By Helen Briggs
Science reporter, BBC News
Chimpanzees have an extraordinary photographic memory that is far superior to ours, research suggests.
Young chimps outperformed university students in memory tests devised by Japanese scientists.
The tasks involved remembering the location of numbers on a screen, and correctly recalling the sequence.
The findings, published in Current Biology, suggest we may have under-estimated the intelligence of our closest living relatives.
Until now, it had always been assumed that chimps could not match humans in memory and other mental skills.
"There are still many people, including many biologists, who believe that humans are superior to chimpanzees in all cognitive functions," said lead researcher Tetsuro Matsuzawa of Kyoto University.
"No one can imagine that chimpanzees - young chimpanzees at the age of five - have a better performance in a memory task than humans.
"Here we show for the first time that young chimpanzees have an extraordinary working memory capability for numerical recollection - better than that of human adults tested in the same apparatus, following the same procedure."
Dr Matsuzawa and colleagues tested three pairs of mother and baby chimpanzees against university students in a memory task involving numbers.
The mothers and their five-year-old offspring had already been taught to "count" from one to nine.
During the experiment, each subject was presented with various numerals from one to nine on a touch screen monitor.
The numbers were then replaced with blank squares and the test subject had to remember which number appeared in which location, then touch the appropriate square.
They found that, in general, the young chimps performed better than their mothers and the adult humans.
The university students were slower than all of the three young chimpanzees in their response.
The researchers then varied the amount of time that the numbers appeared on-screen to compare the working memory of humans and chimps.
Chimps performed much better than university students in speed and accuracy when the numbers appeared only briefly on screen.
HOW FAST CAN YOU COUNT?
Humans can instantly "see" about four or five dots at a glance - but research suggests chimps are sometimes better than humans at counting in this way.
The shortest time duration, 210 milliseconds, did not leave enough time for the subjects to explore the screen by eye movement - something we do all the time when we read.
This is evidence, the researchers believe, that young chimps have a photographic memory which allows them to memorise a complex scene or pattern at a glance. This is sometimes present in human children but declines with age, they say.
"Young chimpanzees have a better memory than human adults," Dr Matsuzawa told BBC News.
"We are still underestimating the intellectual capability of chimpanzees, our evolutionary neighbours."
Dr Lisa Parr, who works with chimps at the Yerkes Primate Center at Emory University in Atlanta, US, described the research as "ground-breaking".
Dr Matsuzawa and chimps Ai and Ayuma
She said the importance of these primates for understanding the skills necessary for the evolution of modern humans was unparalleled.
"They are our closest living relatives and thus are in a unique position to inform us about our evolutionary heritage," said Dr Parr.
"These studies tell us that elaborate short-term memory skills may have had a much more salient function in early humans than is present in modern humans, perhaps due to our increasing reliance on language-based memory skills."
The research is published in Current Biology, a publication of Cell Press.