Alejandro Romero

Fernando V Ramirez-Rozzi

Dr Alejandro Pérez-Pérez

Dental size variability in Central African pygmy hunter-gatherers and Bantuspeaking farmers

Keywords: tooth size, African hunter-gatherers, Bantu-speakers, sexual-dimorphism, phenetic variation

Objectives

Odontometric studies on African populations show high within-group tooth size variation. North Africans exhibit overall smaller dimensions than sub-Saharan groups from eastern and southern regions. However, no previous studies have analyzed dental metrics of the full dentition among extant African pygmy hunter-gatherers and Bantuspeaking farmers and, population variability in tooth crown sizes from equatorial rainforest regions remains to be elucidated.

Materials and Methods

The mesiodistal and buccolingual diameters of the permanent teeth (I1-M2) were measured from in vivo high-resolution replicas in Baka pygmies and, Mvae and Yassa Bantu-speakers from Cameroon (western Africa). Analyses of variance were used to record sex-related and population differences in tooth sizes, and a Principal component analysis (PCA) of geometrically scaled measures was used to plot the odontometric variability among groups.

Results

Cameroonian Baka pygmies differ in dental size from their Bantu-speakers neighbors.

Molar teeth are larger in pygmies than in Bantu individuals, while the anterior dentition is larger in the Bantu. Baka males exhibit significantly larger teeth than females, whereas sexual dimorphism in non-pygmies is only present in the anterior dentition.

Discussion

Odontometric pattern and the degree of sexual dimorphism in dental size differ among Central African groups adapted to a different forager and farmer lifestyle. Singularly, admixture with Bantu-speakers in Baka populations is smaller than in other western pygmy groups. Dental phenetic diversity in Baka compared to the smaller-toothed farmers suggests that eco-genetic and micro-evolutionary factors influencing a particular divergence scenario.

Introduction

Odontometrics provide an important insight into the biological diversity and phylogenetic affinities within and among modern human populations [START_REF] Hanihara | Geographic structure of dental variation in the major human populations of the world[END_REF][START_REF] Pilloud | Dental metrics in biodistance analysis[END_REF]. Tooth crown dimensions have been shown to correlate with the geographic distribution on a worldwide scale of large-toothed and medium to small-toothed groups [START_REF] Harris | Ethnic differences in the apportionment of tooth size[END_REF]. For example, the sub-Saharan African and Australian Aboriginal populations possess the least reduced teeth, whereas the Eurasian populations show smaller dentitions [START_REF] Hanihara | Metric dental variation of major human populations[END_REF]. However, dental measurements show low between-populations variances (Harris & Rathburn, 1991) since less than 20% of the overall odontometric variation in humans occurs between geographic regions, while most of the dental diversity exists within regions [START_REF] Hanihara | Geographic structure of dental variation in the major human populations of the world[END_REF][START_REF] Hanihara | Metric dental variation of major human populations[END_REF].

Studies on dental size variability within African populations are scarce.

Odontometric reports indicate that the smaller, sex-pooled, tooth dimensions in northern African ethnic groups markedly differ from those inhabiting sub-Saharan regions [START_REF] Irish | Size does matter: variation in tooth size apportionment among major regional North and sub-Saharan African populations[END_REF]. The diachronic dental homogeneity recorded in both northeast and northwest African populations suggests a clinal trend in tooth size apportionment [START_REF] Irish | An odontometric investigation of Canary Islander origins[END_REF]. The sub-Saharan populations, mainly from African eastern and southern regions show large teeth compared to western Eurasian populations [START_REF] Hanihara | Metric dental variation of major human populations[END_REF]. Further, the inter-regional comparisons reveals greater degree of dental variation in the sub-Saharan populations compared to those in northern regions [START_REF] Hanihara | Geographic structure of dental variation in the major human populations of the world[END_REF]. Nonetheless, the scarcity of well-documented populations within equatorial African regions prevents drawing conclusive interpretations of the microevolutionary events affecting dental phenetic diversity.

Among populations in the Congo Basin, modern-day African pygmy huntergatherers coexist with neighboring Bantu-speaking farmers [START_REF] Bauchet | Cultural diversity of African pygmies[END_REF]. Based on cultural, genetics and phenotypic criteria, a large diversity exists within populations in this area [START_REF] Bauchet | Cultural diversity of African pygmies[END_REF][START_REF] Marquer | Nouvelle contribution à l'étude du squelette des Pygmées occidentaux du Centre africain comparé à celui des Pygmées orientaux[END_REF][START_REF] Quintana-Murci | Maternal traces of deep common ancestry and asymmetric gene flow between Pygmy Hunter-Gatherers and Bantu-Speaking Farmers[END_REF][START_REF] Ramirez-Rozzi | Diversity among African pygmies[END_REF][START_REF] Ramirez-Rozzi | Growth pattern from birth to adulthood in African pygmies of known age[END_REF].

Particularly, population genomic data suggest an early divergence between pygmies and Bantu-speakers at roughly 50,000-65,000 years ago [START_REF] Patin | Inferring the demographic history of African farmers and pygmy hunter-gatherers using a multilocus resequencing data set[END_REF][START_REF] Quintana-Murci | Maternal traces of deep common ancestry and asymmetric gene flow between Pygmy Hunter-Gatherers and Bantu-Speaking Farmers[END_REF][START_REF] Verdu | Origins and genetic diversity of Pygmy hunter-gatherers from Western Central Africa[END_REF]. However, genetic admixture is highly asymmetrical and most of the genetic variation observed is derived from the forest-dwelling foragers [START_REF] Quintana-Murci | Maternal traces of deep common ancestry and asymmetric gene flow between Pygmy Hunter-Gatherers and Bantu-Speaking Farmers[END_REF][START_REF] Verdu | Origins and genetic diversity of Pygmy hunter-gatherers from Western Central Africa[END_REF][START_REF] Verdu | Sociocultural behavior, sex-biased admixture, and effective population sizes in Central African Pygmies and non-Pygmies[END_REF]. Further, the bulk of the extensive genetic exchange among populations has occurred within the past 1,000 years, suggesting that admixture took place at the later stage of the demographic expansion of early Bantu-speakers and the agriculture diffusion [START_REF] Patin | The impact of agricultural emergence on the genetic history of African rainforest hunter-gatherers and agriculturalists[END_REF].

