Vocal complexity influences female responses to gelada male calls

Abstract

Extensive research indicates that inter-sexual selection drives the evolution of complex vocal communication in birds, but parallel lines of evidence are almost entirely absent in mammals. This dearth of evidence, particularly among primates, limits our understanding of the link between sociality and vocal complexity. Here, we use a playback experiment to quantify how wild female geladas (Theropithecus gelada) respond to three call types that are 'derived' (i.e., unique to geladas) and made by males during various affiliative contexts. These derived calls appeared to be highly salient and preferable to females: they looked longer towards and spent more time in proximity to playbacks of male vocal sequences containing one of the derived calls than to sequences containing only common and less elaborate 'grunt' calls. Our results provide the first experimental evidence for vocal elaboration as a male-specific strategy to maintain social bonds with females in non-human primates.

Figure 1. Example playback stimuli of a (a) grunt-only vocal sequence and derived call vocal sequences that include either a (b) moan, (c) wobble, or (d) yawn.

‘G’ refers to exhaled grunts, ‘I’ refers to inhaled grunts, ‘M’ refers to a moan, ‘W’ refers to a wobble, and ‘Y’ refers to a yawn. All of the vocal sequences include exhaled and inhaled call types. Spectrograms were made with Avisoft SAS Lab Pro.

Figure 2. Visual orientation towards the speaker following the conclusion of unfamiliar male vocal sequences.

Vocal sequences were composed of only grunts (grunt) or included one of the derived calls (moan, wobble, yawn). Behaviors measured included (a) the duration of the first visual orientation towards the speaker, (b) the duration of the total visual orientation towards the speaker, and (c) the number of distinct visual orientations towards the speaker. Lines connect trials carried out with the same female subject. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3. Time spent in proximity to the speaker following the conclusion of unfamiliar male vocal sequences.

Vocal sequences were composed of only grunts (grunt) or included one of the derived calls (moan, wobble, yawn). Lines connect trials carried out with the same female subject. *p < 0.05, **p < 0.01, ***p < 0.001.