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Brain volume and cheek-tooth size have traditionally been considered as two traits that show opposite evolutionary trends during the evolution of

Brain volume and tooth size, particularly those aspects concerning the dimensions of the postcanine dentition, have long been thought as crucial for hominin evolution [

Brain volume has a direct reflection in cognitive abilities [

Evolutionary changes in the dietary preferences of extinct hominins have been inferred from changes in the dimensions of the postcanine dentition, as cheek teeth are involved in food processing in the mouth. For this reason, the marked differences in tooth size between the robust australopithecines and both

This relationship is particularly important, because dietary changes (and, more specifically, an increase in dietary quality) could be involved in the negative correlation between the development of postcanine teeth and neurocranium volume. From the publication of the influential “expensive-tissue hypothesis” by Aiello and Wheeler [

However, larger primates tend to have larger brains and larger teeth than do smaller bodied ones. For this reason, and given the trend to body size increase in the course of hominin evolution, it is necessary to rule out that the negative correlation between molarization and encephalization of

It is well known that brain mass and postcanine tooth size follow specific allometric relationships with body mass, which depend on the taxonomic groupings analyzed (see reviews in [

Consequently, this paper has two main goals: (1) to study in a sample of living primate species if there is a statistically significant, intertaxa allometric relationship between postcanine tooth size and dietary quality, as it has long been presumed that those species with poor-quality diets would require larger postcanine teeth and

In order to evaluate in a comparative context the evolutionary trend for brain volume and postcanine tooth size, 56 living primate species (Table S1) and 17 taxa of australopiths and

The variables used include the upper postcanine tooth area (PCTA, in mm^{2}), estimated as the cumulative occlusal areas of P^{4}, M^{1}, and M^{2}; brain mass (BrM, in g); body mass (BM, in kg); and dietary quality (DQ) (for additional details and bibliographic sources, see Tables S1 and S2). The last variable was defined as follows:

In order to achieve normality, all variables were log-transformed prior to statistical analyses. Linear regression functions were adjusted using two methods, Reduced Major Axis (RMA) and Ordinary Least Squares (OLS). Regressions adjusted by OLS assume that the independent variable (_{BMres}, BrM_{BMres}, and DQ_{BMres}) were used for obtaining “size-free” adjustments. DQ effects on BrM and PCTA and BrM effects on PCTA and DQ were avoided following a similar procedure.

Given that the species analyzed are part of a hierarchically structured phylogeny, data collected from them do not necessarily satisfy the condition of statistical independence, thus hindering traditional (i.e., ahistorical) statistical analyses [

Composite trees, including branch length estimates, were developed for reconstructing the phylogenetic relationships among the primate species studied, following the consensus tree from 10KTrees Project (Primates) (Figure S1) [

The present study does not focus separately on either molarization or encephalization nor on the relationships of dietary quality with body mass and with brain mass. However, we were forced to control for body mass and therefore to study how brain mass, cheek-tooth area, and dietary quality change with body size.

First, it is worth noting that the results obtained do not differ according to the regression method used (i.e., RMA or OLS) (Tables S3 and S4), except for all primates and catarrhines in the regression of logPCTA-logBrM (Table S4). In the case of primates, isometry may be discarded when OLS is used, but not for RMA. In the case of catarrhines, isometry is rejected with RMA and not with OLS. Thus, in general terms we can affirm that the results obtained are not sensitive to the method of adjustment used. For this reason, only the regressions adjusted by OLS are described in this section (see SI for complete regression statistics).

Secondly, although Gould [

In the case of the regressions of brain mass on body mass, the slopes obtained for the different groupings and taxonomic levels considered are all significantly lower than one, with the only exception of the categories extinct hominins and

Bivariate plot for the logarithms of body mass (BM, in kg) and brain mass (BrM, in g). The regression line and its 95% confidence interval (grey shadow) were adjusted with anthropoids. Aafa:

The relationship between postcanine tooth area and body mass provides quite diverse results in the primate groups analyzed (Figure

Bivariate plot for the logarithms of body mass (BM) and postcanine tooth area (PCTA, in mm^{2}). The regression line and its 95% confidence interval (grey shadow) were adjusted with anthropoids. For species abbreviations, see legend of Figure

Table S4 (upper part) and Figure

Bivariate plot for the logarithms of brain mass (BrM) and postcanine tooth area (PCTA). The solid regression line and its 95% confidence interval (grey shadow) were adjusted with anthropoids and the dashed line and its 95% confidence interval (grey shadow) were adjusted with

Bivariate plot of the residuals of postcanine tooth area (PCTA) on body mass (BM) over the residuals of brain mass (BrM) on BM. The regression line and its 95% confidence interval (grey shadow) were adjusted with data for fossil hominins. For species abbreviations, see legend of Figures

Bivariate plot of the residuals of postcanine tooth area (PCTA) on dietary quality (DQ) over the residuals of brain mass (BrM) on DQ. The regression line and its 95% confidence interval (grey shadow) were adjusted with data for nonhuman primates.

