The first example of whispering by non-human primates has been observed amongst a group of tamarin monkeys in captivity in New York City’s Central Park Zoo.
The group of cotton-top tamarins were being observed in order to study their use of human-directed mobbing calls (signals made by groups of prey to confuse and intimidate predators).
However, scientists observed that when a particularly disliked member of zoo staff entered the tamarins’ habitation the primates did not make mobbing calls but instead communicated with one another using “low amplitude vocalizations”.
The research concluded that “the tamarins were reducing the amplitude of their vocalizations in the 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.
The zoo keeper in question had been involved in the capture of the tamarins, with the family of five previously exhibiting "a strong mobbing response" in his presence.
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.
Cotton-tail tamarins (which typically weigh around 0.5kg each) have been extensively studied, with the research describing the species as “highly cooperative and vocal” with command of “a repertoire of call types [used] in contexts of feeding, group cohesion, investigation, mild alarm, and high arousal”.
The paper, published in the journal of Zoo Biology, describes the tamarins' range of calls as including “whistles, chirps, squeaks, chevron chatters, and trills”.
The primates under observation were cotton top tamarins, a species that is known to have a range of anti-predator tactics including “alarm calling, freezing, escape, and mobbing”. However, this was the first time that whispering has been observed in any non-human primate.