That's Not a Yawn. It's a Scream Humans Can't Hear.

http://youtu.be/BTN2W-uZzLk The tarsiers of the Philippines are the smallest primates on the planet, at about five inches tall. They tend to keep their hind legs, which are twice as long as their bodies, folded up frog-style, except when leaping on their insect prey. And a tarsier eyeball, at just over half an inch wide, is as large as a tarsier brain. But the weirdness doesn't stop there. No, it most certainly does not. Scientists had previously remarked that tarsiers were unusually quiet. And they also seemed to yawn quite a lot. Aww, cute, right? Sweepy wittle pwimates! But then, some scientists studying tarsiers made a startling discovery. Zoe Corbyn at New Scientistsums it up well: "Placing 35 wild animals in front of an ultrasound detector revealed that what [the scientists] assumed to be yawns were high-pitched screams beyond the range of human hearing." Turns out tarsiers are shrieking their brains out while their predators in the jungle, including birds and snakes, obliviously goes about their business. (And if you were already freaked out by them, as many YouTube commenters on the above video seem to be, we apologize for adding to the creepy.) It seems like a pretty handy, if eerie, adaptation: tarsiers, which are tasty little prey animals for many jungle dwellers, can probably communicate without other creatures noticing. Are there other primates out there that use ultrasonic screams to communicate? The lead researcher isn't sure, but she is raring to find out, she told ScienceNOW: "I want everyone to go out with their bat detectors." Roger that.