Origin of smile in animals could be trying to appear as of smaller, less threating body

Smiles as Multipurpose Social Signals. Jared Martin et al. Trends in Cognitive Sciences, https://doi.org/10.1016/j.tics.2017.08.007

Abstract: The human smile is highly variable in both its form and the social contexts in which it is displayed. A social-functional account identifies three distinct smile expressions defined in terms of their effects on the perceiver: reward smiles reinforce desired behavior; affiliation smiles invite and maintain social bonds; and dominance smiles manage hierarchical relationships. Mathematical modeling uncovers the appearance of the smiles, and both human and Bayesian classifiers validate these distinctions. New findings link laughter to reward, affiliation, and dominance, and research suggests that these functions of smiles are recognized across cultures. Taken together, this evidence suggests that the smile can be productively investigated according to how it assists the smiler in meeting the challenges and opportunities inherent in human social living.

Trends

Smiles are highly variable across a number of dimensions. Predominant approaches to smile categorization do not sufficiently explain this variability. Their ubiquity and social impact make smiles a critical topic for affective and cognitive science.

A social-functional analysis, categorizing smiles by how they resolve the challenges and opportunities required by social living, suggests three types of smiles: reward smiles that reinforce desired behavior; affiliation smiles that form and maintain social bonds; and dominance smiles that manage social hierarchies.

Recent evidence supports this typology: distinct morphological features communicate each functional intent and motivations to smile are predictably variable across culture based on factors related to the salient social tasks in a given culture.

Keywords: facial expression; social functionalism; social hierarchies; social bonding; behavioral reinforcement


Check also: Tennis grunts communicate acoustic cues to sex and contest outcome. Jordan Raine, Katarzyna Pisanski & David Reby. Animal Behaviour, Volume 130, August 2017, Pages 47-55, https://doi.org/10.1016/j.anbehav.2017.06.022

And: Volitional exaggeration of body size through fundamental and formant frequency modulation in humans. Katarzyna Pisanski et al. Scientific Reports,  2016; 6: 34389. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5043380/

Abstract: Several mammalian species scale their voice fundamental frequency (F0) and formant frequencies in competitive and mating contexts, reducing vocal tract and laryngeal allometry thereby exaggerating apparent body size. Although humans’ rare capacity to volitionally modulate these same frequencies is thought to subserve articulated speech, the potential function of voice frequency modulation in human nonverbal communication remains largely unexplored. Here, the voices of 167 men and women from Canada, Cuba, and Poland were recorded in a baseline condition and while volitionally imitating a physically small and large body size. Modulation of F0, formant spacing (∆F), and apparent vocal tract length (VTL) were measured using Praat. Our results indicate that men and women spontaneously and systemically increased VTL and decreased F0 to imitate a large body size, and reduced VTL and increased F0 to imitate small size. These voice modulations did not differ substantially across cultures, indicating potentially universal sound-size correspondences or anatomical and biomechanical constraints on voice modulation. In each culture, men generally modulated their voices (particularly formants) more than did women. This latter finding could help to explain sexual dimorphism in F0 and formants that is currently unaccounted for by sexual dimorphism in human vocal anatomy and body size.