Body phenotype significantly correlates with levels of genetic admixture in contemporary pygmies; less admixed individuals exhibit shorter statures [START_REF] Becker | Indirect evidence for the genetic determination of short stature in African Pygmies[END_REF], and significant differences in cranial shape between pygmies and non-pygmies have been described [START_REF] Ramirez-Rozzi | Diversity among African pygmies[END_REF]. However, to date, dental studies in central African pygmies are limited, mainly because samples are rarely available for analysis [START_REF] Marquer | Nouvelle contribution à l'étude du squelette des Pygmées occidentaux du Centre africain comparé à celui des Pygmées orientaux[END_REF][START_REF] Ramirez-Rozzi | Diversity in tooth eruption and life history in humans: illustration from a Pygmy population[END_REF][START_REF] Ramirez-Rozzi | Diversity among African pygmies[END_REF]. Early reports on dental morphological traits and metrics involved very small samples and none of these reports provide data about the magnitude of sexual dimorphism. For instance, [START_REF] Irish | Characteristic high-and low-frequency dental traits in sub-Saharan African populations[END_REF] found that western Pygmies (BaBinga and BaBongo) are indistinguishable dentally in non-metric traits compared with Bantu groups from Cameroon, Congo or Gabon. Descriptive studies by [START_REF] Marquer | Nouvelle contribution à l'étude du squelette des Pygmées occidentaux du Centre africain comparé à celui des Pygmées orientaux[END_REF] and [START_REF] Thilmans | Etude de quelques craânes de pygmées Bambuti[END_REF] denote variation in molar crown sizes among western and eastern pygmy populations suggesting a divergence in dental characters. Regarding tooth metrics, analyses by [START_REF] Brabant | Observations sur la denture des Pygmées de l'Afrique centrale[END_REF] reveal larger permanent tooth sizes in Hutu from Rwanda than in eastern pygmies, and [START_REF] Shea | Tooth scaling and evolutionary dwarfism: an investigation of allometry in human Pygmies[END_REF] show a strongly negative allometric relationship between the size of the maxillary check teeth and skull size between Central African pygmies, including eastern and western pygmies, and nonpygmy Bantu groups (Angola, Democratic Republic of Congo, and Mali). Although smaller molar dimensions were found in non-pygmies, scaling patterns of molar shape seem not to change with dwarfing events (Shea & Gómez, 1988).

Here, we provide an extensive in vivo odontometric analysis, both for the anterior and post-canine dentition, for known-sex, Baka rainforest hunter-gatherers compared with Mvae and Yassa Bantu-speaking agriculturalists from Cameroon (western Africa). We chose to analyze these populations because they show distinct and well-documented subsistence strategies [START_REF] Bahuchet | Spatial mobility and access to the resources among the African Pygmies[END_REF][START_REF] Koppert | Food consumption in three forest populations of the southern coastal Cameroon[END_REF][START_REF] Vallois | Les Pygmées Baká du Cameroun: Anthropologie et ethnographie avec une annexe démographique[END_REF][START_REF] Yamauchi | Nutritional status, activity pattern, and dietary intake among the Baka hunter-gatherers in the village camps in Cameroon[END_REF].

Further, among western pygmies, the Baka consist of a single, homogeneous population, regarding their origin, showing low admixture levels with their non-pygmy neighbors [START_REF] Patin | Inferring the demographic history of African farmers and pygmy hunter-gatherers using a multilocus resequencing data set[END_REF][START_REF] Verdu | Sociocultural behavior, sex-biased admixture, and effective population sizes in Central African Pygmies and non-Pygmies[END_REF]. Tooth eruption in Baka, for all tooth classes, occurs earlier compared with other sub-Saharan African populations [START_REF] Ramirez-Rozzi | Diversity in tooth eruption and life history in humans: illustration from a Pygmy population[END_REF]. The large size of check teeth in African pygmies has been attributed to developmental factors (Shea & Gómez, 1988). However, further studies, including the full dentition, are needed to characterize dental size evolution in western-pygmies. We provide new odontometric data useful to assess the phenetic affinities between Central African pygmy and non-pygmy populations.

Material and Methods

Studied populations

The Baka pygmies are semi-nomadic groups living mostly in densely forested areas from southeastern Cameroon. Their culture is based largely on a hunting and foraging lifestyle [START_REF] Bahuchet | Spatial mobility and access to the resources among the African Pygmies[END_REF][START_REF] Romero | Diet-related buccal dental microwear patterns in Central African Pygmy foragers and Bantu-Speaking farmer and pastoralist populations[END_REF][START_REF] Vallois | Les Pygmées Baká du Cameroun: Anthropologie et ethnographie avec une annexe démographique[END_REF]. Geographical subgroups share the same language and a common origin, displaying low levels of genetic differentiation [START_REF] Verdu | Origins and genetic diversity of Pygmy hunter-gatherers from Western Central Africa[END_REF][START_REF] Verdu | Sociocultural behavior, sex-biased admixture, and effective population sizes in Central African Pygmies and non-Pygmies[END_REF]. Individuals included in the study come from Moange-le-Bosquet locality (Lomié district). Various Bantu-speaking groups coexist in the coastal region in south-west Cameroon; the Mvae and Yassa ethnic groups mostly inhabit the Campo district, living under identical climatic and phytogeographic conditions [START_REF] Koppert | Food consumption in three forest populations of the southern coastal Cameroon[END_REF]. We recruited Bantu individuals across the Ebodje and Boandjo villages. In contrast to the foraging activities among the Baka people, the Mvae and Yassa are agriculturalists, primarily on millet cultivation, and fishing populations [START_REF] Yamauchi | Nutritional status, activity pattern, and dietary intake among the Baka hunter-gatherers in the village camps in Cameroon[END_REF]. Mvae and Yassa individuals were grouped since they are closely related groups on the basis of both geographic proximity and ethno-linguistic criteria [START_REF] Koppert | Food consumption in three forest populations of the southern coastal Cameroon[END_REF].

