Muzall et al. 2014.3 8 Dental Anthropology 2014 │ Volume 27 │ Issues 01 and 02 2014 Dahlberg Award Winner: The effects of dietary toughness on occlusopalatal variation in savanna baboons Evan Muzzall 1 , Ryan M. Campbell 1 , Meadow Campbell 1,2 , and Robert S. Corruccini 1 1Department of Anthropology, Southern Illinois University, Carbondale, IL 62901 2Department of Anatomy, Southern Illinois University School of Medicine, Carbondale, IL 62901 ABSTRACT This study investigates the relationship between dietary toughness and craniofacial varia- tion in two groups of savanna baboons. Standard craniofacial and malocclusion data were collected from a captive, soft-diet experiment group (n=24) and a sample of wild-captured baboons, raised on tougher, natural foods (n=19). We tested the hy- pothesis that in the absence of normal masticatory stress experienced during the consumption of wild foods, the captive baboons would exhibit higher levels of facial and dental structural irregularities. Principal component analysis indicates separation of the two samples. The soft-diet sample exhibits significantly shorter palates, greater variability in palate position, and higher frequencies of occlusal irregularities that correlate with the shorter pal- ates. Results offer further support that long-term dietary chewing stresses have a measurable effect on adult craniofacial variation. Keywords: Diet, occlusopalatal variation, savanna baboons Correspondence to: Evan Muzzall, Dept. of Anthropology, Southern Illinois University, Faner Building, Rm. 3525, 1000 Faner Dr., Carbondale, IL, 62901 Email: muzzall@siu.edu Telephone: (618) 536-6651 Malocclusions are the improper growth, posi- tioning, and/or alignment of the teeth and jaws that lead to irregularities in occlusal surface contact and abnormalities of the surrounding bony struc- tures. These deviations are due to multiple factors, but the reduced masticatory demands of modern diets have shown considerable influence (Corruccini, 1984; 1999; Corruccini et al., 1983; Cor- ruccini and Lee, 1984; Varrela, 1990, 1992, 2006; Evensen and Øgaard, 2007). Notably, alterations in the proper growth trajectories of these areas due to decreased chewing forces are not unique to hu- mans. By controlling for diet, laboratory animal studies have contributed to a broader understand- ing of occlusofacial variability (Beecher and Cor- ruccini, 1981a, b; Corruccini and Beecher, 1982; Larsson et al., 2005; Grünheid et al., 2009; Jašarević et al., 2010; Ravosa et al., 2010; Dias et al., 2011; Makedonska et al., 2012). This study expanded on the research of Cor- ruccini and Beecher (1984), who found reduced facial growth, decreased structural correlations, narrower faces, and more occlusal irregularities in savanna baboons fed a soft diet. Using the same soft-diet sample as Corruccini and Beecher (1984), but a different research design and a wild compar- ative sample, the present study contrasted craniofa- cial and occlusal data between two groups of sa- vanna baboons fed diets that differed in their me- chanical properties. This study tested the hypothe- sis that in the absence of natural food consumption, the soft-diet baboon sample would exhibit higher levels of craniofacial variation due to their reduced chewing demands. MATERIALS AND METHODS The soft-diet experiment group consisted of 24 male Papio cynocephalus skulls housed at Southern Illinois University Carbondale. As part of a bio- medical study in the 1970s, these individuals were fed "a very soft, atherogenic diet consisting of cho- lesterol, lard, butter, egg yolks, and powdered chow" for the last 27 months of their dental matura- tion (Corruccini and Beecher, 1984:136). Eighteen male P. anubis and one male P. cynocephalus indi- vidual housed at the Field Museum of Natural His- tory were selected to be the wild-diet control sam- ple because of their wild African origin. Although their exact diet was not known, their natural wild foods consist of grasses, roots, plants, leaves, bark, gums, seeds, fruit, berries, corn, small inverte- brates, and even sheep and goats (Post, 1981; Bar- ton, 1993; Wahungu, 1998; Akosim et al., 2010; Johnson et al., 2012). Visually, all individuals were dentally mature and had erupted third molars to suggest ages around 6-8 years (Phillips-Conroy and Jolly, 1988). 