Rhesus monkeys are aware of the limits of their knowledge, new research shows. According to scientists at Harvard and Yale, the monkeys realized when they didn't know something and needed outside expertise.

"Metacognition -- the ability to think about our own thoughts -- has long been considered a hallmark of being human," Yale University psychologist Laurie Santos explained in a news release. "We all know the difference between feeling like we know something for sure and feeling like we're not all that certain. We know when we need to Google something."

Previous studies have shown animals exhibit signs of uncertainty when exposed to a novel problem over a period of time, but researchers wanted definitive evidence of metacognition -- proof that monkeys quickly recognize, on the spot, when a new problem requires information they don't have.

To isolate this type of thinking, researchers enticed free ranging monkeys by depositing food into one of two cylinders, arranged in a V shape. When monkeys could see which cylinder the food went into, they immediately rushed to the proper opening to retrieve their treat. When they couldn't, they went to a specific vantage point where the contents of both cylinders could be viewed -- the bottom of the V, where the two openings joined. The rhesus monkeys rarely rushed to the center of the V, where they were unable to tell which cylinder contained food.

The findings show that monkeys are aware of when they have the knowledge they need, and when they need to gain new perspective.

Santos and her research partner from Harvard University, evolutionary biologist Alexandra Rosati, detailed their findings in the journal Psychological Science.

"Our human understanding of when we need more information is such a ubiquitous behavior that we never give it a thought," Santos explained. "When navigating a new city, we know the difference between knowing where we're going and realizing we need a map. When considering grabbing an umbrella, we already know it's raining or that we need to look outside. Our results hint that monkeys have that same feeling of certainty and uncertainty themselves, and it guides their behavior."