Model of vocal production in mammals. (A) Relationships between call patterns within two models of vocal production. (Top) Vocal output that is synchronized with the respiratory cycle and (bottom) vocalizations produced by a vocal pattern generator (see text for further explanations). Graphs with red lines on the right-hand side represent correlations between CD and CI as well as CD and ICI that result from each corresponding vocal production mechanism shown on the left hand side. Briefly, when synchronized to respiration, CI is determined by respiratory period and thus independent of CD. Moreover, the longer the CD, the shorter the ICI. On the other hand, with a pattern generator, CI will depend on CD whereas ICI will not. (B) Spectrogram of an example for several consecutive mouse vocalizations. CD call duration, ICI inter pulse interval, CI call interval.