Data collection

Full-mouth dental molds were obtained from 110 Baka pygmies (37 males, 73 females) and 31 Bantu-speaking (21 males, 10 females) Mvae and Yassa individuals during several fieldworks from 2007 to 2012. Only juvenile or young adult individuals with permanent maxillary and mandibular dentitions, none overly affected by attrition, were selected. All participants provided oral informed consent for the study since they are non-literate populations, and the data were analyzed anonymously. Collection methods for illiterate traditional populations were reviewed and approved by the Ethics Committee of the Centre National de la Recherche Scientifique, Agence National de la Recherche (France) and Institut de Recherche et Development (Ramirez-Rozzi, 2016).

Dental molds were made using hydrophobic dental impression materials (Coltène-Whaledent Corp., Switzerland) following in vivo protocols [START_REF] Romero | Short and long-term in vivo human buccal dental-microwear turnover[END_REF][START_REF] Romero | Diet-related buccal dental microwear patterns in Central African Pygmy foragers and Bantu-Speaking farmer and pastoralist populations[END_REF]. Prior to obtaining silicone-based molds, each individual washed and brushed its teeth with water and they were dried with a manual air compressor.

Impression trays were used to obtain a first-step putty soft maxillary and mandibular tooth mold. From these initial molds (cured ~5 min) a second impression was produced by using a low viscosity polyvinylsiloxane-based material (President's Jet, regular body). Finally, high-resolution polyurethane replicas (Feroca® Composites, Spain) were produced from the molds [START_REF] Galbany | Comparative analysis of dental enamel polyvinylsiloxane impression and polyurethane casting methods for SEM research[END_REF] (Figure 1).

[Figure 1 here]

Tooth measurements

Maximum mesio-distal (MD) and bucco-lingual (BL) crown dimensions (in mm) were obtained for both upper (U) and lower (L) permanent dentitions (I1-M2), except for the third molars, in accordance with the procedures described by [START_REF] Harris | Size matters: discrimination between American Blacks and Whites, males and females, using tooth crown dimensions[END_REF]. The anterior dentition (I1, I2, and C) was measured using a needle-point Helios-Preisser (Germany) digital caliper (0.01 mm precision). Instead, data from premolar (P3-P4) and molar (M1-M2) teeth were collected from occlusal images obtained with a Sony α230 (10.2 megapixel) digital single-lens reflex camera attached to a stand with adjustable camera arm. Each tooth was placed with the cervical line perpendicular to the focal point of the camera using a leveling device. A macro setting was used to obtain calibrated (1 mm) images for each occlusal premolar and molar tooth surfaces separately and the measurements were derived using the SigmaScan Pro® software (SPSS TM , Chicago, IL). The dental measurements were taken on the left side.

However, the antimere was used, if available, when the left tooth was damaged or antemortem missing. The measurements were taken twice at a time interval greater than 5 weeks and the two measurements were averaged. Teeth with pathologies or presence of intentional mutilation in anterior teeth, mostly found in Baka pygmies (see [START_REF] Vallois | Les Pygmées Baká du Cameroun: Anthropologie et ethnographie avec une annexe démographique[END_REF], were not considered and treated as missing data. In order to obtain the 28 crown dimensions per individual (MD and BL dimensions from 7 teeth and both jaws), we used a multiple imputation method with Expectation-Maximization (EM) algorithm [START_REF] Dempster | Maximum likelihood from incomplete data via the EM algorithm[END_REF], in accordance with previous odontometric studies [START_REF] Hemphill | Assessing odontometric variation among populations[END_REF]. Missing values were less than 30% of total data, affecting 21.2% of the studied individuals. The relative technical measurement error (rRME) [START_REF] Harris | Accounting for measurement error: a critical but often overlooked process[END_REF] was calculated for both the MD and BL dimensions on 30 randomly selected maxillary first incisors and molars. The rRME values obtained were smaller than 5%, ranging from 0.44% to 3.86% for both metrics and tooth-type, which indicated that the method was precise and repeatable [START_REF] Górka | First molar size and wear within and among modern hunter-gatherers and agricultural populations[END_REF].

Data analyses

The Kolmogorov-Smirnov test indicated that the crown measurements by sex and group were normally distributed (Z = 0.175-0.903; p > 0.05), allowing for the use of parametric tests. The percentage of sexual dimorphism was calculated as [(M-F)/F]*100 [START_REF] Garn | Genetic control of sexual dimorphism in tooth size[END_REF], where M and F are the mean values for each of the 28 measurements of males and females, respectively. Singleclassification analysis of variance (one-way ANOVA) was computed as needed to determine the sources of significant variation on tooth measurements by sex within and among groups considered.

A principal components analysis (PCA), derived from individual-based geometrically scaled dental measurements, was used for the examination of amonggroup variation in dental dimensions. Geometrically scaled values yield several improvements over raw measurements, since they eliminate the allometric factor that might be influencing tooth size, increasing the amount of non-size-related variance [START_REF] Hemphill | Assessing odontometric variation among populations[END_REF][START_REF] Pilloud | Dental metrics in biodistance analysis[END_REF]. The PCA further provided information about individual tooth variation in size from both jaws [START_REF] Harris | A principal components analysis of human odontometrics[END_REF].

Descriptive statistics and group comparisons were conducted using Systat® 10.2 and IBM SPSS Statistics 19.0 with a significance level of α = 0.05.

Results

Sex-related odontometrics

As expected, both Baka pygmies and non-pygmy Bantu-speaking farmers showed larger tooth crown dimensions in males compared to females (Tables 1 and2; see Figure 1). However, the degree of sexual dimorphism in dental size varied notably within groups. The teeth of Baka males had significantly larger dimensions than those of females in 71.42% of the 28 measurements for both maxillary (78.57%) and mandibular (64.28%) teeth. Upper and lower P4 were the only teeth not showing a significant sex-related dimorphism. On the other hand, Bantu individuals display lower overall tooth-size sexual dimorphism (14.28%), limited to the LI1 and U-LC teeth.

Tooth-size sexual dimorphism was particularly marked in the upper and lower canine teeth for the BL breadth (~12-16%), affecting both populations.