9 Occlusopalatal Variation in Baboons Dental Anthropology 2014 │ Volume 27 │ Issues 01 and 02 Members of genus Papio are possibly populations of a single species and are sometimes referred to as Papio hama- dryas cynocephalus and P. hamadryas anubis to reflect this subspecies distinction. P. cynocephalus and P. anubis have been known to interbreed (Samuels and Altmann, 1986; Alberts and Altmann, 2001; Charpentier et al., 2008; Tung et al., 2008) despite geographical distinctions in their genet- ic compositions (Williams-Blangero et al., 1990; Zinner et al., 2013). Further, Frost et al. (2003) noted cranial morpho- logical clinal organization of genus Papio in Africa. North- ern baboons (like P. anubis) exhibit broader, less flexed cra- nia and rostra compared to the southern forms (such as P. cynocephalus) that display inferiorly flexed and narrower crania and rostra (Frost et al., 2003:1056, 1069). Because of their clinal organization and similar environments, general shape differences between these two groups observed by Frost et al. (2003) likely reflect genetic differences. Linear measurements consisted of standard craniofacial measurements (Moore-Jansen et al., 1994) and posterior airway maximum lengths and breadths (Fig. 1, Table 1). These data were recorded using spreading and sliding Mi- tutoyo calipers calibrated to 0.01mm. Principal component analysis (PCA) was used to identify measurement loadings responsible for driving the observed variation. Pearson's product-moment correlation coefficient was used to ana- lyze the strength of correlation between measures identi- fied by the PCA. Occlusal data (Table 2) consisted of molar class relation- ships (Angle, 1899), posterior crossbite, rotations, displace- ments, and incisor overbite and overjet following the sum- mation in Harris and Corruccini (2008). For the purposes of our study, we reduced occlusal scores to a score of 0 for normal occlusion and 1 for any deviation from normal oc- clusion in each category. These values were summed to estimate the magnitude of occlusal irregularity for each individual, and significant differences between the samples were calculated using a Mann-Whitney U test. Spearman's rank correlation coefficient tested for associ- ations between relevant linear measures and occlusal scores. To attempt to account for potential variation in body size, shape ratios were calculated by dividing all line- ar measurements by foramen magnum breadth (simplified from area calculations found in Radinsky, 1967; Gould, 1975). The raw and scaled datasets produced highly similar results so that the data quality appears to be high, and only the scaled data are reported here. Statistical analysis was conducted by RMC using the R Project for Statistical Com- puting (R Core Team, 2013) and PAST: Paleontological Sta- tistics software (Hammer, Harper, and Ryan, 2001). RESULTS The PCA results indicate that the combined first two prin- cipal components account for 81% of variation within the sample (Fig. 2). The first principal component (PC1) indi- cates a size increase, primarily in the measures with the highest loadings, along that axis (Fig. 3). There was clear separation between the wild-diet (maroon circles) and soft- diet (blue circles) samples primarily along the second prin- cipal component (PC 2). The loadings for PC 2 (Fig. 3) sug- gest that palate length (PAL) and incisivion (most distal point in the incisive foramen) to basion (IFB) distance con- tributed most to variation along this axis. Importantly, the single wild-diet P. cynocephalus (red circle) groups with the wild-diet P. anubis sample rather than the soft-diet P. cyno- cephalus sample, which suggests that the variation along Fig. 1. Illustration of relevant linear measures, palate length (PAL) and incisivion to basion (IFB). 