Ethology 110, 301—321 (2004) Ó 2004 Blackwell Verlag, Berlin ISSN 0179–1613 Vocal Repertoire of Sooty Mangabeys (Cercocebus torquatus atys)in the Taı¨ National Park Friederike Range* & Julia Fischer *Department of Psychology, University of Pennsylvania, Philadelphia, PA, USA; Max-Planck Institute for Evolutionary Anthropology, Leipzig, Germany Abstract Three factors, environment, quality and type of interaction, and phylogeny have been hypothesized to influence the structure of signal repertoires in primates. Much is known about vocal repertoires of terrestrial, savannah-dwelling species and arboreal, rainforest-dwelling species, but very little is known about terrestrial rainforest species.To fill this knowledge gap and to further elucidate how the three factors influence vocal repertoires of primates, we designed a study on sooty mangabeys (Cercocebus torquatus atys), a terrestrial Old World primate that lives in dense rainforests.Recordings of sooty mangabeys in their natural environment were used to compile the vocal repertoire of this species.All calls are described according to the basic acoustical structure and the behavioral context in which they occurred.Descriptions are supplemented by quantitative measurements of call occurrence in all age–sex classes.For the most frequently produced vocalizations, a preliminary acoustical analysis was conducted to test for individual and contextual differences.Finally, we compare vocalizations of sooty mangabeys with vocalizations of several other primate species and discuss how the factors mentioned above might influence the vocal repertoire of sooty mangabeys. Corresponding author: Friederike Range, Department of Psychology, University of Pennsylvania, 3720 Walnut Street, Philadelphia, PA, 19104-6196, USA. E-mail: [email protected] Introduction One key means of understanding the evolution of non-human primate vocal behavior is to investigate the selective pressures that shape the acoustic structure of vocalizations.Several factors appear to affect the structure of a signal repertoire.First, sound propagation, as measured by attenuation, reverberation and amplitude, depends on characteristics of the local habitat (Marten & Marler 1977; Waser & Waser 1977; Waser & Brown 1984, 1986).Thus, it is likely that the U. S. Copyright Clearance Center Code Statement: 0179-1613/2004/11004–301/$15.00/0 www.blackwell-synergy.com 302 F.Range & J.Fischer acoustic properties of vocalizations will be shaped to reduce distortion in the environment through which the signal is normally transmitted (the Ôlocal adaptation hypothesisÕ (Gish & Morton 1981)).Secondly, the quality and type of interactions among group members may affect signal diversity (Maestripieri 1996).However, the way individuals interact is in turn influenced by the species Õ habitat.Marler (1975, 1976 hypothesized that continuous acoustic variation between and/or within signal types should evolve when individuals inhabit relatively open habitat and interact at high rates and in face-to-face interactions with conspecifics.In contrast, signals with no intermediates between call types should be favored when auditory signals must operate without accompanying visual or other contextual cues; for example, in forest habitats or when being broadcast over long distances (Marler 1975, 1976).Thirdly, phylogenetic descent is expected to play a substantial role as the structure of primate calls is largely under genetic control (Ju¨ rgens 1992). To date, much is known about the vocal repertoires of terrestrial monkey species living in open habitat, such as baboons and vervet monkeys (e.g. Struhsaker 1967; Cheney & Seyfarth 1980; Seyfarth et al.1980a; Cheney & Seyfarth 1982a; Cheney et al.1996; Rendall et al.1999; Fischer et al.2001), and of arboreal monkey species living in rainforests, such as Diana monkeys, grey- cheeked mangabeys and spider monkeys (Chalmers 1968; Waser & Waser 1977; Chapman & Weary 1990; Zuberbu¨ hler 1995; Teixidor & Byrne, 1999).Some clear differences in vocal repertoires between these two groups are apparent, but we do not yet know whether the differences are due primarily to habitat (open terrain vs. forest), to locomotion (terrestrial vs.arboreal), or both.To date, very little information is available on the vocal repertoire of terrestrial rainforest species (but see Waser 1982).To fill this gap, we collected data on the vocalizations of the sooty mangabey (Cercocebus torquatus atys), a terrestrial, forest-dwelling monkey species that lives in West Africa. Traditionally, all mangabey species have been combined into a single genus, Cercocebus, (Schwartz 1928; Booth 1956; Napier & Napier 1967) that was commonly divided into two species: the torquatus-group and the albigena-group. However, recent molecular studies suggest