Thursday, December 15, 2011
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 due 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.
UPDATE 5/29/12: This paper appears in the July 2012 issue (subscription required until July 2013). Email me for a PDF (seth dot dobson at dartmouth dot edu).
The purpose of this study is to test the hypothesis that social tolerance drives the evolution of facial expression in macaques. Macaque species exhibit a range of social styles that reflect a continuum of social tolerance. Social interactions in more tolerant taxa tend to be less constrained by rank and kinship than in less tolerant macaques. I predicted that macaques that are more tolerant would exhibit a wider range of facial displays than less tolerant species because interactions that are open to negotiation are characterized by greater uncertainty than interactions that are constrained by rank or kinship. To test this hypothesis, I conducted a phylogenetically informed regression analysis (N = 11) using previously published data on repertoire size and two quantitative measures of social tolerance (conciliatory tendency and counter-aggression). As predicted, macaques with more tolerant social styles tended to have larger repertoires than less tolerant species. These results support the hypothesis that increased social tolerance favors the elaboration of communication to mitigate uncertainty.
UPDATE 2/2/12: This paper appears in the March 2012 issue (subscription required). Email me for a PDF (seth dot dobson at dartmouth dot edu).
Tuesday, May 3, 2011
*UPDATE 6/9/11: The official psych science press release is now online and making the rounds on the interwebs.
*UPDATE 8/17/11: This paper appears in the August 2011 issue. Contact me for a PDF.
*UPDATE 5/29/11: This symposium will be associated with a special issue of Philosophical Transactions of the Royal Society B scheduled for mid-2012 release.
Monday, April 11, 2011
Facial motor nucleus volume co-evolves with both social group size and primary visual cortex volume in catarrhine primates as part of a specialized neuroethological system for communication using facial expressions. Here we examine whether facial nucleus volume also co-evolves with functionally unrelated brainstem motor nuclei (trigeminal motor and hypoglossal) due to developmental constraints. Using phylogenetically informed multiple regression analyses of previously published brain component data, we demonstrate that facial nucleus volume is not correlated with the volume of other motor nuclei after controlling for medulla volume. Our results show that brainstem motor nuclei can evolve independently of other developmentally linked structures in association with specific behavioral ecological conditions. This finding provides additional support for the mosaic view of brain evolution.The title of the special issue is New models and insights into primate evolutionary morphology. The editors are Anne Burrows, Kathleen Muldoon and Adam Sylvester.
*UPDATE 4/18/11: Click here to download the PDF.
Tuesday, March 1, 2011
Click here for more information.
The second annual AnthroTree Workshop aims to make phylogenetic methods more accessible by providing hands-on experience to evolutionary anthropologists interested in learning phylogenetic comparative methods.
The four-day course will feature instructors from around the world, and will cover introductions to a wide array of topics, including:
- Inferring and interpreting phylogenetic trees
- Studying correlated evolution using independent contrasts and PGLS
- Reconstructing ancestral states
- Investigating the factors that influence speciation and extinction
- Computational approaches in Compare, Mesquite, R, and BayesTraits
- Examples from all fields of evolutionary anthropology
In addition to lectures, worked examples and exercises, participants will bring a dataset of their choice to work on with the instructors at the workshop.
Wednesday, February 16, 2011
It has been suggested that primates with more specialized visual systems tend to have relatively large brains due to selection for enlarged cortical visual processing areas. This “visual specialization hypothesis” is supported by several comparative studies. However, the behavioral bases of these co-evolutionary patterns remain unclear. If the visual specialization hypothesis is correct, then variation in visually-oriented behaviors should correlate with variation in brain size. The purpose of this study is to test this prediction by examining the co-evolution of facial expression complexity and relative brain size in extant anthropoids. Facial expression complexity is a function of facial mobility, or the number of visually distinct facial movements a species can produce. Data on facial mobility are currently available for 12 species. These data were combined with published estimates of endocranial volume and body mass. Phylogenetically-informed partial correlation analyses were used to examine the association between facial mobility and endocranial volume after controlling for body mass. Male and female data were analyzed separately. The results of this study provide broad support for the visual specialization hypothesis. Facial mobility is positively correlated with endocranial volume after controlling for body mass in males (partial r = 0.65; p = .031). However, females do not exhibit a significant partial correlation between facial mobility and endocranial volume (partial r = 0.10; p = .396). These findings suggest that male, but not female, brain size evolution is influenced by selection for facial expression processing.This presentation is part of an invited poster symposium entitled Ears, Eyes, and Noses: Revisiting the Evolution and Ecology of the Primate Special Senses. The session is organized by Carrie Veilleux, Eva Garrett, and Rachel Jacobs.