Plain faces are more expressive: comparative study of facial colour, mobility and musculature in primates

Abstract

Facial colour patterns and facial expressions are among the most important phenotypic traits that primates use during social interactions. While colour patterns provide information about the sender's identity, expressions can communicate its behavioural intentions. Extrinsic factors, including social group size, have shaped the evolution of facial coloration and mobility, but intrinsic relationships and trade-offs likely operate in their evolution as well. We hypothesize that complex facial colour patterning could reduce how salient facial expressions appear to a receiver, and thus species with highly expressive faces would have evolved uniformly coloured faces. We test this hypothesis through a phylogenetic comparative study, and explore the underlying morphological factors of facial mobility. Supporting our hypothesis, we find that species with highly expressive faces have plain facial colour patterns. The number of facial muscles does not predict facial mobility; instead, species that are larger and have a larger facial nucleus have more expressive faces. This highlights a potential trade-off between facial mobility and colour patterning in primates and reveals complex relationships between facial features during primate evolution.

Are badges of status adaptive in large complex primate groups?

“…Interestingly, facial mobility is more developed among primates with less complex facial coloration, suggesting a trade-off between ornamentation and signalling behavioral intent via facial expression (Santana et al, 2014).…”

The navigational nose: a new hypothesis for the function of the human external pyramid

“…Catarrhines rely heavily on color vision to detect physiological state changes (e.g. sexual skin) in social interactions (Dixson, 2012) and to discriminate the faces of closely related species (Allen et al, 2014;Santana et al, 2014Santana et al, , 2013.…”

“…Note the lateral orientation of the nostril in Platyrrhini. Reproduced with permission fromSantana et al (2014).…”

Evolution of Facial Musculature

“…To date, the evolutionary connections between external (coloration, facial shape) and internal (musculature) facial traits are poorly known. In a recent study (Santana et al, 2014), we contrasted two major hypotheses that could explain the evolution of primate facial diversity when these traits are integrated. First, if the evolution of facial displays has been primarily driven by social factors, highly gregarious primates would possess both complexly colored and highly expressive faces as two concurrent means for social communication.…”