For all of their hollering and shrieking, it turns out some monkeys know how to be quiet too. Science World Report notes on Sept. 26, that cotton-top tamarin monkeys have the ability to whisper and they use that ability to avoid being overheard.
Researchers were originally attempting to know more about the wide range of sounds the monkeys could made. In one particular test aimed at testing their alarm call, a distrusted zoo employee approached the moneys, but rather than sound a loud alarm, they didn't seem to be making noise at all. However, when psychology researchers Rachel Morrison and Diana Reiss listened to the recordings, they learned the animals had lowered their voices to a whisper-level volume, or low-volume vocalizations
Phys.org reports that though scientists don't know what the monkeys were saying, it's likely they were communicating the threat of the zoo employee and wanted to do it in a way that wouldn't alarm the man or draw attention to themselves.
This finding points at humans not being the only primate with the ability to whisper and to the fact that it is likely that this behavior can occur in other species, but has gone unnoticed. This also shows that these monkeys have the ability to use whisperin to avoid being overheard, which is similar to the way humans it.