Using underwater microphones called hydrophones, researchers found that males would make low-frequency sounds when confronted with receptive females. And females preferred males associated with playbacks of these courtship sounds over males that were associated with no sound, or a neutral noise. Subordinate males were also subtly more sensitive to certain frequencies than dominant males. Although the reasons for this change are less clear, Studier suggests the subordinate males may "use the frequency of the sound to determine which males to fight with and attempt to take over their territory." Because the frequency of the courtship sound is related to body size, subordinate males may listen to these courtship sounds to identify the smallest – and most vulnerable – males in their area.