to retrieve food from under a series of plastic cups, the answer was mostly "no."

"About half of the squirrels and gerbils did well and inhibited the direct approach in more than seven out of 10 trials," explained UC Berkeley doctoral student Mikel Delgado. "The rest didn't do so well."

Scientists tested a variety of species at their respective universities, including: bonobos, chimpanzees, gorillas, olive baboons, stump-tailed macaques, golden snub-nosed monkeys, brown, red-bellied and aye-aye lemurs, coyotes, dogs, gray wolves, Asian elephants, domestic pigeons, orange-winged amazons, Eurasian jays, western scrub jay, zebra finches and swamp sparrows.

The conclusion: animals with larger brains, like chimps, have much better self control than those with small noggins, like rodents.

"The study levels the playing field on the question of animal intelligence," said UC Berkeley psychologist Lucia Jacobs, co-author of the self-control study, which was published this week in the Proceedings of the National Academy of Sciences.

The study was organized and led by evolutionary anthropologists Evan MacLean, Brian Hare and Charles Nunn of Duke University.