You can't hear me scream (Image: David Haring/Getty)
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.
Slowed-down recording of an ultrasound tarsier call
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
http://www.newscientist.com/
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