While researchers have rigorously tested chimpanzee intelligence for years, they have paid far less attention to gorillas.
That's because gorillas rarely use tools, and scientists had assumed the great apes are not as mentally astute.
But ongoing research at the Lincoln Park Zoo in Chicago suggests otherwise.
Four years ago, scientists there attached a touch-screen computer terminal to the side of the enclosure of a female gorilla named Rollie.
As the gorilla approached, it saw the numeral one displayed on the screen. When Rollie touched the symbol, a chime sounded and the machine dispensed a frozen blueberry.
It did not take long for the gorilla to work out that pressing the number had benefits.
After a while, the computer screen presented Rollie with two symbols, the numerals one and two. Through trial and error, Rollie learned to press them in the right order to receive a blueberry.
Last year zoo primatologist Steve Ross reported that Rollie could sequence up to seven numbers at a time, and that chimpanzees at the facility were taking twice as long to learn the sequence.
"Gorillas rarely use tools and have rarely been cognitively studied as a result. So we did not expect them to perform very well at this," Ross said.
Despite Rollie's success, Ross and his colleagues wondered whether the gorilla was just one very sharp ape, or if such intellect could be found in other gorillas.
The scientists started testing other gorillas at their facility. The youngest of the group, a five-year-old named Azizi, is also proving to be a quick study.
So far the male gorilla has only learned to sequence five numbers at a time, but has progressed as rapidly as Rollie.
In Japan similar studies are being conducted with chimpanzees, mandrills, and gibbons. None have made it past the number five.
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