[Table 1 here] [Table 2 here] Moreover, the inter-group comparison showed that the Bantu exhibited greater coefficients of variation (~2-15%) in the size of the anterior (incisors and canines) teeth than the Baka pygmies (~6-10%), whereas for the post-canine (premolars and molars) dentition they were small in both group (Baka ~4-8%; Bantu ~4-10%) samples (Tables 1 and2). Odontometric significant differences between Baka and Bantu-speakers are recorded almost exclusively in the upper dentition (Table 3). Regarding sex-related variation, both sexes in the Baka develop greater canine tooth dimensions (>0.3mm in size) than Bantu individuals. The Baka males were also characterized for greater upper molars (UM1-2); their mesio-distal (MD) dimensions exceed in values greater than 0.6mm than that exhibit the Bantu males for UM1-2 teeth. Instead, this molar size trends for females of both groups does not reach significance level. For incisor teeth, Bantu females present a greater breadth (BL) for UI1. However, with the exception of greater BL size of the LI1 found in Baka females, no other within sex differences was observed for the lower dentition. Finally, when differences among groups are considered with pooled sex, we found that maxillary incisors (UI1-2) were larger in the Bantu-speakers, while the Baka showed larger average dimensions for the UC and UM1-2 teeth.

[Table 3 here]

Inter-group tooth-size variation

Five unrotated components of geometrically scaled variables were obtained with eigenvalues greater than one and accounting for 50.38% of the total variance. The first two principal components (PC1-2) explained 28.64% of total variance (Figure 2 and Table 4). The first principal component (PC1: 16.12%) featured significant negative loadings (r = -0.1 to -0.6; p < 0.05) in MD and BL dimensions for both the upper and lower anterior teeth, whereas significant positive loadings were observed in the postcanine dentition. This trend for PC1 draws a clear sex-related population distinction between the anterior and the post-canine teeth of both jaws. Females showed smaller crown dimensions than males for both the Baka pygmies (F1,108 = 22.787; p < 0.001) and the non-pygmy Bantu (F1,29 = 6.101; p = 0.020) group.

[Table 4 here] [Figure 2 here] The second principal component (PC2: 12.51%) showed clear inter-group differences in dental metrics (F1,139 = 51.657; p < 0.001), although only the Baka pygmies showed significant differences between males and females (F1,108 = 6.528; p = 0.012). For this component, significant positive loadings (r > 0.3; p < 0.01) were observed for the UC, UM1, and UM2, whereas significant negative loadings (r ≤ -0.4; p < 0.05) were observed for the maxillary and mandibular incisors and for most of the lower post-canine teeth (see Table 4).

Discussion

Population variability in tooth size

Significant differences in overall crown dimensions have been observed between African pygmies and non-pygmy Bantu-speaking farmers. Sex-related differences in dental size are only present in some tooth-types and greatly vary among groups. Cross-population studies have already shown that sexual dimorphism is a population-specific phenomenon, with dental crown size for males exceeding on average that for females [START_REF] Garn | Genetic control of sexual dimorphism in tooth size[END_REF][START_REF] Harris | Size matters: discrimination between American Blacks and Whites, males and females, using tooth crown dimensions[END_REF][START_REF] Zorba | Sexual dimorphsim in permanent teeth of modern Greeks[END_REF].

Populations with larger teeth tend to show greater sexual dimorphism [START_REF] Garn | Genetic control of sexual dimorphism in tooth size[END_REF][START_REF] Harris | Size matters: discrimination between American Blacks and Whites, males and females, using tooth crown dimensions[END_REF]. The observed sex-related differences in dental dimensions within the Baka pygmies and the Bantu-speakers from Cameroon also show this dimorphic trend, with the Baka showing larger and more dimorphic teeth. The intertooth levels of sexual dimorphism observed among the Baka individuals are similar to those seen in the Australian Aborigine foragers and in African Americans [START_REF] Zorba | Sexual dimorphsim in permanent teeth of modern Greeks[END_REF]. However, the reduced sexual dimorphism found in the Bantu sample studied appears to stand apart from that observed in the Baka and other African populations [START_REF] Kieser | Human adult odontometric: the study of variation in adult tooth size[END_REF]. For instance, San hunter-gatherers show a microdontic dental pattern in comparison to southern African Bantu-speakers [START_REF] Haeussler | Morphological and metrical comparison of San and Central Sotho dentitions from southern Africa[END_REF][START_REF] Van Reenen | Dentition, jaws and palate of the Kalahari Bushman[END_REF]. Likewise, San people exhibit also greater sexual dimorphism in maxillary teeth than those Bakoni and Batau farmer groups [START_REF] Haeussler | Morphological and metrical comparison of San and Central Sotho dentitions from southern Africa[END_REF].

Further, odontometrics of other southern African Bantu-speaking groups reveal a closely related sexual dimorphic trend with male teeth generally larger and more variable in dimensions [START_REF] Kieser | An odontometric analysis of the early Griqua dentition[END_REF]. Thereby, the unusual lack of dental sexual dimorphism found in the Bantu groups analyzed could be a distinct phenetic trait resulting from a reduction in tooth size in males [START_REF] Harris | Tooth size of Ticuna Indians, Colombia, with phenetic comparisons to other Amerindians[END_REF]. The relative sizes of the anterior and post-canine maxillary dentitions effectively discriminated pygmy and non-pygmy groups and illustrate that the anterior and post-canine segments of the dental arch function as independent units [START_REF] Harris | A principal components analysis of human odontometrics[END_REF][START_REF] Kieser | Allometric relations of teeth and jaws in man[END_REF]. In our study, Baka pygmies show smaller anterior teeth and larger cheek teeth than the Bantu speakers. This difference in the apportionment of dental size across the dentition has already been described in sub-Saharan Africans and their derivative groups [START_REF] Harris | Ethnic differences in the apportionment of tooth size[END_REF]. When the anterior and post-canine teeth act as two separate, negatively correlated units [START_REF] Kieser | Allometric relations of teeth and jaws in man[END_REF], molar size enlargement, an ancestral character in the Baka pygmies, may be accompanied by a reduction in the incisor sizes. Morphometric studies have shown that western pygmies have developed smaller facial length and height dimensions than non-pygmy populations (Ramirez-Rozzi & Sardi, 2010), which might be associated to the reduction in size of the anterior dentition and enlargement of molar size dimensions. Indeed, [START_REF] Marquer | Nouvelle contribution à l'étude du squelette des Pygmées occidentaux du Centre africain comparé à celui des Pygmées orientaux[END_REF] reported that the cranium of western pygmies shows a well-develop macrodont premolar-molar pattern compared to the length of the skull base. Instead, the relatively small post-canine teeth in the Bantu farmers might be related to changes in the masticatory apparatus due to dietary and morphogenetic factors [START_REF] Calcagno | Human dental reduction: natural selection or the probable mutation effect[END_REF][START_REF] Hanihara | Metric dental variation of major human populations[END_REF].

