First example of whispering observed in non-human primates

Scientists have observed the first instance of whispering among non-human primates in a group of tamarin monkeys in captivity in the Central Park Zoo here.
Researchers Rachel Morrison and Diana Reiss of The City University of New York have recorded vocalisations of captive tamarin monkeys and found that when threatened they sometimes revert to whispering to one another to avoid being overheard.
Whispering is a common strategy used by people to communicate with one or more people while simultaneously trying to avoid having others hear, 'Phys.Org' reported.
Other animals have been found to lower the volume of their communications as well under certain circumstances, but never before has any primate other than humans been found to do so.

Researchers were studying cotton-top tamarins, hoping to learn more about the kinds of calls the monkeys make to one another under different circumstances.
Previous research has found that tamarins are capable of vocalising a wide range of noises. Morrison and Reiss were most interested in what are known as mobbing calls - sounds members of a group make to confuse or intimidate predators.

The researchers recorded sounds a group made when a known threat entered the vicinity - a supervisor that had been part of the team that had captured them in the wild.
Before the research, the monkeys had used mob calls whenever the supervisor came into their view.

The researchers did not notice anything unusual as recordings were made, however, later during playback analysis they found the monkeys were whispering to one another.
The animals were engaging in vocalisations that were at such low amplitude that people in the area couldn't hear them at all, the report said.
The researchers said it seems pretty clear from observation that the monkeys were reminding one another of the threat the man posed and were doing it in a way that wouldn't alert the threat to the calls they were making to each other.
The discovery of whispering by a non-human primate likely means that it occurs in other species as well, researchers said.

The study was published in the journal Zoo Biology.