a) Analysis of the individual signature in cries: each point in the central graph represents the centroid of a baby’s cries as a function of the first two discriminant variables that maximize individual separation. The mean fundamental frequency and the maximal value of the fundamental frequency are the two main factors that separate individuals on the first function (DF1, Table 1). The second function (DF2, Table 1) relies mostly on the maximal value of the fundamental and the periodic quality of the signal (parameter ‘jitter’, see Methods section). Spectrograms on the sides illustrate the differences in acoustic structure between cries of different babies. Two cries coming from two different recording sessions are illustrated for the babies 1 and 2, showing the individual consistency of individual cries (to hear the sounds: Supplementary Audio 1–4 corresponding to babies 1–4). ( b) Playback experiments testing parental recognition of their baby’s cry: correct recognition (in blue) and false positive (in red) by parents spending at least 4 h a day with their baby. Circles represent the values for each of the tested parents ( N=29 mothers and 14 fathers). Box and whisker plots report the median, 25th and 75th percentiles, and the lowest and highest data, which are no more than 1.5 × interquartile range from the box. Both fathers and mothers reliably identify their babies on the basis of cries only. Differences between fathers and mothers are not statistically significant (see main text and Tables 3, 4, 5 for statistics).