10 Occlusopalatal Variation in Baboons Dental Anthropology 2014 │ Volume 27 │ Issues 01 and 02 Measure Description MWCond Mediolateral width of the mandibular condyle taken at the longest ML axis MLCond Maximum AP length of the mandibular condyle, perpendicular to MWCond MDC Maximum depth of the mandibular corpus from the superior edge of the alveolar MWCorp Maximum width of the mandibular corpus from the labial to lingual side of the mandibular cor- pus at the midpoint of M1 MMdW Maximum mandibular arch width at M1 taken on alveolar bone with calipers at the midpoint of LM1 and RM1 NMdW Minimum mandibular arch width at M3 taken on the alveolar bone with calipers at the mid- point of LM3 and RM3 MandL Mandibular length from the anterior point of projection on the alveolar bone of the mandible to the most posterior projection of the mandibular condyles (infradentale to condylion) MMW Maximum maxillary arch width taken at the widest point of the alveolar bone on the maxilla regardless of field or adjacent tooth NMW Minimum maxillary arch width at M3 taken on the alveolar bone (lingual surface) at the mid- points of LM3 & RM3 NSB Minimum snout breadth between L & R maxilla, with calipers in the fossae PAL The length of the palate measured from prosthion to the plane of the posterior projection of the maxilla (using a rubber band to delineate the posterior border) IFB From the most posterior point on the incisive foramen to basion (incisivion was estimated in poorly mascerated soft-diet individuals) PAB The greatest medio-lateral breadth of the posterior airway, taken with the calipers held just pos- terior to the palate PAH The anterio-posterior length of the internal nares, from the posterior margin of the palate to the anterior margin of the opening BZB The widest breadth across L & R zygomatic arches (zygion to zygion) FB The breadth of the frontal bone across brows FMW The medio-lateral breadth of the foramen magnum, measured from within the margins of the occipital with the “inside” arms of the caliper TABLE 1. Metric variables PC 2 is not the result of genetic differences. A two-sample t-test demonstrates that mean differences in PAL were significantly smaller (P<0.000), and an F statis- tic indicates that IFB was significantly more variable (p<0.010) in the soft-diet sample (Fig. 4, Table 3). Again, the bivariate plot of PAL and IFB (Fig. 5) implies that the wild- diet P. cynocephalus individual groups with the wild-diet P. anubis group, as a this is a reflection of the PCA measures responsible for driving the observed variation. Although a Pearson's correlation coefficient for the soft-diet group (r=0.752) was only slightly lower than the wild-diet sample (r=0.780), results suggest that the soft-diet sample displays significantly shorter palate lengths relative to IFB distances. A Mann-Whitney U test (Table 4) indicates that the soft- diet sample exhibits significantly greater overall occlusal scores than the wild-diet group. Spearman’s correlation coefficient (Table 5) demonstrates a relatively weak yet sig- nificant (p<0.050) negative correlation between PAL and occlusal scores and suggests that occlusal patterns become more variable as palate length reduces. DISCUSSION The hypothesis that the soft-diet baboon sample would exhibit higher levels of craniofacial variation due to de- creased masticatory loading during ontogeny is supported. Specifically, the soft-diet group exhibits greater occlusopal- atal variation. The single wild-diet P. cynocephalus offers support that our results are not the mere reproduction of clinal shape differences of genus Papio as noted by Frost et al. (2003). Although genetics undoubtedly play a considera- ble role (Carlson, 2005; Harris, 2008; Koussoulakou