You can't hear me scream

"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."
  via New Scientist /read more