Perched in a lush, mountainous Madagascar jungle, Jessica Cantlon sat and watched wild ring-tailed lemurs go about their day, leaping around branches and munching on tamarind fruits and leaves. She came to Africa to study female dominance in lemurs—one of only two mammalian species with that kind of social structure, the other being hyenas. She found herself staring at her stopwatch, writing down what the lemurs were doing every five minutes.
Trained as an anthropologist, this kind of work was right up her alley. But from time to time her mind wandered to the thoughts and emotions behind the tick marks she made on her clipboard.
"I became very interested in what kinds of thoughts they were having," she says.
She spent the next year in Rwanda studying the ways multi-male gorilla groups sort out mating rights. Again, she wondered about the minds that drove that behavior.
By the time she returned to the United States, she was hooked on primate psychology. She joined a primate cognition lab at Columbia University and moved to Duke University two years later, where she became fascinated with monkeys' abilities to do math. It's not the kind of arithmetic you learned in grade school, but monkeys have their own system of counting, adding and subtracting.
Now, instead of counting the minutes watching lemurs, she watches monkeys and human children count dots on a screen to find the connections in all primates' mathematical abilities.
"We're very different from other animals—we talk, build skyscrapers, drive cars, use money, make computers," Cantlon says. "But at the same time, we still do some things the same way as animals."
One of those commonalities seems to be the way we quickly come up with rough approximates of numbers. Traditionally, researchers have thought that monkeys might exclusively use size comparisons to judge quantity. If that were the case, a monkey would think that three large dots is a greater quantity than four small dots. But Cantlon wasn't so sure, so she took a monkey who hadn't been trained in counting and gave it a numerical comparison test.
The monkey would see an image with some number of red squares on it, then choose another image that, to the monkey's mind, represented the same thing. Early in the task, Cantlon would show the monkey four large red squares, then have her choose between either another image with four large squares or one with two small squares. In all these tests, the monkey picked the correct image, but so far she could have based her decision either on the size of the squares or the number of them. So Cantlon introduced tests where four large squares would be pitted against two large squares or four small ones. The monkey went for the correct number, not the size, according to a study published in the Journal of Experimental Psychology: Animal Behavior Processes (Vol. 33, No. 1).
"So even without any training, monkeys can single out number as an important property of the world," Cantlon says.
That's not the only mathematical trait monkeys share with humans. In a 2007 study published in Public Library of Science – Biology (Vol. 5, No. 12) Cantlon found that both humans and monkeys are equally bad at adding quickly. In the study, Cantlon set up an experiment that asked both groups to rapidly sum two sets of dots. Then she showed them one of two possible solutions: one with the correct sum of the dots, or one with either two times or three times as many dots. The people and monkeys had to decide whether the solution was right or not. When faced with an incorrect answer that was three times larger than the correct one, both monkeys and humans usually got it right. But when it was only twice the correct answer, both species frequently got it wrong.
Since there wasn't enough time for either species to count all the dots and mechanically add them together, Cantlon says they must both be using some intuitive estimating system.
"If humans were counting to solve the addition problems, we wouldn't see this pattern—they would be nearly perfect no matter how different the choices were," she says. "This tells us that humans have something in common with monkeys in terms of how they think about numbers."
Cantlon's ability to design experiments that can be used with both monkeys and humans may help other researchers study cognition across the species gap, says Francys Subiaul, PhD, director of the Mind, Brain and Evolution Center at George Washington University in Washington, D.C.
"Her work has been influential not just in terms of her novel methods, but also in translating her work to child cognition," he says.
Through these experiments, Cantlon hopes to unearth the fundamental tools that humans and other primates use to deal with numbers. Some of these tools include deciding which of two sums is larger, determining an approximate number of things in a scene, and rudimentary counting abilities.
"Perhaps these kinds of numerical abilities serve as a foundation for the more complex math concepts that modern humans use," she says. "This kind of work gives us information about where we come from."
Don't bore the animals
The origin of how we think resonates in Cantlon's research. Her other interest area, studying the numerical abilities of 3- to 5-year-old children, looks at how humans develop their extraordinary math skills.
"How do we become so smart?" she wonders.
While that question remains yet unanswered, other researchers have a pretty good notion of what makes Cantlon so successful. Her adviser, Elizabeth Brannon, PhD, a primate cognition researcher at Duke University in Durham, N.C., says that she's never seen a student as motivated and passionate as Cantlon. As soon as Cantlon assigns herself a task, Brannon says, she works at it tirelessly until it's finished.
"I had high expectations for her," says Brannon, who first met Cantlon when they were colleagues at Columbia, "but she's surpassed every expectation I've ever had for a student. She's pushing the frontiers of science."
Cantlon says she draws her motivation from the research subjects themselves. They've taught her how to think creatively about designing experiments and to ask provoking questions.
"Monkeys, in particular, are so curious and wily," she says. "If [a task] is too boring, their eyes will kind of glaze over and they zone out. If it's too difficult, they'll attempt to disconnect the machine. But if the task is challenging but doable, they're focused, quiet and super-motivated to work."
Cantlon has adopted the mantra of "challenging but doable" as her research philosophy, keeping an eye on the big questions in cognition while focusing on smaller research goals that pave the way to bigger answers. The greater the challenge, the more Cantlon seems to do, Brannon says.
"She's unique among students to be able to accomplish so much as a graduate student," she says, noting Cantlon's 12 first-author published papers.
Brannon plans on guiding Cantlon through the next step in her career, too: starting her own lab at the University of Rochester in New York. There, she'll broaden her focus slightly, expanding from just studying numerical skills to looking at the origins of conceptual knowledge in primates and preschool children—though she'll always have a special commitment to the origins of math and numbers.
Cantlon says she hopes to benefit from Brannon's own experience opening a lab, and Brannon is more than happy to oblige.
"I'll be with her every step of the way to consult when she needs me," she says, "but ultimately she'll succeed on her own."
By Michael Price