et al., 2009), our study supports the potential for environmental factors to alter developmental trajectories. Incisivion (Mew, 1974; Frost et al., 2003) should be uti- lized when investigating basicranial flexion. By using inci- sivion to construct multivariate ratios, it may be possible to test for the functional and taxonomic significance of the palate's effect on basicranial flexion (Corruccini, 1978; Oxnard, 1983). Through dietary manipulation of living ani- mals, radiographs could be used to investigate the relation- ships between ontogenetic shape changes, adult cranial form, allometric scaling, heterochrony, and differential 11 Occlusopalatal Variation in Baboons Dental Anthropology 2014 │ Volume 27 │ Issues 01 and 02 Measure Description Anterior Overjet The maximum distance between the most inferior point on the upper central incisors, and the most superior point on the lower central incisors Anterior Overbite The maximum distance between the labial surface of the lower central inci- sors and the labial surface of the upper central incisors Posterior Crossbite The buccolingual interrelationship between upper and lower first molar an- tagonists Normal occlusion The buccal cusp of the upper molars overhang the lower buccal cusps, with the lowers reaching proper centric occlusion Buccal crossbite The upper molars are atypically buccally located, such that the lowers do not reach proper centric occlusion Lingual crossbite The upper molars are atypically lingual, such that the buccal cusps of the up- pers do not overhang the lowers Buccal Segment Relationship The interrelationships between the upper and lower first molars in the para- sagittal plane Class 1 The mesiobuccal cusp of the M1 is parasagittal to the buccal groove of M1 Class 2 The mesiobuccal cusp of M1 is mesial to the buccal groove of M1 Class 3 The mesiobuccal cusp of M1 is distal to the buccal groove of M1 Rotation Refers to a tooth in its normal position in the dental arcade but rotated about its long axis. The sum of rotated teeth are recorded for each side. Recorded for both maxilla and mandible 0 Unrotated 1 Rotated < 45° 2 Rotated > 45° Displacement Refers to a tooth that is out of ideal alignment. The summed value is record- ed. Recorded for both maxilla and mandible 0 Not displaced 1 Displaced < 2mm 2 Displaced > 2mm TABLE 2. Occlusal variables growth (Frost et al., 2003; Leigh, 2006; Trenouth and Joshi, 2006). This could broaden allometric understanding, as Gilbert (2011) reminds us that a large percentage of shape information is lost during allometric correction, and our results suggest that masticatory behavior also confounds shape analyses. These implications are also important for humans. Many authors have used the hunter-gatherer/agricultural transition to illustrate how changes in dietary selective pressures produced skull morphologies able to cope with new masticatory functional demands (Carlson, 1976a, b; Carlson and Van Gerven, 1977; Hinton and Carlson, 1979; Paschetta et al., 2010). Clinically, Haskell et al. (2009) noted correlations between snoring, sleep apnea, and the struc- tures of the face and mouth. However, this study cannot conclude about the taxonomic significance of airway di- mensions in the two samples specifically, nor the relation- ships between dental variation and airway dimensions in general. There could be multiple reasons why smaller air- way dimensions were not found in the soft-diet group. Alt- hough the tight confines of the airway's location could have prevented the caliper arms from accurately touching the landmarks, there also exists the possibility that there does not exist significant morphological differences in this area. Anthropologically, it should be remembered that diet influences our reconstructions of biodistance, phylogeny, and taxonomy whether we account for it or not. Eshed et al. (2006) and Halcrow et al. (2013) rightfully remind us that simple linear relationships between diet and the denti- 12 Occlusopalatal Variation in Baboons Dental Anthropology 2014 │ Volume 27 │ Issues 01 and 02 Fig. 2. PCA on scaled data. Maroon circles = wild diet sample; blue circles = soft diet sample; red circle = the single wild-diet P. cynocephalus. Fig. 4. Boxplot for scaled data. WD = wild-diet sample; SD = soft-diet sample. PAL = palate length; IFB = incisivion to basion. Fig. 3. Loadings for PC1 (top) and PC2 (bottom). Note the inverse relationship between PAL and IFB for PC2. 