Individual recognition of infant vocalizations by the mother is described in several species. Beyond the information about the context (e.g. lost calls), the calls have to contain information about individual specificity, extracted by acoustic parameters. If these acoustic parameters underlie a trend within a calling sequence, it describes a part of the intra-individual variability. The aim of this study was to determine the temporal dynamic of those acoustic parameters which describe in the best way the individual differences in vocalization. In two captured groups of Japanese macaques Macaca fuscata vocalizations of infants (6 and 7 months old) were recorded in defined contexts. A compound call from the infant vocal repertoire, consisting of two acoustic elements, was selected to measure 18 parameters. These parameters were tested according to their ability to separate the recorded calls of nine infants. The procedure of a discriminance analysis was used. The maximal correct assignment was reached by using 9 out of 18 parameters. An increase in the number of parameters above 9 could not improve the result. The temporal development of these 9 parameters within a calling sequence was determined by trend analysis. It reveals an unrelated dynamic of the 9 parameters, e.g. the starting fundamental frequency was constant over time in all calling sequences of all animals, while other frequency points underlie trends. A relationship between the parameter dynamic and the detailed behavioural response is shown.
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