London, Sep 24 (IANS): Researchers have observed the first instance of whispering by non-human primates amongst a group of tamarin monkeys in captivity in New York City's Central Park Zoo.

Tamarins are squirrel-sized monkeys found in the tropical rainforests of Central and South America. They can live for up to 18 years in groups of up to 40 individuals.

The group of cotton-top tamarins were being observed to study their use of human-directed mobbing calls (signals made by groups of prey to confuse and intimidate predators).

Scientists, however, noticed that when a disliked member of the zoo staff entered the tamarins' habitation, the primates did not make mobbing calls. They instead communicated with one another using "low amplitude vocalisations".

The research showed that "the tamarins were reducing the amplitude of their vocalisations in context of exposure to a potential threat''.

In other words, the primates - like many humans - felt it was prudent to keep quiet when a disliked or dangerous individual was around.

This particular zoo keeper, apparently on the tamarins disliked list, had been involved in the tamarins' capture with the family of five previously exhibiting "a strong mobbing response" in his presence.

The paper, published in the journal of Zoo Biology, describes the tamarins' range of calls as including "whistles, chirps, squeaks, chevron chatters and trills".