Age- and rank-related changes in groans. Spectrograms (above) and oscillograms (below) of three groans produced by the same male at six, seven and eight years old. Pulses are visible both on the spectrograms and oscillograms and are indicated as black vertical lines. Formants are visible as horizontal frequency bands on the spectrograms. Age-related changes to F0min, and rank-related changes to F6min are shown: F0min was measured for each groan as the inverse of the maximum interpulse interval (grey rectangle) on the oscillogram and increases with age (grey rectangle decreases); F6min is indicated with a black arrow and increases when the male becomes lower-ranking.