The silent scream
In the forests of the Philippines lives a primate the size of a human fist that has evolved a secret channel of communication. The Philippine tarsier (Carlito syrichta) was long considered a quiet animal by scientists. Researchers observed them opening their mouths as if calling out, but no sound could be heard. The solution to this puzzle came in 2012, when a study published in Biology Letters confirmed that these tiny primates communicate using pure ultrasound.
The upper limit of human hearing is about 20 kilohertz (kHz); any sound above this frequency is classified as ultrasound. Field recordings of 35 wild tarsiers on the islands of Bohol and Leyte revealed purely ultrasonic calls with a dominant frequency of approximately 70 kHz. Further auditory tests, which measured brainstem responses to sound, showed that the tarsier's hearing range extends up to 91 kHz. This auditory sensitivity surpasses that of all other known primates and most terrestrial mammals, placing them in a category with bats and dolphins. This form of communication is an evolutionary advantage, creating a private channel that remains undetected by most predators, prey, and competing species.
An anatomy for extremes
The Philippine tarsier is a living fossil, having remained largely unchanged for 45 million years. Its body is a collection of extreme adaptations for a nocturnal and insectivorous life. Each of its enormous eyes is about 16 mm in diameter, as large as its entire brain, but they lack a reflective layer for night vision called a tapetum lucidum. To compensate, the tarsier can rotate its head nearly 180 degrees in either direction.
Its auditory system is similarly specialized. The ability to produce and perceive such high-frequency sounds is rare among land mammals. While other primates may produce calls with ultrasonic components, The Philippine tarsier's 70 kHz call is pure ultrasound, meaning its dominant frequency is entirely above the range of human hearing. This ability likely helps them locate insect prey that may also produce high-frequency sounds. The discovery was made by a team of researchers, including Marissa Ramsier and Nathaniel Dominy, who used specialized microphones and bat detectors to finally record the tarsiers' "silent" calls. Their work showed that what was once mistaken for silence was actually a scream at the upper limits of animal communication.