It was already known that mice make high-pitched noises that are too high for the human ear to hear. For example, baby mice make noises to attract their mothers' attention, and the smell of females causes male mice to squeak. But scientists did not realize that these ultrasonic sounds were significant in any way until two scientists at Missouri's Washington University School of Medicine accidentally discovered the mouse songs. Scientists Tim Holy and Zhongsheng Guo were conducting an experiment to test the reactions of male mice to the smell of female mice. They put 45 male mice in their own "rooms," each with a urine-soaked cotton swab. Some of the swabs were soaked in the urine of female mice, while others were soaked in male mouse urine or a combination of male and female mouse urine. When the male mice sniffed the swabs soaked with the urine of the female mice, they began to sing. They did not sing when they sniffed the other swabs.