Socially Meaningful Vocal Plasticity in Adult Campbell’s Monkeys (Cercopithecus campbelli) Alban Lemasson and Martine Hausberger Universite ´ de Rennes 1 Klaus Zuberbu ¨hler University of St. Andrews Campbell’s monkeys (Cercopithecus campbelli) frequently exchange vocalizations, the combined- harmonic calls, with individuals responding to one another’s calls. Previous work has shown that these calls can be grouped into several structural variants. Adult females differ in their variant repertoires, which may change during their adult life, particularly after changes in the group composition. Playback of females’ currently produced variants triggered vocal responses from other group members, whereas the same females’ former, no longer used variants and those of stranger females never did. In contrast, former variants caused long-term cessation of vocal behavior, whereas stranger variants had no effect. Data showed that monkeys were able to distinguish between the different types of variants, indicating that these calls form part of a long-term social memory. Plasticity in vocal production is a widespread phenomenon in songbirds and some marine mammals, but for nonhuman primates comparably little evidence is available. Current theory suggests that primates have little influence over the acoustic structure of their calls and that vocal repertoires are under strong genetic control. If they occur, ontogenetic changes in call structure are usually explained as the results of maturational effects (Fischer, 2002; Hauser, 1989; Janik & Slater, 1997; Seyfarth & Cheney, 1996; Snowdon & Hausberger, 1997). Support for this view comes from studies that unsuccessfully attempted to condition macaques (Macaca mulatta) to alter the acoustic structure of their calls and from cross-fostering experiments and hybridization studies (Gei- ssmann, 1984; Owren, Dieter, Seyfarth, & Cheney, 1992; Sutton, Larson, Taylor, & Lindeman, 1973). These findings have contrib- uted to the general and widely accepted notion that human speech is fundamentally different from primate vocal production and in some ways more similar to bird song or cetacean communication (e.g., Doupe & Kuhl, 1999; Janik & Slater, 2000; Snowdon & Hausberger, 1997). For example, both young children and song- birds go through a babbling phase, in which developmental progress is dependent on social feedback (Goldstein, King, & West, 2003), a phenomenon not reported for nonhuman primates. More recently, the strong dichotomy between innately guided primate vocalizations and human speech has encountered a num- ber of challenges (Riede, Bronson, Hatzikirou, & Zuberbu ¨hler, 2005). For example, it has been documented that the trill vocal- izations of pygmy marmosets (Cebuella pygmaea) change in acoustic structure after pairing and remain highly stable thereafter (Snowdon & Elowson, 1999). Second, pant hoot vocalizations of chimpanzees (Pan troglodytes) are more similar within than be- tween groups, regardless of the individuals’ genetic relatedness (Crockford, Herbinger, Vigilant, & Boesch, 2004; Marshall, Wrangham, & Clark Arcadi, 1999; Mitani & Brandt, 1994; Mitani & Gros-Louis, 1998; Mitani, Hunley, & Murdoch, 1999). A vari- ety of other evidence suggests that nonhuman primates may have some control over elements of their vocal repertoire (Macaca fuscata: Hihara, Yamada, Iriki, & Okanoya, 2003; Masataka & Fujita, 1989; Sugiura, 1993, 1998; Pan paniscus: Taglialatela, Savage-Rumbaugh, & Baker, 2003). Recently, it has been shown that some vocalizations produced by male baboons (Papio cyno- cephalus ursinus) change as males acquire and lose dominance (Fischer, Kitchen, Seyfarth, & Cheney, 2004). Although these studies are interesting, little is still known about the perceptual abilities of monkeys to discriminate such subtle variations and whether the described acoustic variation is socially meaningful to them (Fichtel & Hammerschmidt, 2003; Rendall, Seyfarth, & Cheney, 1999; Semple & McComb, 2000). Recent work with captive Campbell’s monkeys (Cercopithecus campbelli) has provided evidence that significant acoustic varia- tion is present in at least one call type, the combined-harmonic call (Lemasson, Gautier, & Hausberger, 2003; Lemasson & Haus- berger, 2004; Lemasson, Richard, & Hausberger, 2004). Adult females frequently exchange combined-harmonic calls (or cohesion-contact calls; Gautier & Gautier, 1977) as part of their daily social interactions. In their native, visually dense West Af- rican forests, these calls are crucial in maintaining proximity to other group members and in providing information about impor- tant ongoing events, such as the arrival of a neighboring group or the desire to initiate a progression (Uster & Zuberbu ¨hler, 2001; Alban Lemasson and Martine Hausberger, Centre National de la Re- cherche Scientifique, Ethologie–Ecologie–Evolution, Universite ´ de Rennes 1, Rennes, France; Klaus Zuberbu ¨hler, School of Psychology, University of St. Andrews, St. Andrews, Scotland. Funding was provided by the Royal Society (International Joint Projects Grant to Klaus Zuberbu ¨hler and Martine Hausberger), the Centre National de la Recherche Scientifique. (Origine de l’Homme, du Langage et des Langues to Martine Hausberger), and the Universite ´ de Rennes 1 (profes- seur invite ´, Klaus Zuberbu ¨hler). We are grateful to Vincent Janik, Julia Fischer, and Jennifer McClung for comments on the article and to Philippe Bec and Cathrine Blois-Heulin for their support throughout the study. Correspondence concerning this article should be addressed to Klaus Zuberbu ¨hler; School of Psychology, University of St. Andrews, St. An- drews KY16 9JP, Scotland. E-mail: kz3@st-andrews.ac.uk Published in Journal of Comparative Psychology, Vol. 119, Issue 2, 2005, p. 220-229 which should be used for any reference to this work 1