PYGMY MARMOSETS, CEBUELLA-PYGMAEA, MODIFY VOCAL STRUCTURE IN RESPONSE TO CHANGED SOCIAL-ENVIRONMENT

Pygmy marmosets displayed plasticity in vocal structure over a 5-month study of nine individuals. Changes in acoustical structure of a contact call, the trill, were investigated when two unfamiliar, captive populations of monkeys were placed together in a common acoustical environment. Monkeys representing four age categories (infant, juvenile, subadult and adult) were studied in three time blocks: 9 weeks of no acoustical interaction, the first 4 weeks after contact and 6-10 weeks after contact. Analyses of variance showed that the monkeys made coextensive shifts in two frequency measures (bandwidth and peak frequency) making parallel vocal changes rather than converging on or diverging from one another's call form. Call duration initially showed distinct population differences that disappeared after acoustic contact between the populations. Comparable analyses of trills given by pygmy marmosets that did not experience novel social companions did not show significant changes. These results suggest greater vocal plasticity across age ranges than has been hitherto described for a non-human primate and suggest the importance of social factors in vocal architecture. © 1994 The Association for the Study of Animal Behaviour.