From “Tiny Primate Is Ultrasonic Communicator” (ScienceDaily, Feb. 8, 2012) , we learn,
Tarsiers are pint-size primates from Southeast Asia who produce some of the most extreme ultrasonic calls in the animal kingdom, well beyond the threshold of human hearing.
They belong to a relict lineage of primates that gave rise to monkeys and apes about 60 million years ago, and for the past 45 million years tarsiers have been largely unchanged.
It was unclear how the tarsiers of Borneo and the Philippines communicated because they were “ordinarily silent.”
This apparent lack of vocalizations led investigators to suspect that the animals were indeed engaged in these critical communications. We just couldn’t hear them.
So researchers decided to test them using audio equipment:
The researchers observed that tarsiers emitted their ultrasonic call when humans were near, suggesting they were voicing alarm. “Ultrasonic alarm calls can be advantageous to both the signaler and receiver as they are potentially difficult for predators to detect and localize,” they write.
Or actually difficult, as in the case of human researchers, who never even discovered it until now.