Tooth scaling revisited

Most intra-population studies on tooth size allometry in humans have shown a lack of significant correlations between tooth size and stature or body size [START_REF] Perzigian | Allometric analysis of dental variation in a human population[END_REF]; but see [START_REF] Kieser | Human adult odontometric: the study of variation in adult tooth size[END_REF]. Earlier reports have suggested that epigenetic factors controlling tooth and body sizes during early growth stages are independents [START_REF] Kieser | Allometric relations of teeth and jaws in man[END_REF][START_REF] Shea | Tooth scaling and evolutionary dwarfism: an investigation of allometry in human Pygmies[END_REF]. The low growth rates during the earliest infancy in Baka pygmies of know ages are accompanied by changes in tooth-eruption rates [START_REF] Ramirez-Rozzi | Diversity in tooth eruption and life history in humans: illustration from a Pygmy population[END_REF][START_REF] Ramirez-Rozzi | Growth pattern from birth to adulthood in African pygmies of known age[END_REF]. Teeth in Baka pygmies erupt earlier than in any other sub-Saharan African Bantu population. Crown formation of their permanent first molars (M1) occurs during a period of reduced body growth rates (Ramirez-Rozzi, 2016), likely affecting the plasticity of dental developmental, since during this precalcification epigenetic and environmental factors can influence the size and shape of the developing tooth crown [START_REF] Townsend | Morphogenetic fields within the human dentition: a new, clinically relevant synthesis of an old concept[END_REF].

Our observation that the maxillary molars of the Baka are larger than those of the Bantu groups is consistent with inter-population scaling patterns. Intra-specific allometric reports by [START_REF] Lavelle | Relationship between tooth and long bone size[END_REF] and [START_REF] Perzigian | Allometric analysis of dental variation in a human population[END_REF] found a negative maxillary tooth size to body size association. Studies by [START_REF] Shea | Tooth scaling and evolutionary dwarfism: an investigation of allometry in human Pygmies[END_REF] reveal that the covariation between body size and tooth dimensions, in both African pygmies and large-sized non-pygmies, was only present in the absolute size of upper molars (M1-2), with the dwarfed forms exhibiting larger dimensions and a negative allometry of tooth size to body size. Since stature in Baka pygmies from Cameroon is much lower than that in the non-pygmy neighbors [START_REF] Becker | Indirect evidence for the genetic determination of short stature in African Pygmies[END_REF][START_REF] Vallois | Les Pygmées Baká du Cameroun: Anthropologie et ethnographie avec une annexe démographique[END_REF], our odontometric results support the scaling prediction for the maxillary molars by [START_REF] Shea | Tooth scaling and evolutionary dwarfism: an investigation of allometry in human Pygmies[END_REF] of a strong negative allometry on tooth-size to body height in African pygmy individuals.

Sex-related tooth-scaling patterns among populations can also be explained by the odontogenesis during growth [START_REF] Townsend | Genetic, environmental and epigenetic influences on variation in human tooth number, size and shape[END_REF]. The variability in the degree and patterning of sex dimorphism in the size of the permanent teeth is the consequence of the population-specific variability in the array of genes that control the apportionment of overall tooth size of the permanent dentition. In this regard, tooth enlargement in Baka pygmy males seems to be strongly correlated with the overall degree of sexual dimorphism in body size, since Cameroonian pygmy and non-pygmy males show greater adult statures than females [START_REF] Becker | Indirect evidence for the genetic determination of short stature in African Pygmies[END_REF].

Dental phenetic diversity

The odontometric variability recorded among ethnic groups from Cameroon appears to be consistent with genetic signatures and population admixture. Overall, geographic affinities do not correspond to dental phenetic similarities. The demographic expansion of the early Bantu speakers, practicing slash-and-burn agriculture, from the Nigeria-Cameroon borderland to southern Africa (~5,000 years ago) was confronted by the dense and humid forests of Central Africa [START_REF] Campbell | The evolution of human genetic and phenotypic variation in Africa[END_REF][START_REF] De Filippo | Bringing together linguistic and genetic evidence to test the Bantu expansion[END_REF][START_REF] Grollemund | Bantu expansion shows that habitat alters the route and pace of human dispersals[END_REF]. Population genetics have shown that the expansion of the early farming societies into the forests triggered the fragmentation of western pygmies ~2,800 years ago [START_REF] Verdu | Origins and genetic diversity of Pygmy hunter-gatherers from Western Central Africa[END_REF]. Genetic and linguistic signatures show that geographically distant Bantu-speakers are closely related [START_REF] De Filippo | Bringing together linguistic and genetic evidence to test the Bantu expansion[END_REF][START_REF] Quintana-Murci | Maternal traces of deep common ancestry and asymmetric gene flow between Pygmy Hunter-Gatherers and Bantu-Speaking Farmers[END_REF], which provides evidence of the demic diffusion of farmer populations through the Congo Basin during the Bantu expansion. Substantial admixture between pygmies and non-pygmies occurred only in the last ~1,000 years, at later phases of the farming communities' expansion [START_REF] Patin | The impact of agricultural emergence on the genetic history of African rainforest hunter-gatherers and agriculturalists[END_REF]. In this genetic scenario, the levels of admixture across the pygmy populations were high and largely variable [START_REF] Quintana-Murci | Maternal traces of deep common ancestry and asymmetric gene flow between Pygmy Hunter-Gatherers and Bantu-Speaking Farmers[END_REF][START_REF] Verdu | Origins and genetic diversity of Pygmy hunter-gatherers from Western Central Africa[END_REF]. For instance, the Baka groups show substantially less admixture with non-pygmy neighbors than other western pygmy populations [START_REF] Verdu | Sociocultural behavior, sex-biased admixture, and effective population sizes in Central African Pygmies and non-Pygmies[END_REF]. A strong signal of isolation by distance has been reported among the Baka pygmies [START_REF] Verdu | Limited dispersal in mobile hunter-gatherer Baka Pygmies[END_REF], a pattern owing to limited parent-offspring dispersal. This genetic model could explain the phenotypic differences in dental size found between the Baka and Bantu-speaking populations.