that these species-groups are paraphyletic (Barnicott & Wade 1970; Barnicott & Hewett-Emmett 1972; Cronin & Sarich 1976; Groves 1978; Disotell 1994, 1996; van der Kuyl et al.1995; Fleagle & McGraw 1999) and should be placed in two separate genera.Accordingly, the torquatus-group, including the sooty mangabey, is assigned to the genus Cercocebus, which is most closely related to forest-dwelling mandrills and drills (Mandrillus spp.), whereas the albigena-group belongs to the genus Lophocebus, which is most closely related to open-terrain baboons (Papio spp.) and geladas (Theropithecus).Species of both genera are mainly forest dwelling, but while the Cercocebus group is mainly terrestrial, the Lophocebus group is arboreal. As virtually nothing is known about the vocal repertoire of sooty mangabeys, and even less about the communication of their closest relatives, the mandrills and drills, this study fills a gap in our knowledge about the vocal behavior of an important group of Old World monkeys.We attempt to understand how the Vocal Repertoire of Sooty Mangabeys 303 structure of the signal repertoire of free-ranging sooty mangabeys is influenced by environment, quality and type of interaction, and phylogeny, by (1) describing the vocal repertoire and presenting sonograms of the most frequently produced vocalizations, (2) examining the behavioral context in which specific call types are produced, (3) conducting a preliminary acoustic analysis of the most frequent vocalizations (grunts, twitters and copulation calls) to test if vocalizations vary between the sexes, between individuals and between contexts, thus giving us an idea about the quality of interactions, and finally (4) by comparing these aspects of sooty mangabey vocalizations with the same aspects of vocalizations of other, arboreal and terrestrial, savannah- and forest-dwelling non-human primate species. Methods Study Site and Subjects The study was conducted over a 16-month period (April 2000–December 2001) on free-ranging sooty mangabeys in the Taı¨ National Park in Ivory Coast (6°20¢Nto5°10¢N and 4°20¢Wto6°50¢W).The park is the last remaining major block of primary forest in West Africa and covers approximately 4 54 000 ha. Visibility ranges from 5 to 20 m throughout the home range of our study group. During the study period, group size ranged between 98 and 122 animals.The group has been under study since 1997.All animals are well habituated to human observers and can be recognized individually.Because of political problems in Ivory Coast, this study was terminated earlier than planned and thus sample sizes are small.However, we feel that the present data give a good preliminary overview of the vocal repertoire of the sooty mangabey. Data Collection Behavioral data to determine use and hourly rates of vocalizations were collected by focal animal sampling (Altmann 1974).Focal samples were 15 min long with at least 60 min between consecutive samples of the same individual. During observations, we used instantaneous sampling (Altmann 1974) to record the individual’s general activity (foraging, traveling, social interaction or resting). Social interactions and vocalizations were recorded continuously according to a detailed ethogram (Range & Noe¨ 2002). Behavioral data were recorded by F.Range on 24 adult females from April– August 00 and on 12 juveniles (seven females, five males) from May– December 01.Y.Meystre also collected behavioral data on 12 adult males (October 00–April 01).As there was no temporal overlap between the two observers, no inter-observer reliability tests were possible. Vocalizations were tape-recorded only from May–December 2001 using a Sony-DAT PCM-M1 recorder and a Sennheiser directional microphone (ME 68). Vocalizations were collected opportunistically.Whenever a call was recorded, we 304 F.Range & J.Fischer also noted the caller’s identity, its activity and social behavior (for details of the ethogram see Range & Noe¨ 2002). Data Analysis For each call type, we calculated vocalization time as the percentage of minutes during focal animal sampling when the focal individual used a specific call type at least once.Hourly rates of vocalizations for individuals were calculated by dividing the total number of calls given by the individual during focal samples by the sum of the total observation time for that individual.Spectrograms were generated with Avisoft-SASLab (R.Specht, Berlin, Germany). To document the acoustic features of the sooty mangabey vocal repertoire we used only vocalizations tape-recorded from known adult individuals.For a summary of all call types used for acoustic analysis and sample sizes see Table 1. All statistical analyses were performed on individual means. Tape-recorded vocalizations of sooty mangabeys were categorized