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Review
. 2012 Jul 5;367(1597):1901-8.
doi: 10.1098/rstb.2011.0224.

Face to face with the social brain

Seth Dobson  1
Affiliations
Review

Face to face with the social brain

Seth Dobson. Philos Trans R Soc Lond B Biol Sci. .

Abstract

Recent comparative evidence suggests that anthropoid primates are the only vertebrates to exhibit a quantitative relationship between relative brain size and social group size. In this paper, I attempt to explain this pattern with regard to facial expressivity and social bonding. I hypothesize that facial motor control increases as a secondary consequence of neocortical expansion owing to cortical innervation of the facial motor nucleus. This is supported by new analyses demonstrating correlated evolution between relative neocortex size and relative facial nucleus size. I also hypothesize that increased facial motor control correlates with enhanced emotional expressivity, which provides the opportunity for individuals to better gauge the trustworthiness of group members. This is supported by previous evidence from human psychology, as well as new analyses demonstrating a positive relationship between allogrooming and facial nucleus volume. I suggest new approaches to the study of primate facial expressivity in light of these hypotheses.

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Figures

Figure 1.
Figure 1.
Three basic types of facial expression in the gelada monkey (Theropithecus gelada): (a) brow-raise display, (b) bared-teeth display and (c) lip-smack display.
Figure 2.
Figure 2.
Residual plot depicting the relationship between log group size and log facial nucleus volume holding log medulla volume constant. Data points are ordinary least-squares (OLS) residuals. Filled circles are catarrhines. Open circles are platyrrhines. OLS trend lines are presented for illustrative purposes.
Figure 3.
Figure 3.
Residual plot depicting the relationship between log non-V1 neocortex ratio and log facial nucleus volume holding log medulla volume constant. Data points are OLS residuals.
Figure 4.
Figure 4.
Residual plot depicting the relationship between log grooming time and log facial nucleus volume holding log medulla volume constant. Data points are OLS residuals.
Figure 5.
Figure 5.
Scatter plot depicting the relationship between social group size and neocortex ratio in New World monkeys. The reference line highlights the qualitative difference between species that live in groups of more than 10 as opposed to species that live in groups of less than 10 individuals. Within these two sets of species, the relationship between group size and neocortex ratio appears to break down.
See this image and copyright information in PMC

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