A saucer-eyed Philippine tarsier opens its mouth wide, squints, and lets out a great burst of . . . silence. The gape-mouthed expression of these primates has long been considered an act of yawning or stretching. But Sharon Gursky-Doyen, a biological anthropologist at Texas A & M University, became convinced the pint-size creatures were actually making ultrasonic screeches. She set up microphones at the Philippine Tarsier Sanctuary and found the animals were emitting sounds up to 75 kilohertz; humans cannot hear past 20 kilohertz. “It was mind-boggling,” she says. “It makes you reevaluate everything you’ve done, heard, and observed.” The shrill call may serve as an alarm signal that is undetectable to approaching predators.