Gorillas may use their laryngeal air sacs for whinny-type vocalizations and male display

Abstract

Great apes and siamangs-but not humans-possess laryngeal air sacs, suggesting that they were lost over hominin evolution. The absence of air sacs in humans may hold clues to speech evolution, but little is known about their functions in extant apes. We investigated whether gorillas use their air sacs to produce the staccato 'growling' of the silverback chest beat display. This hypothesis was formulated after viewing a nature documentary showing a display by a silverback western gorilla (Kingo). As Kingo growls, the video shows distinctive vibrations in his chest and throat under which the air sacs extend. We also investigated whether other similarly staccato vocalizations-the whinny, sex whinny, and copulation grunt-might also involve the air sacs. To examine these hypotheses, we collected an opportunistic sample of video and audio evidence from research records and another documentary of Kingo's group, and from videos of other gorillas found on YouTube. Analysis shows that the four vocalizations are each emitted in rapid pulses of a similar frequency (8-16 pulses per second), and limited visual evidence indicates that they may all occur with upper torso vibrations. Future research should determine how consistently the vibrations co-occur with the vocalizations, whether they are synchronized, and their precise location and timing. Our findings fit with the hypothesis that apes-especially, but not exclusively males-use their air sacs for vocalizations and displays related to size exaggeration for sex and territory. Thus changes in social structure, mating, and sexual dimorphism might have led to the obsolescence of the air sacs and their loss in hominin evolution.

Sexual selection and the loss of laryngeal air sacs during the evolution of speech

“…If air sacs were lost because they interfered with speech, then the evidence from hyoids would suggest that highly complex vocalizations must have been present by more than 500000 years ago (Martínez et al, 2008). However, as recently noted by Perlman and Salmi (2017), this figure significantly pre-dates some estimates about the timing of the evolution of speech, as some authors have argued the gesture may have been the principal form of linguistic communication up until 150000 (Tomasello, 2008), or even 50000 years ago (Corballis, 2002). It is, of course, possible that laryngeal air sacs were not lost because they interfered with speech, but because they became obsolete and the selection pressure maintaining them was no longer acting.…”

“…Given this, air sacs are likely to be more common, and perhaps larger, in species under strong sexual selection. For example, recent evidence suggests that gorillas may use their laryngeal air sacs for whinny-type vocalizations and male display (Perlman and Salmi, 2017). Similarly, reindeer show pronounced sexual dimorphism in the size of the air sac, and males use air sacs when vocalizing during the rutting period (Frey et al, 2007).…”

“…One significant change in vocal anatomy during hominin evolution was the loss of laryngeal air sacs (Fitch, 2000a;de Boer, 2007de Boer, , 2012Perlman and Salmi, 2017). All of the great apes, and many other non-human primates, have large, inflatable, soft-walled air sacs that connect to the larynx (Starck and Schneider, 1960;Gautier, 1971;Fitch and Hauser, 1995;Hewitt et al, 2002).…”

“…Thus, comparative evidence suggests that air sacs were an ancestral condition, lost during hominin evolution. Several authors have argued that understanding this change in vocal anatomy may provide important clues as to the drivers and timing of the evolution of speech (Fitch and Hauser, 1998;Fitch, 2000a;Hewitt et al, 2002;de Boer, 2007de Boer, , 2008de Boer, , 2012Nishimura et al, 2007;Riede et al, 2008;Perlman and Salmi, 2017).…”

Chest beats as an honest signal of body size in male mountain gorillas (Gorilla beringei beringei)

“…1 ). The chest beat is sometimes preceded by a hooting vocalization and accompanied by slurred growling but not always 22 – 24 . The chest beat has both acoustic and visual components, and therefore is an example of a multimodal non-vocal signal.…”