The distribution of hominin taxa on the morphospace defined by the logarithms of brain mass and postcanine tooth area (Figure

A number of regressions were adjusted for evaluating the statistical relationships between postcanine tooth area and dietary quality (Table S5). Negative slopes were obtained in some taxonomic groupings such as nonhuman primates, anthropoids, and platyrrhines (Figure

Bivariate plot of dietary quality (DQ) on postcanine tooth area (PCTA). The regression line and its 95% confidence interval (grey shadow) were adjusted with data for nonhuman primates.

Bivariate plot of the residuals of dietary quality (DQ) on body mass (BM) over the residuals of postcanine tooth occlusal area (PCTA) on BM. The regression line and its 95% confidence interval (grey shadow) were adjusted with data for platyrrhines.

Bivariate plot of the residuals of dietary quality (DQ) on brain mass (BrM) over the residuals of postcanine tooth occlusal area (PCTA) on BrM. The regression line and its 95% confidence interval (grey shadow) were adjusted with data for platyrrhines.

The relationship between postcanine tooth size, metabolic requirements, and dietary preferences is an old topic that periodically experiences a renewed interest in both primatology [

One common feature of previous studies is that they rely on indirect inferences, basically those derived from the “equal exponent” hypothesis for the scaling of tooth size and body mass. For example, several researchers have proposed that cheek-tooth area reflects the energetic demands of primates, because the values obtained for the slope of the bilogarithmic regression between tooth size and body mass are positive [

The metabolic interpretation of postcanine tooth area is based on functional grounds, as cheek teeth are used for chewing food before swallowing. For this reason, it has long been proposed that those species adapted to a poor diet (i.e., one containing a high amount of fibrous foods and/or leaves, which are difficult to digest) will show cheek teeth with a well-developed occlusal surface [

The results obtained in this study do not clarify if there is an isometric or metabolic scaling for postcanine tooth area on body mass, because no common, unambiguous pattern was obtained for all primate species. This agrees with the absence of a universal pattern of scaling for tooth size on body mass in mammals [

The lack of parallelism between

In this way, the size of the postcanine dentition is related to dietary quality with independence of brain size in the case of prosimians and platyrrhines. This is also the case for anthropoids, although the correlation is not significant for catarrhines. For this reason, we can conclude that the relationship detected in anthropoids emerges because they include platyrrhines. In any case, such relationship is biased by phylogeny. For this reason, from the results obtained in this study we can affirm that the relationship between tooth size and dietary quality is, at least partially, dependent on brain size in nonhuman primates.

Therefore, if dietary quality, including cooking and nonthermal, extraoral food processing [

The first considers the relative development of the temporal muscles as such causal link. In fact, given that the dimensions of the chewing teeth and of the masticatory muscles scale isometrically in hominins [

The second hypothesis arises from a recent discovery. Inhibition of SRGAP2 gene function by its human-specific paralogs (SRGAP2C) has contributed to the evolution of the human neocortex and plays an important role during human brain development [

The results of our study show that although there is a significant relationship between postcanine tooth area and dietary quality in some taxonomic groupings, even after phylogenetic correction, in most cases this correlation vanishes when the effects derived from differences in body mass are removed. Prosimians and platyrrhines are the only exception. In the case of prosimians, our results agree with those of Vinyard and Hanna [

Two issues must be raised for explaining particular aspects of several taxa included in this study. The first is the anomalous position of

Whatever it takes, a number of aspects not considered in this study could be also influencing the relationship between tooth size and dietary quality, including the morphology of the dental cusps, the mechanical properties of food (external and internal), or the amount of food that is processed simultaneously into the mouth [

The authors declare that there is no conflict of interests regarding the publication of this paper.

The authors gratefully acknowledge J. M. Plavcan and J. M. Bermúdez de Castro for their insightful comments and helpful criticism of the original paper. This research has been funded by the Spanish Ministry of Science and Innovation (Projects CGL2010-18124, CGL2011-30334, and HAR2008-04577) and supported by the Department of Economy, Innovation and Science, Junta de Andalucía, Spain (Project P11-HUM-7248 and Research Groups HUM-607 and RNM-146). This study has been possible thanks to a Return Contract (University of Granada, Spain) to Juan Manuel Jiménez-Arenas. And, last but not least, the authors also acknowledge constructive remarks and suggestion form the anonymous reviewers.