13 Occlusopalatal Variation in Baboons Dental Anthropology 2014 │ Volume 27 │ Issues 01 and 02 n x̅ SD F p t p PAL WD 18 1.12 0.07 1.35 0.500 8.49 2.16E-10** PAL SD 23 0.94 0.06 IFB WD 18 1.46 0.08 3.668 0.008* -0.13 0.90 IFB SD 23 1.47 0.15 TABLE 3. Summary statistics for relevant linear measurements scaled for body size *significant at P<0.010, **significant at p<0.000; WD = wild-diet sample, SD = soft-diet sample; PAL = palate length, IFB = incisive foramen to basion n Median U p Occlusal WD 19 0 122 0.035* Occlusal SD 20 1 TABLE 4. Mann-Whitney U Test for wild-diet and soft-diet occlusal scores *significant at p<0.050 r’s p PAL + Occlusal -0.3782 0.017* IFB + Occlusal -0.2260 0.172 TABLE 5. Spearman correlation for relevant linear measures and occlusal scores *significant at p<0.050 Fig. 5. Biplot showing the relationship between PAL and IFB. Maroon circles = wild-diet sample; blue circles = soft-diet sample; red circle = the single wild-diet P. cynocephalus. tion should not be assumed as the ambiguity of genetic, environmental, and cultural influences have the poten- tial to produce a multitude of skeletal adaptations and alterations. By expanding on the research of Corruccini and Beecher (1984), we were able to demonstrate that a variety of research designs can strengthen discussions about the gene-environment interaction and other com- plex anthropological topics. Luckily, a stronger under- standing of genetic influences will better contextualize environmental factors of phenotypic variation. Kuang et al. (2013) have documented the involvement of regulato- ry genes in mice with long-term, laboratory induced malocclusions. New discoveries such as this continue to enable anthropology to pioneer explanations of ob- served skeletal variation. ACKNOWLEDGEMENTS Our gratitude is owed to Bill Stanley, Lawrence Haney, and Anna Goldman at the Field Museum of Natural History (Division of Mammals) for access to the wild- diet sample. Thanks to the Southern Illinois University Carbondale Department of Anthropology for providing the soft-diet sample. Jeremiah E. Scott is thanked for providing helpful comments on the manuscript. 14 Occlusopalatal Variation in Baboons Dental Anthropology 2014 │ Volume 27 │ Issues 01 and 02 LITERATURE CITED Akosim C, Joseph J, Egwumah PO. 2010. Assessment of feeding behavior of baboons (Papio anubis) in Hong Hills Adamawa State, Nigeria. J Res Forest Wldl Env 2:60-72. Alberts SC, Altmann S. 2001. Immigration and hybridi- zation patterns of yellow and anubis baboons in and around Amboseli, Kenya. Am J Primatol 53:139-154. Angle EH. 1899. The classification of malocclusion. Dent Cosmos 41:248-264. Barton RA. 1993. Sociospatial mechanisms of feeding competition in female olive baboons, Papio anubis. Anim Behav 46:791-802. Beecher RM, Corruccini RS. 1981a. Effects of dietary con- sistency on craniofacial and occlusal development in the rat. Angle Orthod 51:61-69. Beecher RM, Corruccini RS. 1981b. Effects of dietary con- sistency on maxillary arch breadth in macaques. J Dent Res 60:68. Carlson DS. 1976a. Temporal variation in prehistoric Nu- bian crania. Am J Phys Anthropol 45:467-484. Carlson DS. 1976b. Patterns of Morphological Variation in the Human Midface and Upper Face. In: McNama- ra JA, editors. Factors affecting the growth of the mid- face: Proceedings of a sponsored symposium honor- ing professor Robert E. Moyers, held February 6 and 7 in Ann Arbor, Michigan. Ann Arbor: Center for Hu- man