Let's be honest: tarsiers look odd. Among the smallest of all primates, most species of tarsier would fit easily in the palm of your hand. They have long, slender, largely hairless tails and elongated fingers with knobby knuckles and mushroom-cap finger pads.

To fully confront the tarsier's bizarre anatomy, you must stare it in the face. It will stare back at you with the largest eyes relative to body size of any mammal—eyes that shimmer in the daylight like peeled grapes. Surely such eyes—each of which is as big as the tarsier's brain—belong on the face of a frog or a squid or an alien, rather than that of a furry tree-climber.

Now, scientists have discovered that the tarsier is even stranger than we realized. Apparently, these tiny primates can send and receive ultrasonic calls, joining a select club of mammals with the same acoustic talent—namely, whales, dolphins, cats, rats and bats. Researchers already knew that tarsiers make at least 15 distinct calls—all of which are audible to people—but until now no one had good evidence that they also communicate with ultrasonic shrieks, although some scientists guessed they might.

As part of a larger study on primate hearing, Marissa Ramsier of Humboldt State University, Nathaniel Dominy of Dartmouth and their colleagues traveled to Mindanao island in the Philippines, where they found and captured six wild Philippine tarsiers (Tarsius syrichta). After sedating the animals, Ramsier stuck small needle electrodes just beneath their scalps and tucked the primates inside a makeshift sound booth constructed from sound-deafening boards lined with a heavy rubber and foam. While the tarsiers slept, Ramsier played tones of varying pitches—just like a hearing test at the doctor's office—and watched their brain activity on a laptop. When she saw a characteristic spike in the recordings of neuronal chatter, she knew the primates had heard the tone she played.

Astonishingly, the tarsiers were able to hear high-pitched tones that no other primates can hear, as far as we know—sounds with frequencies as high as 91 kilohertz (kHz), which are well out of the audible range for people and all terrestrial mammals except some bats and rats. (We call anything above 20 kHz ultrasound.)

Sharon Gursky-Doyen of Texas A&M University, one of the co-authors of the new study, had previously noticed that tarsiers sometimes open their mouths without making any audible sounds. Although it looked a lot like yawning, Gursky-Doyen wondered if the tarsiers were in fact making sounds people can't hear. When she recorded tarsiers with an ultrasound microphone—also known as a ‘bat detector’—she found that they were in fact screeching out of the human audible range. The primates probably produce ultrasonic calls by rapidly opening and closing their tiny vocal cords or by simultaneously expelling air from the lungs and constricting the larynx, a technique called laryngeal braking. If you're feeling courageous, click here to listen to a recording of a tarsier's ultrasonic call slowed down 8X so we can hear it. Warning: this is an unpleasant, high-pitched sound.

Because the tarsiers made ultrasonic calls frequently when being handled by a researcher, Ramsier and her colleagues speculate that some of the calls are alarms. Unlike many other nocturnal mammals, the tarsier lacks a reflective layer of tissue in the eye called the tapetum lucidum, which improves night vision. As such, ultrasonic yelps might come in handy when trying to find friends and avoid foes in the dark. Since the tarsier's main predators cannot hear ultrasonic calls, they also provide private channels of communication. Like many bat species, tarsiers are fond of insects, and it is also possible that high-pitched hoots help them snag a meal. The new study appears in the February 8 issue of Biology Letters.

But, so far, the evidence suggests only that tarsiers send and receive ultrasonic calls. Ramsier hopes to return to the Philippines and secretly record tarsiers in the complete absence of people.