The diffusion of an agriculture-based lifestyle during the Bantu expansion is a significant event for our odontometric evidence. Teeth may be under strong selective pressures related to both cultural factors and dietary changes favoring a reduction in dental size [START_REF] Calcagno | Human dental reduction: natural selection or the probable mutation effect[END_REF][START_REF] Hanihara | Metric dental variation of major human populations[END_REF]. The Baka huntergatherer lifestyles are associated with a large post-canine dentition, while their Bantu neighbors, engaged in agricultural subsistence practices, show a significant reduction in molar sizes. Thereby, selection upon phenotypic plasticity was a likely non-random factor causing the observed strong between-populations differences in tooth size.

Further studies on dental size variability within African pygmies are yet needed, but the evidence that the western pygmies -with high and largely variable levels of admixture [START_REF] Quintana-Murci | Maternal traces of deep common ancestry and asymmetric gene flow between Pygmy Hunter-Gatherers and Bantu-Speaking Farmers[END_REF][START_REF] Verdu | Origins and genetic diversity of Pygmy hunter-gatherers from Western Central Africa[END_REF]) and short stature [START_REF] Becker | Indirect evidence for the genetic determination of short stature in African Pygmies[END_REF] also show particular odontometric patterns compared with Bantu-speakers, could suggest that genetic signatures on dental phenotypes have probably evolved independently in different African pygmy populations.

  Figure 2

Table 1 .

 1 Descriptive statistics of tooth crown dimensions for Baka hunter-gatherers by sexual dimorphism.

	Tooth			Males			Females		All		Sex difference 1
	Maxil	Meas		σ	CV		σ	CV		σ	%	F1,10	p
	la	ure	𝒙 ̅			𝒙 ̅			𝒙 ̅			8
		MD	9.1	0.7	8.6	8.6	0.6	7.6	8.7	0.7	6.1	14.	0.0
	UI1		48	92	58	15	57	26	94	46	87	037	00*
		BL	8.4	0.8	10.	7.7	0.7	9.3	7.9	0.8	9.5	22.	0.0
			69	81	403	29	26	93	78	53	74	097	00*
		MD	6.9	0.5	7.7	6.5	0.5	7.9	6.7	0.5	5.7	12.	0.0
	UI2		71	41	61	94	26	77	21	58	17	384	01*
		BL	7.4	0.5	7.5	6.9	0.6	9.6	7.1	0.6	8.3	20.	0.0
			96	69	91	20	66	24	14	89	24	215	00*
		MD	8.0	0.5	6.3	7.5	0.4	5.5	7.7	0.5	7.3	37.	0.0
	UC		83	12	34	29	17	39	15	21	58	062	00*
		BL	9.2	0.7	7.8	8.2	0.7	9.6	8.6	0.9	12.	42.	0.0
			88	25	06	69	97	38	12	10	323	562	00*
		MD	7.7	0.5	6.9	7.5	0.6	7.9	7.6	0.5	2.7	3.2	0.0
	UP3		85	40	36	73	00	23	44	87	99	96	72
		BL	9.8	0.6	6.6	9.5	0.5	6.0	9.6	0.6	3.3	6.8	0.0
			50	56	60	32	74	22	39	19	36	46	10*
		MD	7.5	0.5	7.3	7.3	0.5	7.9	7.4	0.5	2.9	3.4	0.0
	UP4		57	54	31	41	87	96	14	82	42	53	66
		BL	9.6	0.6	6.9	9.4	0.6	6.7	9.4	0.6	2.1	2.3	0.1
			18	72	87	19	34	31	86	51	13	25	30
		MD	12.	0.6	5.5	11.	0.5	5.1	11.	0.6	4.6	18.	0.0
	UM1		013	61	02	479	90	40	659	62	52	578	00*
		BL	11.	0.6	5.2	11.	0.7	6.3	11.	0.7	2.7	5.1	0.0
			945	30	74	624	39	58	732	18	62	18	26*
		MD	11.	0.7	6.3	10.	0.7	7.1	11.	0.8	8.6	36.	0.0
	UM2		680	38	18	752	67	34	064	73	31	798	00*
		BL	11.	0.8	7.2	11.	0.6	5.7	11.	0.7	3.8	9.0	0.0
			998	68	35	548	68	85	699	68	97	64	03*
	Mand	Meas		σ	CV		σ	CV		σ	%	F1,10	p
	ible	ure	𝒙 ̅			𝒙 ̅			𝒙 ̅			8
		MD	5.4	0.3	6.8	5.3	0.5	9.5	5.3	0.4	1.6	0.8	0.3
	LI1		45	72	32	59	10	17	88	68	05	35	63
		BL	6.7	0.6	9.2	6.3	0.6	9.6	6.5	0.6	5.8	9.1	0.0
			70	25	32	93	15	20	19	41	97	12	03*
		MD	6.0	0.3	6.1	5.9	0.4	7.5	5.9	0.4	2.0	2.0	0.1
	LI2		61	75	87	40	47	25	81	26	37	11	59
		BL	6.9	0.6	9.0	6.5	0.5	8.3	6.6	0.6	7.0	15.	0.0
			67	30	43	08	41	13	62	10	53	843	00*
		MD	7.2	0.4	5.8	6.7	0.4	6.0	6.9	0.4	7.5	37.	0.0
	LC		56	25	57	45	10	79	17	79	76	187	00*
		BL	8.0	0.6	8.0	7.2	0.5	7.8	7.5	0.7	12.	55.	0.0
			95	49	17	00	66	61	01	29	431	573	00*
		MD	7.6	0.5	7.4	7.4	0.4	6.5	7.5	0.5	3.0	4.6	0.0
	LP3		69	70	33	44	88	56	20	25	23	16	34*
		BL	8.0	0.5	6.4	7.8	0.4	6.3	7.9	0.5	2.5	3.9	0.0
			54	21	69	51	98	43	19	13	86	75	49*
		MD	7.9	0.5	6.6	7.7	0.5	7.5	7.7	0.5	2.6	3.2	0.0
	LP4		27	25	23	21	85	77	90	72	68	57	74
		BL	8.4	0.6	7.9	8.3	0.6	7.9	8.3	0.6	0.5	0.1	0.7
			00	71	88	54	62	24	69	63	51	19

Table 2 .

