Strength can be measured from both speech & roars, & strength is more reliably gauged from roars; the acoustic structure of roars explains 40-70% of the variance in actual strength within adults of either sex

Predicting strength from aggressive vocalisations versus speech in African bushland and urban communities. Karel Kleisner et al. Philosophical Transactions of the Royal Society B: Biological Sciences, in press Jun 2021. https://jdleongomez.info/es/publication/kleisner2021b/Kleisner2021b.pdf

Abstract: The human voice carries information about a vocaliser’s physical strength that listeners can perceive, and that may influence mate choice and intrasexual competition. Yet, reliable acoustic correlates of strength in human speech remain unclear. Compared to speech, aggressive nonverbal vocalisations (‘roars’) may function to maximise perceived strength, suggesting that their acoustic structure has been selected to communicate formidability, similar to the vocal threat displays of other animals. Here, we test this prediction in two non-WEIRD African samples: an urban community of Cameroonians and rural nomadic Hadza hunter-gatherers in the Tanzanian bushlands. Participants produced standardised speech and volitional roars and provided handgrip strength measures. Using acoustic analysis and informationtheoretic multi-model inference and averaging techniques, we show that strength can be measured from both speech and roars, and as predicted, strength is more reliably gauged from roars than vowels, words or greetings. The acoustic structure of roars explains 40-70% of the variance in actual strength within adults of either sex. However, strength is predicted by multiple acoustic parameters whose combinations vary by sex, sample and vocal type. Thus, while roars may maximally signal strength, more research is needed to uncover consistent and likely interacting acoustic correlates of strength in the human voice.

Keywords: nonverbal vocalisation; acoustic communication; Hadza; handgrip strength, aggression

4. Discussion

Our results support the prediction that vocal signals to physical strength in humans are maximised in aggressive nonverbal vocalisations (‘roars’) compared to speech. While this prediction has been supported in a Western population (UK drama students: [25,26]), here we extend this research to two African samples, one from the relatively urbanised municipality of Buea (students at the local university), the other from a rural and nomadic small-scale population of Hadza hunter-gatherers. Applying a bottom-up information-theoretic modelling approach, we show that the nonverbal acoustic structure of roars best predicts physical strength. Indeed, predicted strength based on vocal parameters in roars explained the most variance in actual strength for Cameroonian men and women (explaining 40% of the variance in measured hand grip strength) and for Hadza men (explaining 63% of the variance), and explained generally two to four times more variance in strength than did speech (vowels, words, or phrases). While roars relative to greetings predicted strength better in men than in women, roars produced by Hadza women explained an impressive 71% of the variance in their actual physical strength, though this was comparable to the predictive power of their greeting speech (77%). Thus, in contrast to speech, nonverbal roars appear to most effectively encode functional cues to physical strength, as also observed in nonhuman mammals [29]. However, despite our finding that roars and, to a lesser extent, speech, encode information about physical strength in non-WEIRD samples of men and women of African origin, our analyses did not identify a single vocal parameter nor a consistent combination of vocal parameters that predicted strength in both sexes and in both speech and roars. The complex combinations of acoustic predictors revealed by our models, and their high variability across sex, sample, and vocal stimulus type, corroborates the discrepancies of past studies conducted in Western samples [20,22–24,26]. In an attempt to overcome the mixed and null results of this past work, we (1) employed an information-theoretic approach [61,66,67] in order to more extensively explore potential acoustic predictors of strength; (2) examined these predictors in both speech and roars, wherein the latter was predicted to carry more information about physical formidability [25,26]; and (3) tested for acoustic indices of strength in two non-WEIRD African samples. In both samples, but particularly among the Hadza, physical strength may significantly contribute to the biological fitness of an individual given that it positively affects hunting outcomes [44]. Therefore, acoustic communication may be an optimal way to mediate social dominance hierarchies and maintain resource-control without engaging in risky physical confrontation. Indeed, we found that Hadza men and women were physically stronger than our more urban sample of Cameroonian men and women (on average by 16-31%) and that roars predicted strength better in Hadza men and women than in Cameroonian men and women. However, we also found that acoustic predictors of actual strength were more difficult to identify and less stable in the Hadza sample. The reasons for this could be ecological. For instance, Hadza are bush-living people who often communicate at long distances using loud vocalisations or speech, whereas our Cameroonian sample are urbanized, and more often communicate at shorter distances and at a lower volume. The two samples also speak different languages. While Cameroonians from Southwest and Northwest regions speak fluent English, alongside a variety of local native languages, the Hadza speak Swahili and/or Hadzane, a click language consisting of three types of click consonants that may be produced in voiceless oral, voiced nasal, or voiceless nasal, and glottalised variant [60]. Despite these differences, we cannot rule out the possibility that sample-level differences emerged due to a small sample size in the Hadza. Indeed the small sample size of the Hadza is a key limitation of this study. While data from extreme non-WEIRD samples are rare and difficult to obtain, the small sample size may have contributed to inconsistencies in the predictive power of vocal parameters and these results thus should be interpreted with caution. Regarding specific acoustic parameters, it is difficult to derive a clear generalisation of their independent contributions due to the lack of consistency in the pattern of acoustic predictors included in each final average model. However, unlike in studies based on assessments of formidability in voice perception (e.g., [72]), and evidence that relatively low fo can predict strength in the speech of peri-pubertal Bolivian Tsiname males (but not females; [22]), we did not find a consistent relationship between low male fundamental frequency (fo) and strength across samples and different vocal types. In fact, in several cases, for example in the short speech and roars of Hadza men, higher mean fo signalled strength. As increased subglottal pressure will cause an increase in voice pitch [73] this result could be due to greater lung capacity and/or louder vocalisations produced by stronger men, a prediction that can be directly tested in future work. Notably a recent meta-analysis showed, using data from 8 studies and 845 adult men, that mean fo explains a mere 0.005% of the variance (r = -0.07) in men’s upper-body strength [24]. The present study is, to our knowledge, the first to examine whether nonlinear acoustic phenomena (NLPs) predict strength in human roars. While we find preliminary evidence to support this, the positive relationship between NLPs and strength was most evident in Cameroonian women’s roars. In order to reduce the number of terms in our statistical models, we computed a single cumulative proportion (%NLP) combining side-bands, subharmonics and deterministic chaos. This cumulative proportion has previously been shown to reliably index ostensible pain level in volitional human pain vocalisations [74]. However, we cannot rule out the possibility that specific NLP sub-types (e.g., deterministic chaos, which is typically the most strongly associated with affective intensity [33]) may predict strength more effectively than others. This possibility can be tested in future studies that employ larger samples of vocalisers to ensure adequate sampling of various sub-types of nonlinear phenomena in nonverbal vocalisations, and adequate statistical power to test their relative roles.