Growth and Development. p 277-279. Carlson DS, Van Gerven DP. 1977. Masticatory function and post-Pleistocene evolution in Nubia. Am J Phys Anthropol 46:495-506. Carlson DS. 2005. Theories of craniofacial growth in the postgenomic era. Semin Orthod 11:172-183. Charpentier MJE, Tung J, Altmann S, Alberts SC. 2008. Age at maturity in wild baboons: Genetic, environ- mental, and demographic influences. Mol Ecol 17:2026-2040. Corruccini RS. 1978. Morphometric analysis: uses and abuses. Yrbk Phys Anthropol 21:134-150. Corruccini RS. 1984. An epidemiologic transition in den- tal occlusion in world populations. Amer J Orthodon- tics 86:419-426. Corruccini RS. 1999. How anthropology informs the or- thodontic diagnosis of malocclusion's causes. Lewis- ton: Edwin Mellen Press. Corruccini RS, Beecher RM. 1982. Occlusal variation re- lated to soft diet in a nonhuman primate. Science (New Series) 218:74-76. Corruccini RS, Potter RHY, Dahlberg AA. 1983. Chang- ing occlusal variation in Pima Amerinds. Am J Phys Anthropol 62:317-324. Corruccini RS, Beecher RM. 1984. Occlusofacial morpho- logical integration lowered in baboons raised on soft diet. J Cran Genet Dev Bio 4:135-142. Corruccini RS, Lee GTR. 1984. Occlusal variation in Chi- nese immigrants to the United Kingdom and their offspring. Arch Oral Biol 29:779-782. Dias GJ, Cook RB, Mirhosseini M. 2011. Influence of food consistency on growth and morphology of the man- dibular condyle. Clin Anat 24:590-598. Eshed V, Gopher A, Hershkovitz I. 2006. Tooth wear and dental pathology at the advent of agriculture: New evidence from the Levant. Am J Phys Anthropol 130:145-159. Evensen JP, Øgaard B. 2007. Are malocclusions more prevalent and severe now? A comparative study of medieval skulls from Norway. Am J Orthod Dentofac 131:710-716. Frost SR, Marcus LF, Bookstein FL, Reddy DP, Delson E. 2003. Cranial allometry, phylogeography, and sys- tematics of large-bodied Papionins (Primates: Cerco- pithecinae) inferred from geometric morphometric analysis landmark data. Anat Rec Part A 275A: 1048- 1072. Gilbert CC. 2011. Phylogenetic analysis of the African Papionin basicranium using 3-D geometric morpho- metrics: The need for improved methods to account for allometric effects. Am J Phys Anthropol 144:60-71. Gould SJ. 1975. Allometry in primates, with emphasis on scaling and the evolution of the brain. In: Szalay FS, editor. Approaches to Primate Paleobiology. Basel: S. Karger. p 244-292. Grünheid T, Brugman P, Zentner A, Langenbach GEJ. 2009. Changes in rabbit jaw-muscle activity parame- ters in response to reduced masticatory load. J Exp Biol 213:775-781. Halcrow SE, Harris NJ, Tayles N, Ikehara-Quebral R, Pietrusewsky M. 2013. From the mouths of babes: Dental caries in infants and children and the intensifi- cation of agriculture in mainland Southeast Asia. Am J Phys Anthropol 150:409-420. Hammer Ø, Harper DAT, Ryan PD. 2001. Past: Paleonto- logical Statistics Software Package for Education and Data Analysis. Palaeontol Electron 4:, vol. 4, issue 1, a r t . 4 : 9 p p . , 1 7 8 k b . h t t p : / / p a l a e o - electronica.org/2001_1/past/issue1_01.htm. Harris EF. 2008. Interpreting heritability estimates in the orthodontic literature. Semin Orthod 14:125-134. Harris EF, Corruccini RS. 2008. Quantification of dental occlusal variation: A review of methods. Dental An- thropology 21:1-11. Haskell JA, McCrillis J, Haskell BS, Scheetz JP, Scarfe WC, Farman AG. 2009. Effects of Mandibular Ad- vancement Device (MAD) on airway dimensions as- sessed with cone- beam computed tomography. Sem Orthod 15:132-158. Hinton RJ, Carlson DS. 1979. Temporal changes in hu- man temporomandibular joint size and shape. Am J Phys Anthropol 50:325-333. 