 2 Descriptive statistics of tooth crown dimensions for Bantu-speaking farmers by sexual dimorphism.

	Tooth			Males			Females		All		Sex difference 1
	Maxil	Meas		σ	CV		σ	CV		σ	%	F1,29	p
	la	ure	𝒙 ̅			𝒙 ̅			𝒙 ̅
		MD	9.3	0.5	6.0	9.0	0.4	4.9	9.2	0.5	3.7	2.7	0.1
	UI1		69	71	95	27	51	96	58	52	89	48	08
		BL	8.6	0.6	6.9	8.5	0.9	11.	8.5	0.7	0.6	0.0	0.8
			14	00	65	61	87	529	96	30	19	35	54
		MD	7.1	0.6	9.2	6.7	0.4	7.1	7.0	0.6	6.2	3.1	0.0
	UI2		66	66	94	47	83	59	31	37	10	36	87
		BL	7.2	0.7	10.	7.0	1.0	14.	7.2	0.8	2.5	0.3	0.5
			79	40	166	99	63	974	21	44	36	01	87
		MD	7.6	0.5	7.6	7.1	0.3	5.1	7.5	0.5	6.5	5.4	0.0
	UC		56	84	28	84	71	64	04	65	70	11	27*
		BL	8.4	0.6	7.8	7.4	0.3	4.4	8.0	0.7	13.	20.	0.0
			26	60	33	13	29	38	99	45	665	797	00*
		MD	7.6	0.5	7.1	7.4	0.3	5.1	7.5	0.5	2.5	0.9	0.3
	UP3		30	47	69	39	86	89	69	02	68	79	31
		BL	9.4	0.8	8.6	9.4	0.5	5.8	9.4	0.7	0.2	0.0	0.9
			74	15	02	54	57	92	68	32	12	05	45
		MD	7.3	0.4	6.2	6.9	0.4	7.1	7.2	0.4	4.9	3.6	0.0
	UP4		33	59	59	87	99	42	21	92	52	37	66
		BL	9.5	0.7	8.0	9.4	0.6	6.9	9.5	0.7	1.7	0.3	0.5
			89	68	09	23	57	72	35	28	62	43	63
		MD	11.	0.6	5.9	11.	0.6	5.3	11.	0.6	0.9	0.1	0.6
	UM1		382	80	74	279	05	64	349	48	13	64	89
		BL	11.	0.6	5.5	11.	0.6	5.3	11.	0.6	2.2	1.1	0.2
			494	40	68	238	01	48	411	30	78	28	97
		MD	10.	0.8	8.4	9.9	1.0	10.	10.	0.9	6.0	2.7	0.1
	UM2		533	93	78	35	38	448	340	67	19	36	09
		BL	11.	0.8	7.4	11.	0.8	7.1	11.	0.8	3.3	1.2	0.2
			571	67	93	200	05	88	451	53	13	97	64
	Mand	Meas		σ	CV		σ	CV		σ	%	F1,29	p
	ible	ure	𝒙 ̅			𝒙 ̅			𝒙 ̅
		MD	5.5	0.2	4.9	5.3	0.1	2.1	5.4	0.2	3.0	3.2	0.0
	LI1		30	74	55	65	18	99	76	46	75	99	80
		BL	6.6	0.8	12.	5.9	0.6	11.	6.4	0.8	13.	6.4	0.0
			79	52	756	01	50	015	28	65	184	78	17*
		MD	6.1	0.4	6.9	5.9	0.4	8.1	6.0	0.4	3.7	1.7	0.2
	LI2		65	26	10	41	85	64	93	51	70	20	00
		BL	6.8	0.8	12.	6.4	0.4	6.4	6.7	0.7	6.0	1.9	0.1
			38	29	123	49	17	66	13	38	32	41	74
		MD	7.0	0.5	7.4	6.7	0.2	3.7	6.9	0.4	3.4	1.8	0.1
	LC		21	21	21	85	55	58	45	62	78	21	88
		BL	8.0	0.8	10.	6.8	0.6	9.3	7.6	0.9	16.	14.	0.0
			18	41	489	86	43	38	53	41	439	106	01*
		MD	7.7	0.5	7.1	7.4	0.6	8.3	7.6	0.5	3.7	1.5	0.2
	LP3		07	49	23	32	19	29	18	77	00	62	21
		BL	8.3	0.7	8.5	7.9	0.6	7.9	8.2	0.7	5.7	2.9	0.0
			60	12	17	05	27	32	13	09	56	66	96
		MD	7.8	0.5	7.1	7.5	0.7	9.4	7.7	0.6	4.1	1.7	0.1
	LP4		40	58	17	27	13	73	39	18	58	86	92
		BL	8.6	0.8	9.4	8.3	0.7	8.3	8.5	0.7	2.8	0.6	0.4
			18	12	22	79	01	66	41	75	52	38

Table 3 .

 3 Tooth size differences by sex between Baka hunter-gatherers and Bantu-speaking farmers.Size indicates differences in tooth dimensions (mm) between Baka pygmies and Bantu-speakers. Negative values denote greater sizes in Bantu-speakers. Statistically significant differences (one-way ANOVA) at p < 0.05 (*). See Tables1 and 2for odontometric values.

