A tarsier could be screaming its head off and you would never know it. Uniquely among primates, some of the diminutive mammal's calls are made up of pure ultrasound.
Marissa Ramsier of Humboldt State University in California and her colleagues were puzzled to sometimes hear no sound when Philippine tarsiers (Tarsius syrichta) opened their mouths as if to call. Placing 35 wild animals in front of an ultrasound detector revealed that what they assumed to be yawns were high-pitched screams beyond the range of human hearing.
While some primates can emit and respond to calls with ultrasonic components, none are known to use only ultrasonic frequencies in a call. The dominant frequency of the Philippine tarsier's ultrasonic call was 70 kilohertz, amongst the highest recorded for any terrestrial mammal. They can hear up to 91 kHz, well beyond the 20 kHz limit of human hearing. Whales, dolphins, domestic cats and some bats and rodents are the only other mammals known to communicate in this way.
Having the equivalent of a private communication channel could help tarsiers warn others of predators such as lizards, snakes and birds which can't detect such frequencies, says Ramsier. Eavesdropping on insects could also help them locate their prey.
Although the tarsier's hidden talent may be unique, future studies could reveal that more primates use pure ultrasound calls. "Many primatologists have observed 'silent' mouth-opening behaviours in other primates," says Ramsier. "It is certainly possible that some of these behaviours are accompanied by ultrasonic vocalisations."
Journal reference: Biology Letters, DOI: 10.1098/rsbl.2011.1149
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