Sabina, an olive baboon at the Seneca Park Zoo, participates in a study led by cognitive scientist Jessica Cantlon.
Opposable thumbs, expressive faces, complex social systems: it’s hard to miss the similarities between apes and humans. Now 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 our primate cousins.
“The human capacity for complex symbolic math is clearly unique to our species,” says coauthor Jessica Cantlon, assistant professor of brain and cognitive sciences. “But where did this numeric prowess come from? In this study we’ve shown that nonhuman primates also possess basic quantitative abilities. In fact, nonhuman primates can be as accurate at discriminating between different quantities as a human child.
“This tells us that nonhuman primates have in common with humans a fundamental ability to make approximate quantity judgments,” says 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.”
Cantlon, her research assistant Allison Barnard, postdoctoral fellow Kelly Hughes, and other colleagues at the University and the Seneca Park Zoo reported their findings online in Frontiers in Psychology.
The study tracked eight olive baboons, ages 4 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 received all the peanuts in the cup they chose, whether it was the cup with the most goodies or not. The baboons guessed the larger quantity roughly 75 percent of the time on easy pairs when the relative difference between the quantities was large, such as two versus seven. But when the ratios were more difficult to discriminate, say six versus seven, their accuracy fell to 55 percent.
The pattern, argue the authors, helps to resolve a standing question about how animals understand quantity. Scientists have speculated that animals may use two different systems for evaluating numbers: one based on keeping track of discrete objects—a skill known to be limited to about three items at a time— and a second approach based on comparing the approximate differences between counts. The baboons’ choices, conclude the authors, clearly relied on this latter “more than” or “less than” cognitive approach, known as the analog system.
The baboons were able to consistently discriminate pairs with numbers larger than three as long as the relative difference between the peanuts in each cup was large. 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.
A final experiment tested two baboons over 130 more trials. The monkeys showed little improvement in their choice rate, indicating that learning did not play a significant role in understanding quantity.
“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, the authors conclude.
Coauthors on the study include former undergraduate Regina Gerhardt, and Louis DiVincenti, a veterinarian and senior instructor in comparative medicine.
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