Monkey maths! Apes and humans not only share traits such as opposable thumbs, expressive faces and social systems, they also have in common the ability to understand numbers, researchers say.
A new study with a troop of zoo baboons and lots of peanuts shows that a less obvious trait - the ability to understand numbers - also is shared by humans and their primate cousins.
"The human capacity for complex symbolic math is clearly unique to our species," said co-author Jessica Cantlon, assistant professor of brain and cognitive sciences at the University of Rochester.
"In this study we've shown that non-human primates also possess basic quantitative abilities. In fact, non-human primates can be as accurate at discriminating between different quantities as a human child," Cantlon said.
"This tells us that non-human primates have in common with humans a fundamental ability to make approximate quantity judgements," said Cantlon.
"Humans build on this talent by learning number words and developing a linguistic system of numbers, but in the absence of language and counting, complex math abilities do still exist."
The study tracked eight olive baboons, aged four to 14, in 54 separate trials of guess-which-cup-has-the-most-treats.
Researchers placed one to eight peanuts into each of two cups, varying the numbers in each container.
The baboons guessed the larger quantity roughly 75 per cent of the time on easy pairs when the relative difference between the quantities was large.
But when the ratios were more difficult to discriminate, say six versus seven, their accuracy fell to 55 per cent.
That pattern, researchers say, helps to resolve a standing question about how animals understand quantity.
The baboons' choices relied on this latter "more than" or "less than" cognitive approach, known as the analog system, researchers said.
Research has shown that children who have not yet learned to count also depend on such comparisons to discriminate between number groups, as do human adults when they are required to quickly estimate quantity.
The study relied on zoo baboons with no prior exposure to experimental procedures. Additionally, a control condition tested for human bias by using two experimenters - each blind to the contents of the other cup.
"What's surprising is that without any prior training, these animals have the ability to solve numerical problems," says Cantlon.
The results indicate that baboons not only use comparisons to understand numbers, but that these abilities occur naturally and in the wild, researchers conclude.
The study was published in the journal Frontiers in Comparative Psychology.