Beluga whales live in the world's cold northern seas, where they endure months of perpetual darkness. "So they have to use sound, rather than sight, in order to find their way around." That's Aran Mooney, a marine biologist at Woods Hole Oceanographic Institution, in Massachusetts. He says belugas have really fast hearing, too: "Sound underwater travels five times faster than it does in air. And so those guys have to basically perceive and utilize sound five times faster than we do." He and his colleagues traveled to Bristol Bay, Alaska, to test belugas' hearing in the wild. They captured seven belugas for routine physicals. And played them a series of frequencies, while measuring the whales' brain activity with electrodes. Turns out the whales' hearing was sharp, and similar to that of captive belugas. Which means they could appreciate sound ranging from about 4 kilohertz.