Hear the ultrasonic scream of this pint-sized primate!

At just five inches long, tarsiers are among the world's smallest primates. And yet they communicate much like humans, using a wide range of calls to communicate. That's why it's so weird that some tarsiers apparently stay completely silent.

Tarsiers are like living fossils — a tiny link back to the ancient beginnings of primate evolution. Though they once roamed the world, they're now only found in the islands of Southeast Asia. The Borneo and Phillippine tarsiers have been called "ordinarily silent", meaning they don't make the same sounds we have observed in all the other species.

Their silence was a mystery to scientists, as their talkative counterparts were using their audible calls to convey some crucial information, such as warning others of danger, telling rivals to keep away, and just generally as a tool of social interaction with other tarsiers. How could some species depend on this communication, while other species apparently go completely without?

Dartmouth anthropologist Nathaniel Dominy set to work on this conundrum, and he soon discovered the answer: these tarsiers were only silent to our ears. They were communicating in ultrasound, meaning they used frequencies that are too high for human hearing. While lots of animals from dogs to fish to insects can hear ultrasonic frequencies, it's much more unusual for animals, particularly mammals, to actually communicate at these frequencies.

Dolphins and bats use ultrasound in their echolocation as part of their navigation, but the Borneo and Phillippine tarsiers are doing something very different. This isn't about creating a natural sonar — these primates actually use ultrasonic frequencies to communicate, and they possess an audible range that the researchers say is far beyond any other primates, not to mention most other animals. While human hearing tops out at about 20 kilohertz, these tarsiers can hear just fine all the way up to 91 kilohertz, and the researchers recorded tarsiers emitting pure ultrasonic screams at 70 kilohertz.

The ultrasonic calls possessed the same tone-like structure heard in the audible calls of the other tarsier species, suggesting they really are using these frequencies for the same sorts of communication. You can hear a recording of one of these ultrasonic calls below, as it's been slowed down to an eighth of its normal speed so that it's audible to human ears. Though I'll warn you now - it is not a pleasant sound.

In their paper, the researchers speculate that these ultrasonic calls effectively provides the tarsiers with a secret form of communication that gives them an evolutionary leg-up on all other animals in the areas:

"Ultrasonic alarm calls can be advantageous to both the signaler and receiver as they are potentially difficult for predators to detect and localize. [The calls are] private channels of communication with the potential to subvert detection by predators, prey, and competitors."

Original paper to appear in Biology Letters. Image and audio courtesy of Nathaniel Dominy.