Horses have a vocal trick no one fully understood until now. Scientists have discovered that when a horse whinnies, it produces two completely different sounds at the same time. One is a deep tone created by vibrating the vocal folds, similar to how humans sing. The other is a high-pitched whistle generated inside the larynx, something never before confirmed in a large mammal. This rare ability, known as biphonation, likely helps horses send multiple emotional signals in a single call.