Experimental evidence for limited vocal recognition in a wild primate: implications for the social complexity hypothesis

Abstract

Although monitoring social information is a key aspect of the social complexity hypothesis, surprisingly little work has compared social knowledge across different species of wild animals. In the present study, I use playback experiments to test for individual recognition in wild male geladas (Theropithecus gelada) to compare with published accounts of social knowledge in chacma baboons (Papio ursinus). Geladas and baboons are closely related primates living in socially complex groups that differ dramatically in group size-geladas routinely associate with more than 10 times the number of conspecifics than do baboons. Using grunts from non-rival males to simulate approaches, I examined the strength of a subject male's response when the 'approach' was from the direction of (i) non-rival males (control), or (ii) rival males (a more salient stimulus if playback grunts are not recognized by the subject). I compared responses separately based on the degree of social overlap between the caller and the subject. Responses indicate that male geladas, unlike baboons, do not use vocalizations to recognize all of the individuals they regularly encounter. This represents, to my knowledge, the first documented evidence of 'missing' social knowledge in a natural primate population. The sharp distinction between baboons and geladas suggests that geladas are either unable or unmotivated to keep track of the individual identity of other males in their multi-level society-even males with whom they have a large degree of social overlap. Thus, these results are consistent with the central assumption of the social complexity hypothesis that social cognition is costly.

Ultrasonic Vocalizations by the Spectral Tarsier, Tarsius spectrum

“…This is almost twice the total number of ultrasonic vocalizations that were observed as given by the Philippine tarsiers. The greater number of ultrasonic vocalizations used by the spectral tarsier might reflect the greater social complexity exhibited by this species relative to the Philippine tarsier; vocal complexity has been linked to social complexity in numerous birds and mammals [Bergman, 2010].…”

Evidence for tactical concealment in a wild primate

“…Geladas' social knowledge in their large, fluid societies has been shown to be limited: males do not know the identities of, or relationships between, individuals outside their immediate reproductive unit or all-male (bachelor) group 41,54 . However, the occurrence of punishment and deceptive behaviour gives us the opportunity to investigate other cognitive skills, such as temporal discounting and higher order intentionality 55,56 (that is, awareness of the mental state of the individuals they are deceiving).…”

Gelada vocal sequences follow Menzerath’s linguistic law

“…The remaining three study males were bachelors from allmale groups in the study bands, and one of these males was a leader and a bachelor at different times during the study period (65). This population has been under intensive behavioral study since January 2006, and study subjects are fully habituated to human observers on foot (approach distances less than 3 m) (66).…”