Ontogeny of bat sonar: paired FM signalling

Throughout the animal's life, starting as early as age three days, the big brown bat (Eptesicus fuscus) produces pairs of tonal, frequency‐modulated (FM) vocalizationS. The interpulse interval (i.e., the interval from the beginning of one signal to the beginning of the next signal) within pairs is on the order of 20–40 msec. Depending on the age of the infant, the two signals of the pair differ in duration, initial frequency, terminal frequency, degree of FM, and harmonic content. Both communication and echolocation FM signals exhibit the same paired temporal pattern. When used in echolocation, the paired signals occur during the approach phase. The transition from the approach phase to the terminal phase of echolocation is indicated by the production of FM signals with interpulse intervals shorter than those within pairs. This vocal transition supports the view that the inferior colliculus, in which a very significant number of neural units have slow recovery times of the order of the interpulse interval...