Researchers have discovered the first example of whispering in non-human primates. Cotton-top tamarins (Saguinus oedipus) were observed to whisper when threatened or when a perceived threat was around.

Researchers Diana Reiss and Rachel Morrison, from Hunter College of the City University of New York, were observing the primates at New York’s Central Park Zoo and researching the “human-directed mobbing calls” of cotton-top tamarins. The behavior, loud shrieks and aggressive behavior, was directed at a supervisor the monkeys perceived as a threat. Reiss and Morrison would record the behavior of the monkeys when the supervisor entered their enclosure and discovered that, instead of mobbing calls, the primates displayed whisper-like behavior.

The research was published in the journal Zoo Biology and is the first instance of whisper-like behavior in a non-human primate. According to the abstract, “A post-hoc analysis of the data was conducted to test a new hypothesis — the tamarins were reducing the amplitude of their vocalizations in the context of exposure to a potential threat. Consistent with whisper-like behavior, the amplitude of the tamarins' vocalizations was significantly reduced only in the presence of the supervisor.”

The whisper-like behavior was a series of chirps that were inaudible to humans, reports National Geographic. These chirps were meant to signal, and coordinate, a response to a perceived threat. The researchers discovered this behavior by analyzing spectrograms, a visual graph of sound, and they said the observed behavior in tamarins may also occur in other social primates. “Low amplitude signaling” could be an effective communication strategy and would prevent potential enemies from listening in, giving the animals an advantage over a predator.

The researchers conclude, “Increasing reports of subtle communication in diverse species suggests that whispering may be a more widespread phenomenon than previously thought. Ironically, whisper-like behavior in non-human primates may have eluded prior detection due to its inherent subtlety.”

You can listen to the tamarins whisper to each other, courtesy of io9, below. Reiss is an expert in animal intelligence and had previously studied the signaling behavior of bottlenose dolphins as well as self-recognition in Asian elephants and bottlenose dolphins. She was recently involved in a TED Talk on using the Internet to communicate with animals.