Divergent acoustic properties of gelada and baboon vocalizations and their implications for the evolution of human speech

Faces and Voices Processing in Human and Primate Brains: Rhythmic and Multimodal Mechanisms Underlying the Evolution and Development of Speech

While influential works since the 1970s have widely assumed that imitation is an innate skill in both human and non-human primate neonates, recent empirical studies and meta-analyses have challenged this view, indicating other forms of reward-based learning as relevant factors in the development of social behavior. The visual input translation into matching motor output that underlies imitation abilities instead seems to develop along with social interactions and sensorimotor experience during infancy and childhood. Recently, a new visual stream has been identified in both human and non-human primate brains, updating the dual visual stream model. This third pathway is thought to be specialized for dynamics aspects of social perceptions such as eye-gaze, facial expression and crucially for audio-visual integration of speech. Here, we review empirical studies addressing an understudied but crucial aspect of speech and communication, namely the processing of visual orofacial cues (i.e., the perception of a speaker's lips and tongue movements) and its integration with vocal auditory cues. Along this review, we offer new insights from our understanding of speech as the product of evolution and development of a rhythmic and multimodal organization of sensorimotor brain networks, supporting volitional motor control of the upper vocal tract and audio-visual voices-faces integration.

Linguistic laws in biology

Linguistic laws, the common statistical patterns of human language, have been investigated by quantitative linguists for nearly a century. Recently, biologists from a range of disciplines have started to explore the prevalence of these laws beyond language, finding patterns consistent with linguistic laws across multiple levels of biological organisation, from molecular (genomes, genes, and proteins) to organismal (animal behaviour) to ecological (populations and ecosystems). We propose a new conceptual framework for the study of linguistic laws in biology, comprising and integrating distinct levels of analysis, from description to prediction to theory building. Adopting this framework will provide critical new insights into the fundamental rules of organisation underpinning natural systems, unifying linguistic laws and core theory in biology.

Vocal modulation in human mating and competition

The human voice is dynamic, and people modulate their voices across different social interactions. This article presents a review of the literature examining natural vocal modulation in social contexts relevant to human mating and intrasexual competition. Altering acoustic parameters during speech, particularly pitch, in response to mating and competitive contexts can influence social perception and indicate certain qualities of the speaker. For instance, a lowered voice pitch is often used to exert dominance, display status and compete with rivals. Changes in voice can also serve as a salient medium for signalling a person's attraction to another, and there is evidence to support the notion that attraction and/or romantic interest can be distinguished through vocal tones alone. Individuals can purposely change their vocal behaviour in attempt to sound more attractive and to facilitate courtship success. Several findings also point to the effectiveness of vocal change as a mechanism for communicating relationship status. As future studies continue to explore vocal modulation in the arena of human mating, we will gain a better understanding of how and why vocal modulation varies across social contexts and its impact on receiver psychology. This article is part of the theme issue 'Voice modulation: from origin and mechanism to social impact (Part I)'.

Inter-sexual multimodal communication during mating in wild geladas: the leading role of females

Female primates can emit vocalizations associated with mating that can function as honest signals of fertility. Here, we investigated the role of mating calls and visual signals in female geladas (Theropithecus gelada). Because females have a central role in the gelada society and seem to solicit sexual interactions, we answered whether they emit vocalizations in conjunction with gazing to increase mating success probability. Before and during copulations, females can emit pre-copulation calls and copulation calls. For the first time, we identified a new female vocalization emitted at the final stage of copulations (end-copulation call), possibly marking the occurrence of ejaculation. We found that longer pre-copulation call sequences were followed by both prolonged copulations and the presence of end-copulation calls, thus suggesting that females use pre-copulation calls to ensure successful copula completion. Moreover, we found that different combinations of female vocal types and gazing had different effects on male vocal behavior and motivation to complete the copula. The analysis of the vocal and visual signals revealed a complex inter-sexual multimodal chattering with the leading role of females in the signal exchange. Such chattering, led by females, modulates male sexual arousal, thus increasing the probability of the copula success.

