September 26, 2013
Monkey See, Monkey Whisper?
Lee Rannals for redOrbit.com - Your Universe Online
Like victims hiding in the closet from a robber, a group of cotton-top tamarin monkeys whisper to each other in the presence of a potential threat.
In a new report published in the journal Zoo Biology, researchers say they witnessed how cotton-top tamarins at the Central Park Zoo in New York lowered the volume of their vocalizations when they feared a predator was nearby.
The team said they conducted an experiment to determine whether the tamarins would produce human-directed mobbing calls when a zoo staff member was inside their enclosure. They predicted that the monkeys would respond with a mobbing response that includes raising their voices and lunging or jumping towards the supervisor. However, what they observed was quite the opposite.
After exhibiting a suite of anti-predator behaviors, the tamarins produced a series of low-amplitude vocalizations that were initially inaudible to the researchers. The team had to refer back to the recording of the incident in order to hear the whispering the primates.
The zoo supervisor who helped with the study had been involved in the monkey's capture and medical procedures, and his presence within their enclosure had elicited a mobbing response in the past. The team expected to record a mobbing response during this current experience as well.
The team recorded sessions 15 minutes prior to the supervisor's arrival, three to five minutes during the presence of the supervisor, and 15 minutes following his departure.
"Behavioral observations of the tamarins' response to the supervisor's presence in all four experimental conditions showed that they did not engage in mobbing nor did they produce loud vocalizations," the authors wrote in the journal. "Instead they performed a suite of anti-predator behaviors previously observed in wild tamarins during potentially threatening situations. These behaviors included piloerection (bristling hair), increased vigilance, cautious approaching, and rapid withdrawing rather than lunging or jumping onto the supervisor."
The tamarins produced chirps and whistles throughout the whole experience, including before and after the supervisor's presence. However, when the supervisor was inside the enclosure, the monkeys whispered their chirps and whistles.
The scientists say this if the first evidence of a whisper-like behavior in cotton-top tamarins. They said that this behavior may be a more widespread phenomenon than previously thought.
Tamarins aren't the only animals other than humans that can whisper. Scientists say that bats are known to whisper, however, a study in 2008 claimed that these whispers are more like shrieks.