Selection pressures acting on both intrasexual competition and intersexual relationships may lead to the emphasis of individual variation and might thus lead to the expression of individual signature. This is particularly true in lek mating systems, where providing information on identity and/or quality to potential mates or congeners of the same sex can be essential for individuals to optimize their reproductive success. Visual and acoustic signals produced during the courtship of the lekking North African Houbara bustard (Chlamydotis undulata undulata), based on field data from wild birds, are investigated here for the first time. Results show that the vocal signals, called booms, are characterized by a very low frequency, a rare phenomenon in birds which should allow booms to propagate over long distances. Results also show that both visual and acoustic signals are individualized and stereotyped between males. Using three methods of analysis, we highlight the acoustic parameters likely to support vocal individual signature and show that such information mainly relies on frequency parameters.

No abstract

The human voice carries information about a vocalizer'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 vocalizations (roars) may function to maximize 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 standardized speech and volitional roars and provided handgrip strength measures. Using acoustic analysis and information-theoretic 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. This article is part of the theme issue ‘Voice modulation: from origin and mechanism to social impact (Part I)’.