The Philippine tarsier, a small nocturnal animal, has the world’s highest pitched primate vocalization ever documented.
"Tarsiers are among only a handful of mammals that are known to communicate in the pure ultrasound,” lead author of a paper in the Royal Society's Biology Letters, Marissa Ramsier, told Discovery News. "No other primate is known to produce and detect signals as high as the tarsier."
Ramsier, co-director of the Biological Anthropology Research Lab at Humboldt State University, and her colleagues, made the determination after studying six tarsiers that were captured in the vicinity of Motorpool, Surigao del Norte, Mindanao, Philippines.
To estimate auditory sensitivities, the researchers first used a minimally invasive brainstem response test. This showed that the primates can hear in the ultrasound range, up to 91 kHz. For comparison, humans have a high-frequency detection limit of only about 20 kHz.
Next, the scientists recorded vocalizations made by the tarsiers. In some cases, the tarsiers were opening and closing their mouths, looking like they were communicating, but no sound was heard. Sensitive high tech recording equipment revealed that the primates were indeed communicating, but in ultrasound frequencies.
Philippine tarsiers join a select group of mammals that have this ability. The group includes certain bats, rodents, cetaceans and even domestic cats. Ramsier explained that "kittens produce a pure ultrasonic call from about 2-6 weeks of life when they are first exploring their environment, and a mother cat produces its own purely ultrasonic call in response to the kitten." Cats at these times of life can therefore communicate in ways not detected by their owners, "unless they follow them around with a bat detector."
All of these ultrasound-producing animals can then communicate within their own private "channel," which could prevent detection by predators, prey and competitors. It could also enhance energetic efficiency and improve detection against low-frequency background noise.
Humans, on the other hand, may have no evolutionary advantage to producing and detecting ultrasound.
"Humans are big and noisy," Ramsier said. "If a predator is nearby, it will likely see and hear us."
Conversely, tarsiers are small and active during the night, so it is relatively easy for them to use masked vocalizations as a covert strategy. Ramsier thinks they do this by constriction of their larynx and rapid opening and closing of their vocal chords. They probably have special auditory features that enable hearing of ultrasound.
Chris Kirk, an associate professor in the department of anthropology at the University of Texas at Austin, told Discovery News that this "study is important because it expands the number of primate species that concentrate a large part of the acoustic energy in their vocal communications within the ultrasonic range."
Kirk continued that one of the documented tarsier calls look to be a classic "ventriloquial" call, meaning a vocalization that conceals the location of the sender.
"In fact, it looks an awful lot like the 'seet' alarm calls or (those of) passerine birds, but scaled up to a higher frequency range," he said.
Ramsier and her team also suspect that the tarsier ultrasound calls are alarms, especially since the little primates only emitted the vocalizations when they were near humans during the study.