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. Although tarsiers are important "living fossils," 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.

Nathaniel Dominy, an associate professor of anthropology at Dartmouth, describes the tarsier's ultrasonic vocalizations as "extreme, and comparable to the highly specialized vocalizations of bats and dolphins, which are used primarily for echolocation."

Dominy and a cadre of colleagues have been studying the hearing and vocalizations of one tarsier species in the Philippines, Tarsius syrichta. Results of their research appeared online in the Royal Society journal Biology Letters, on February 8, 2012.

Some species of tarsier seem rather talkative, with a range of calls audible to humans and capable of "conveying alarm, deterring rivals, and facilitating social interactions," the authors note. In contrast, tarsiers from Borneo and the Philippines have conventionally been described as "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.

Recent technical advances allowed the investigators to test the hearing of six wild tarsiers on the island of Mindanao. They found "an audible range that extended substantially into the ultrasound," reaching a high of 91 kilohertz (kHz), "a value that surpasses the known range of all other primates and is matched by few animals."

They also used a microphone and recording unit capable of registering sounds up 96 kHz. The upper limit of human hearing is generally set at 20 kHz, and frequencies above this limit are classified as ultrasound. In the field, the team recorded the sounds of 35 wild tarsiers from the islands of Bohol and Leyte with this equipment, documenting eight individuals giving out a purely ultrasonic call at approximately 70 kHz. The tone-like structure of the call resembles those of other tarsier species, but none were purely ultrasonic.

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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.

Dominy and his group conclude that there may be selective advantages to vocalizations in the pure ultrasound. They call them "private channels of communication with the potential to subvert detection by predators, prey, and competitors."

"Our findings not only verify that tarsiers are sensitive to the ultrasound, but also that Tarsius syrichta can send and receive vocal signals in the pure ultrasound," Dominy says.

The Tarsier's ultrasonic call

Slowed down to be audible to humans. Warning! This is not a pleasant sound. You may find it uncomfortable to listen to.

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