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Abstract Six squirrel monkeys (Saimiri sciureus) were studied for their vocal development over the first 20 months of life, using a multi-parametric acoustic analysis. Four of the animals were normally raised, one animal ('Kaspar Hauser') was deprived of adult species-specific calls, one animal was congenitally deaf. The study showed that in all 12 call types under investigation, age-related changes in one or the other acoustic parameter occurred. The degree to which vocal structures changed, was call type-specific, with some calls showing dramatic changes and others showing only minor changes. Most changes occurred within the first four months; some lasted up to 10 months. All call types showed a high variability throughout the 20 months study period. Only five of the 12 call types showed a reduction in variability over time. Both acoustically deprived animals remained within the variability range of the normally raised animals, suggesting that the ontogenetic changes found were mainly maturational.
Hammerschmidt et al. (Mon,) studied this question.