	Tooth			Males			Females		Group (pooled sex)
		Measu	Size	F1,56	p	Size	F1,81	p	Size	F1,139	p
		re	(mm			(mm			(mm
	Maxilla		)			)			)
		MD	-0.2	1.25	0.26	-0.4	3.67	0.05	-0.4	10.3	0.00
	UI1		21	6	7	12	0	9	64	77	2*
		BL	-0.1	0.44	0.50	-0.8	10.5	0.00	-0.6	13.4	0.00
			45	3	8	32	60	2*	18	96	0*
		MD	-0.1	1.47	0.23	-0.1	0.76	0.38	-0.3	7.02	0.00
	UI2		95	1	0	53	4	5	10	3	9*
		BL		1.56	0.21	-0.1	0.54	0.46	-0.1	0.53	0.46
			0.217	3	6	79	4	3	07	1	8
		MD		8.41	0.00		6.12	0.01		3.84	0.05
	UC		0.427	7	5*	0.345	8	5*	0.211	3	2
		BL		20.1	0.00		11.1	0.00		8.26	0.00
			0.862	63	0*	0.856	65	1*	0.513	7	5*
		MD		1.09	0.30		0.46	0.49		0.42	0.51
	UP3		0.155	4	0	0.134	5	7	0.075	5	6
		BL		3.69	0.06		0.16	0.68		1.71	0.19
			0.376	1	0	0.078	4	7	0.171	2	3
		MD		2.47	0.12		3.30	0.07		2.82	0.09
	UP4		0.224	6	1	0.354	8	3	0.193	1	5
		BL		0.02	0.88	-0.0	0.00	0.98	-0.0	0.13	0.71
			0.029	3	0	04	0	4	49	2	7
		MD		12.0	0.00		1.00	0.31		5.36	0.02
	UM1		0.631	00	1*	0.200	4	9	0.310	0	2*
		BL		6.79	0.01		2.49	0.11		5.07	0.02
			0.451	9	2*	0.386	5	8	0.321	9	6*
		MD		27.7	0.00		9.12	0.00		15.8	0.00
	UM2		1.147	71	0*	0.817	9	3*	0.724	60	0*
		BL		3.24	0.07		2.27	0.13		2.40	0.12
			0.427	1	7	0.348	3	5	0.248	2	3
	Mandi	Measu	Size	F1,56	p	Size	F1,81	p	Size	F1,139	p
	ble	re	(mm			(mm			(mm
			)			)			)
		MD	-0.0	0.82	0.36	-0.0	0.00	0.97	-0.0	1.02	0.31
	LI1		85	4	8	06	1	2	88	4	3
		BL		0.21	0.64		5.54	0.02		0.41	0.51
			0.091	6	4	0.492	3	1*	0.091	9	9
		MD	-0.1	0.92	0.34	-0.0	0.00	0.99	-0.1	1.62	0.20
	LI2		04	6	0	01	0	7	12	1	5
		BL		0.44	0.50		0.10	0.74	-0.0	0.15	0.69
			0.129	6	7	0.059	7	4	51	1	8
		MD		3.46	0.06	-0.0	0.09	0.76	-0.0	0.08	0.77
	LC		0.235	9	8	40	0	5	28	5	2
		BL		0.15	0.70		2.63	0.10	-0.1	0.91	0.34
			0.077	0	0	0.314	2	9	52	8	0
		MD	-0.0	0.06	0.80		0.00	0.94	-0.0	0.81	0.36
	LP3		38	2	4	0.012	5	4	98	4	9
		BL	-0.3	3.51	0.06	-0.0	0.09	0.75	-0.2	6.64	0.01
			06	4	6	54	9	4	94	3	1*
		MD		0.35	0.55		0.91	0.34		0.18	0.66
	LP4		0.087	0	6	0.194	7	1	0.051	6	7
		BL	-0.2	1.21	0.27	-0.0	0.01	0.91	-0.1	1.50	0.22
			18	2	6	25	3	1	72	4	2

Table 4 .

 4 Unrotated principal component analyses based on geometrically scaled measurements for groups analyzed.

	Tooth	Measure	PC1	PC2
	Maxilla
	UI1	MD	-0.296**	-0.451**
		BL	-0.659**	-0.294**
	UI2	MD	-0.182*	-0.282**
		BL	-0.452**	-0.046
	UC	MD	-0.154	0.429**
		BL	-0.528**	0.535**
	UP3	MD	0.374**	-0.057
		BL	0.428**	0.127
	UP4	MD	0.407**	0.143
		BL	0.523**	0.001
	UM1	MD	0.223**	0.364**
		BL	0.500**	0.113
	UM2	MD	0.037	0.861**
		BL	0.249**	0.425**
	Mandible	Measure	PC1	PC2
	LI1	MD	0.011	-0.225**
		BL	-0.378**	0.044
	LI2	MD	0.176*	-0.225**
		BL	-0.433**	-0.002
	LC	MD	-0.181**	0.041
		BL	-0.615**	-0.013
	LP3	MD	0.513**	-0.228**
		BL	0.050	-0.249**
	LP4	MD	0.503**	-0.126
		BL	0.340**	-0.209*
	LM1	MD	0.329**	-0.103
		BL	0.388**	-0.381**
	LM2	MD	0.422**	-0.119
		BL	0.279**	-0.224**
	Eigenvalue		2.741	2.127
	Variance (%)		16.126	12.516

Correlation (Pearson's r) at p < 0.05 (*) and p < 0.01 (**).

Sexual dimorphism % defined as [(𝑥̅ male -𝑥̅ female) / 𝑥̅ female]*100; F-ratio from one-way ANOVA at p < 0.05 (*).

Acknowledgements

The authors thank X. Garde, B. Bordage and all personnel of IRD Yaounde for their logistical support; the nuns at the Le Bosquet mission for their kind hospitality. Special thanks to P. Kalo and J. B. Etoa for their help, assistance and friendship. We are grateful to all the Baka, Mvae and Yassa people during the field studies for their help and hospitality, whose participation made this research possible.

Grant sponsors: Spanish Ministerio de Economía y Competitividad, grant numbers CGL2010-15340, CGL2011-22999 and CGL2014-52611-C2-1-P; Wenner-Gren Foundation, grant number 7819; National Geographic, grant number 8863-10; The French National Research Agency, 'GrowinAP' program, grant number ANR-11-BSV7-0011.