Adherence to Menzerath's Law is the exception (not the rule) in three duetting primate species

Across diverse systems including language, music and genomes, there is a tendency for longer sequences to contain shorter constituents; this phenomenon is known as Menzerath's Law. Whether Menzerath's Law is a universal in biological systems, is the result of compression (wherein shortest possible strings represent the maximum amount of information) or emerges from an inevitable relationship between sequence and constituent length remains a topic of debate. In non-human primates, the vocalizations of geladas, male gibbons and chimpanzees exhibit patterns consistent with Menzerath's Law. Here, we use existing datasets of three duetting primate species (tarsiers, titi monkeys and gibbons) to examine the wide-scale applicability of Menzerath's Law. Primate duets provide a useful comparative model to test for the broad-scale applicability of Menzerath's Law, as they evolved independently under presumably similar selection pressures and are emitted under the same context(s) across taxa. Only four out of the eight call types we examined were consistent with Menzerath's Law. Two of these call types exhibited a negative relationship between the position of the note in the call and note duration, indicating that adherence to Menzerath's Law in these call types may be related to breathing constraints. Exceptions to Menzerath's Law occur when notes are relatively homogeneous, or when species-specific call structure leads to a deterministic decrease in note duration. We show that adherence to Menzerath's Law is the exception rather than the rule in duetting primates. It is possible that selection pressures for long-range signals that can travel effectively over large distances was stronger than that of compression in primate duets. Future studies investigating adherence to Menzerath's Law across the vocal repertoires of these species will help us better elucidate the pressures that shape both short- and long-distance acoustic signals.

An exploration of Menzerath's law in wild mountain gorilla vocal sequences

Menzerath's law, traditionally framed as a negative relationship between the size of a structure and its constituent parts (e.g. sentences with more clauses have shorter clauses), is widespread across information-coding systems ranging from human language and the vocal and gestural sequences of primates and birds, to the building blocks of DNA, genes and proteins. Here, we analysed an extensive dataset of 'close-call' sequences produced by wild mountain gorillas (Gorilla beringei beringei, no. individuals = 10, no. sequences = 2189) to determine whether, in accordance with Menzerath's law, a negative relationship existed between the number of vocal units in a sequence and the duration of its constituent units. We initially found positive evidence for this but, on closer inspection, the negative relationship was driven entirely by the difference between single- and multi-unit (two to six unit) sequences. Once single-unit sequences were excluded from the analysis, we identified a relationship in the opposite direction, with longer sequences generally composed of longer units. The close-call sequences of mountain gorillas therefore represent an intriguing example of a non-human vocal system that only partially conforms to the predictions of Menzerath's law.

Origin and evolution of human consciousness

The study of the origin and evolution of consciousness presents several problems. The first problem concerns terminology. The word consciousness comes from the Latin term conscĭentĭa that means "knowledge shared with others." However, the term consciousness also refers to several other aspects involving both its levels (sleep, coma, dreams and waking state) and contents (subjective, phenomenal and objective). A second issue is the problem of other minds, namely, the possibility to establish whether others have minds very like our own. Moreover, human consciousness has been linked to three different forms of memory: procedural/implicit, semantic and episodic. All these different aspects of consciousness will be discussed in the first part of the chapter. In the second part, we discuss different neuroscientific theories on consciousness and examine how research from developmental psychology, clinical neurology (epilepsy, coma, vegetative state and minimal state of consciousness), neuropsychology (blindsight, agnosia, neglect, split-brain and ocular rivalry), and comparative neuropsychophysiology contribute to the study of consciousness. Finally, in the last part of the chapter we discuss the distinctive features of human consciousness and in particular the ability to travel mentally through time, the phenomenon of joint intentionality, theory of mind and language.

Baboon vocal repertoires and the evolution of primate vocal diversity

A remarkable and derived trait of humans is the faculty for language, and considerable research effort has been devoted to understanding the evolution of speech. In contrast to spoken language, which constitutes a (learned) symbolic communication system, the acoustic structure of nonhuman primate vocalizations is largely genetically fixed. Yet, appreciable differences between different genera and species may exist. Environmental conditions, sexual selection, and characteristics of the social system have been invoked to explain these differences. Here, we studied the acoustic variation of call types and vocal repertoires in the genus Papio. Because the genus comprises both stable groups as well as multi-level societies, and reveals striking variation in the degree of aggressiveness from south to north, it constitutes a promising model to assess the link between social system characteristics and vocal communication. We found that, the vocal repertoires of the different species were composed of the same general call types. A quantitative analysis of the acoustic features of the grunts and loud calls of chacma (Papio ursinus), olive (P. anubis), and Guinea (P. papio) baboons showed subtle acoustic differences within call types, however. Social system characteristics did not map onto acoustic variation. We found no correlation between the structure of grunts and geographic distance; the same was true for female loud calls. Only for male loud calls from three populations, call structure varied with geographic distance. Our findings corroborate the view that the structure of nonhuman primate vocalizations is highly conserved, despite the differences in social systems. Apparently, variation in rate and intensity of occurrence of signals, probably due to different behavioral dispositions in species, are sufficient to allow for plasticity at the level of the social relationships, mating patterns, and social organization.