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

Abstract

Acoustic signals that reliably indicate body size, which usually determines competitive ability, are of particular interest for understanding how animals assess rivals and choose mates. Whereas body size tends to be negatively associated with formant dispersion in animal vocalizations, non-vocal signals have received little attention. Among the most emblematic sounds in the animal kingdom is the chest beat of gorillas, a non-vocal signal that is thought to be important in intra and inter-sexual competition, yet it is unclear whether it reliably indicates body size. We examined the relationship among body size (back breadth), peak frequency, and three temporal characteristics of the chest beat: duration, number of beats and beat rate from sound recordings of wild adult male mountain gorillas. Using linear mixed models, we found that larger males had significantly lower peak frequencies than smaller ones, but we found no consistent relationship between body size and the temporal characteristics measured. Taken together with earlier findings of positive correlations among male body size, dominance rank and reproductive success, we conclude that the gorilla chest beat is an honest signal of competitive ability. These results emphasize the potential of non-vocal signals to convey important information in mammal communication.

Origins of vocal-entangled gesture

“…Gesture-vocal entanglement can be related to the adaptive functions of indexical cues, and thereby also the derived adaptive functions of "faking" or "representing" such indexical cues. As is well known in non-human animals, acoustics is indexically informative about body size (Fitch and Kelley, 2000;Ghazanfar et al, 2007;Raine et al, 2019;Reby et al, 2005;Wright et al, 2021). Some species might even exaggerate such qualities (de Boer et al, 2015;Hardus et al, 2009).…”

“…We deem it possible that tensioning of the upper limbs may have similarly played a role in early aggressive or territorial vocal displaying, and could play a more general role in emotional expression today. Tensioning gestures provide potential information about action-readiness, bodily activity (Pouw et al, 2020d), and emotional states, much like how chest beats are informative about body size in gorillas (Wright et al, 2021), and much like how tensioning of the vocal folds (and heightened F0) is itself deeply connected to (and thus an indexical cue for) emotional states (Bolinger, 1986). It is not that producing a gesture allows someone to signal their internal state of anger, rather it is by producing this high-tensioned bodily impulse that they enact that anger (Merleau-Ponty, 1945).…”

Origins of vocal-entangled gesture

“…Gesture-vocal entanglement is then comparable to several other indexical phenomenon. The acoustics of manual chest-beating by gorillas are informative about body size (Wright et al, 2021). Further, vocalizations, in general, are often informative about body sizes in a range of animal species (Fitch and Kelley, 2000;Reby et al, 2005), including primates (Ghazanfar et al, 2007;Raine et al, 2019), and some species might even exaggerate such qualities, such as in the case of Orangutangs who cup their hands in front of their mouth to deepen their vocal qualities (Boer et al, 2015;Hardus et al, 2009).…”

“…For example, we deem it possible that tensioning of the upper limbs may have played a role in early aggressive or territorial defensive vocal displaying. Tensioning gestures provide potential information about action-readiness, bodily activity (Pouw et al, 2020d), and perhaps emotional states, much like how chest beats are informative about body size in gorillas (Wright et al, 2021), and much like tensioning of the larynx (and heightened F0) is itself deeply connected (and thus an indexical cue for) emotional states (Bolinger, 1986). It is not that producing a gesture allows someone to signal their internal state of anger, it is by producing this high-tensioned bodily impulse that they enact that anger, and the gesture communicates this process of enacting anger accordingly (Merleau-Ponty, 1945).…”

“…As such it is quite logical that within the frame of grander explanatory enterprises, theorists conclude that the pulse-quality of gesture is simply too narrow in its scope to be of importance. Further, if the pulse quality of gesture is basic in evolutionary terms, we would expect that in non-human apes such as chimpanzees some vocal-entangled gestures would have been found by now (Ravignani and Kotz, 2020), but experts in these field have not recognized any behavior yet that may be understood as similar to human vocal-entangled gestures (Slocombe et al, 2011;Fröhlich et al, 2019), though some behaviors like chest-beating in gorillas come close (Wright et al, 2021). Finally, it is quite possible that definitions that are in some ways protected by researchers blind us of the continuities in behavior within and between species (Pouw et al, 2021).…”