"We were really curious whether monkeys could even detect the common trend found in human language to add sounds to word edges, like adding 'ed' in English to create the past tense," said lead study author Ansgar Endress, a linguist at Harvard University.
Previous research in cotton-top tamarins had shown that the animals can understand basic grammar, for instance, identifying which words logically follow other words in a sentence.
But that same study, published in the journal Science in 2004, found that monkeys did not understand complex grammar, such as when words in a sentence depend on each other but are separated.
While that study suggested monkeys were deaf to complex communication, the new research shows that tamarins can grasp at least one advanced concept: prefixes and suffixes.
For their study, Endress and colleagues played recordings of made-up English words to a population of captive cotton-top tamarins for roughly 30 minutes a day.
Half of the tamarins were exposed to words with a varied stem but a constant suffix (such as bi-shoy, mo-shoy, and lu-shoy). The other half were exposed to a constant prefix followed by a varied stem (such as shoy-bi, shoy-mo, and shoy-lu).
The following day, individual tamarins were brought into an observation enclosure equipped with an audio speaker and video-recording equipment to capture their behavior. These tamarins were then exposed to more words.
Many of the words followed the same language rules that the tamarins had heard the day before, with half hearing "shoy" as a suffix and half hearing it as a prefix.
However, every once in a while, the researchers would play a recording of an "incorrect" word. For instance, the speaker would broadcast "shoy" as a suffix when it had previously been presented as a prefix, or vice versa.
Other biologists who were not aware of the research question were asked to watch and note every time the small mammals turned their heads toward the speaker.
When tamarins were exposed to words that "broke" the rules they had learned, they looked toward the speaker in a startled manner, observers noted.
(Related: "Monkeys Can Subtract, Study Finds.")
The finding is dramatic, Endress explained, because it reveals that our distant cousins seem to have the mental machinery to identify verbal structures like suffixes and prefixes.