Tarsius syrichta, otherwise known as the Philippine Tarsier, belongs to an ancient lineage of primates that gave rise to monkeys and apes about 60 million years ago. For the past 45 million years tarsiers have been largely unchanged, but their size and shyness means that they are difficult to study in the wild. Scarcely five inches high, they are nocturnal and subsist mostly on a diet of insects, along with some small vertebrates such as lizards and snakes.
Other species of tarsiers are known to communicate fairly complex information via distinct types of calls, but scientists have long been puzzled as to why species like T. syrichta appear to be mostly silent. While scientists suspected that the mammals might communicate via ultrasound, this was very difficult to test because tarsiers are endangered animals which are difficult to maintain in captivity.
However, an international team of scientists led by Professor Marissa Ramsier of Humboldt State University used an innovative recording technique to listen to tarsiers in the wild. They were astonished to find that the pint-sized primates were using an extraordinarily high frequency of ultrasound to communicate. Very few mammals are able to communicate using such high frequencies and this is the first time that ultrasound communication has been observed in a primate.
The researchers observed that tarsiers emitted their ultrasonic call when humans were near, suggesting they were using their high-pitched calls as private alarm calls. “Ultrasonic alarm calls can be advantageous to both the signaller and receiver as they are potentially difficult for predators to detect and localize,” the researchers write.
Listen to the Tarsier's call, slowed down to be audible to humans - please note that it is not a pleasant sound and some people may find it uncomfortable to listen to. Web users are advised to ensure that the volume of their speakers / headphones is at an appropriately low level.