Researchers at Harvard University have evidence that monkeys can understand the difference between prefixes and suffixes, showing that animals may have more capacity to learn language than previously thought. In a study released this month in Biology Letters journal, Ansgar D. Endress and his colleagues tested 14 cotton-top tamarins to see if they could learn the linguistic rule that adds '-ed' to create the past tense, as in transforming 'kick' to 'kicked.' The scientists used the nonsense syllable 'shoy' as a base, then they added prefixes and suffixes such as 'ba, pu, di, ki, lu, ro and mo' and played the monkeys recordings of humans saying these 'words.' One group became familiar with prefixes, such as 'ba-shoy' and 'mo-shoy,' and the other group heard suffixes, such as 'shoy-ba' and 'shoy-mo.' On testing day, the researchers played a recording of words, half of which stuck to the monkey's familiarization pattern and half that violated it -- similar to saying 'kicked' and 'edkick.' When the tamarin turned his head at least 60 degrees toward the speaker, the researchers counted this as a response. The human equivalent would be if someone said, 'The girl edkick the ball,' and the listener cocked his head as if to say, 'What?' The tamarins responded 52 percent of the time after hearing a violation, compared with 37 percent for correct grammar. 'One of the big mysteries is why we speak and other animals don't,' Endress said. He said he 'would be surprised' if the ability to understand prefixes and suffixes were limited to humans and primates. 