We are also investigating other sources of selection on geladas’ vocal communication. Geladas have a rich and extensive vocal repertoire, and have unusually high-pitched vocalizations compared to primates of a similar size. We are using a variety of methods to examine the links between geladas’ multi-leveled social structure, relatively simple ecological environment, and their complex vocal repertoire. We are currently examining results from a sound transmission experiment that we have conducted in Awash National Park (Ethiopia) and the SMNP. These experiments were designed to test the Acoustic Adaptation Hypothesis, which proposes that a species’ vocalizations have acoustic features that allows for optimal transmission within that species’ native habitat. In addition to testing the influence of the physical environment on gelada vocalizations, we are collaborating with other researchers to put gelada vocalizations in phylogenetic context.

We are conducting detailed acoustic analyses to tease apart the influence of the social and ecological environment on geladas’ vocal repertoire. Using a library of chacma baboon and gelada vocalizations (collected by Thore Bergman and Aliza le Roux), we are contrasting two theories that link vocal repertoire complexity to (a) the social environment, versus (b) the ecological environment. Based on the social complexity hypothesis, we predict that gelada males, playing more social roles than chacma males, will have a larger vocal repertoire than chacmas. Novel vocalizations that evolved in the gelada lineage should be calls used during social interactions, such as variations on contact grunts. However, if the ecological environment has had a stronger selective pressure than the social environment, we would expect to find more complex vocalizations in chacma baboons’ repertoire. These calls should be linked to non-social aspects of their environment, such as predators and food discovery. In this comparison we are identifying homologous and unique vocalizations in each species’ repertoire. Because they are sister genera, unique calls likely represent new evolutionary events, allowing us to address the origins of new calls.

Our Relevant Publications

• The social functions of complex vocal sequences in wild geladas
• Vascularization underlies differences in sexually selected skin coloration in a wild primate
• The structural and motivational role of the unique lip-flip movement in the gelada (Theropithecus gelada) facial display repertoire
• Evidence for mutual assessment in a wild primate
• Divergent acoustic properties of gelada and baboon vocalizations and their implications for the evolution of human speech
• Gelada vocal sequences follow Menzerath’s linguistic law
• Vocal complexity influences female responses to gelada male calls
• Speech-like vocalized lip-smacking in geladas
• Evidence for tactical concealment in a wild primate
• Derived vocalizations of geladas (Theropithecus gelada) and the evolution of vocal complexity in primates
• Experimental evidence for limited vocal recognition in a wild primate: implications for the social complexity hypothesis
• The face never lies: facial expressions and mimicry modulate playful interactions in wild geladas