15 Occlusopalatal Variation in Baboons Dental Anthropology 2014 │ Volume 27 │ Issues 01 and 02 Jašarević E, Ning J, Daniel AN, Menegaz RA, Johnson JJ, Stack MS, Ravosa MJ. 2010. Masticatory loading, function, and plasticity: A microanatomical analysis of mammalian circumorbital soft-tissue structures. Anat Rec 293:642-650. Johnson CA, Sweddell L, Rothman JM. 2012. Feeding ecology of olive baboons (Papio anubis) in Kibale Na- tional Park, Uganda: preliminary results on diet and food selection. Afr J Ecol 50:367-370. Koussoulakou DS, Margaritis LH, Koussoulakou SL. 2009. A curriculum vitae of teeth: Evolution, genera- tion, regeneration. Int J Biol Sci 5:226-243. Kuang B, Dai J, Wang QY, Song R, Jiao K, Zhang J, Tian XG, Duan YZ, Wang MQ. 2013. Combined degenera- tive and regenerative remodeling responses of the mandibular condyle to experimentally induced dis- ordered occlusion. Am J Orthod Dentofac 143:69-76. Larsson E, Øgaard B, Lindsten R, Holmgren N, Brattberg M, Brattberg L. 2005. Craniofacial and den- tofacial development in pigs fed soft and hard diets. Am J Orthod Dentofac 128:731-739. Leigh SR. 2006. Cranial ontogeny of Papio baboons (Papio hamadryas). Am J Phys Anthropol 130:71-84. Makedonska J, Wright BW, Strait DS. 2012. The effect of dietary adaptation on cranial morphological integra- tion in capuchins (Order Primates, Genus Cebus). PLoS ONE 7 (10): e40398. doi:10.1371/ journal.pone.0040398 Mew JRC. 1974. The incisive foramen - A possible refer- ence point. Br J Orthod 1:143-146. Moore-Jansen PH, Ousley SD, Jantz RL. 1994. Data col- lection procedures for forensic skeletal material. Re- port of investigation no. 48. Department of Anthro- pology. Knoxville: The University of Tennessee. Oxnard CE. 1983. Multivariate statistics in physical an- thropology: testing and interpretation. Z Morphol Anthropol 73:237-278. Paschetta C, de Azevedo S, Castillo L, Martínez- Abadías N, Hernández, Lieberman DE, González- José R. 2010. The influence of masticatory loading on craniofacial morphology: A test case across techno- logical transitions in the Ohio Valley. Am J Phys An- thropol 141:297-314. Phillips-Conroy JE, Jolly CJ. 1988. Dental eruption schedules of wild and captive baboons. Am J Prima- tol 15:17-29. Post DG. 1981. Activity patterns of yellow baboons (Papio cynocephalus) in the Amboseli National Park, Kenya. Anim Behav 29:357-374. R Core Team. 2013. R: A language and environment for statistical computing. R Foundation for Statistical Computing. Vienna, Austria. http://www.R- project.org Radinsky LB. 1967. Relative brain size: A new measure. Science 155:836-838. Ravosa MJ, Ning J, Costley DB, Daniel AN, Stock SR, Stack MS. 2010. Masticatory biomechanics and mas- seter fiber-type plasticity. J Musculoskelet Neuronal Interact 10:46-55. Samuels A, Altmann J. 1986. Immigration of a Papio anubis male into a group of Papio cynocephalus ba- boons and evidence for an anubis-cynocephalus hy- brid zone in Amboseli, Kenya. Int J Primatol 7:131- 138. Trenouth MJ, Joshi M. 2006. Proportional growth of cra- niofacial regions. J Orofac Orthop 67:92-104. Tung J, Charpentier MJE, Garfield DA, Altmann J, Al- berts SC. 2008. Genetic evidence reveals temporal change in hybridization patterns in a wild baboon population. Mol Ecol 17:1998-2011. Varrela J. 1990. Effects of attritive diet on craniofacial morphology: A cephalometric analysis of a Finnish skull sample. Eur J Orthod 12:219-223. Varrela J. 1992. Dimensional variation of craniofacial structures in relation to changing masticatory- functional demands. Eur J Orthod 14:31-36. Varrela J. 2006. Masticatory function and malocclusion: A clinical perspective. Semin Orthod 12:102-109. Wahungu GM. 1998. Diet and habitat overlap in two sympatric primate species, the Tana crested manga- bey Cercocebus galeritus and yellow baboon Papio cynocephalus. Afr J Ecol 36:159-173. Williams-Blangero S, Vandeberg JL, Blangero J, Konigs- berg L, Dyke B. 1990. Genetic differentiation be- tween baboon subspecies: Relevance for biomedical research. Am J Primatol 20:67-81. Zinner D, Wertheimer J, Liedigk R, Groeneveld LF, Roos C. 2013. Baboon phylogeny as inferred from complete mitochondrial genomes. Am J Phys An- thropol 150:133-140.