People, meet your new grammar Nazi: a study by Harvard University linguist Ansgar Endress has revealed that monkeys can recognize poor grammar!
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). [...]
When tamarins were exposed to words that "broke" the rules they had learned, they looked toward the speaker in a startled manner, observers noted.
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.