Evolution of genetically correlated traits: tooth size and body size in baboons

Within a population, only phenotypic variation that is influenced by genes will respond to selection. Genes with pleiotropic effects are known to influence numerous traits, complicating our understanding of their evolution through time. Here we use quantitative genetic analyses to identify and estimate the shared genetic effects between molar size and trunk length in a pedigreed, breeding population of baboons housed at the Southwest National Primate Research Center. While crown area has a genetic correlation with trunk length, specific linear measurements yield different results. We find that variation in molar buccolingual width and trunk length is influenced by overlapping additive genetic effects. In contrast, mesiodistal molar length appears to be genetically independent of body size. This is the first study to demonstrate a significant genetic correlation between tooth size and body size in primates. The evolutionary implications are discussed.     

Static intraspecific allometry of the dentition in Otoletnur crassicaudatus

Adult static intraspecific allometry of tooth size was evaluated in a sample of 66 Otolemur crassicaudatus (34 male, 32 female). Tooth areas were calculated from mesiodistal and buccolingual measurements of canines and postcanine teeth of both arcades and were scaled to four viscerocranial measurements: bimaxillary width; maxillo-alveolar length; mandibular length and bigonial width. Individual tooth crown areas were also scaled to total skull length, body length and body weight. From the log-transformed analyses it is concluded that postcanine tooth size was unrelated to body length or weight, and poorly correlated to skull length or jaw size. Although viscerocranial size appears to be independent of body size, these measures are well correlated to skull length. It is shown that the longer the skull, the shorter and narrower the maxilla, and the longer and broader the mandible. Canines are shown to scale negatively allometric to skull length, hence, large animals will have relatively small canines.     

A differential environmental effect on human anterior tooth size

Factor-analytic studies of human tooth size routinely exhibit separate factors for the mesiodistal and buccolingual dimensions of anterior teeth but a single factor each for premolar and molar size. This observed independence within the anterior field is shown to be attributable to a much larger effect of environmental factors on the buccolingual vs. the mesiodistal diameters, a significant cause of which may be calculus accumulation hitherto unrecognized in the relevant literature. The heritability of dental dimensions is also discussed.     

Static intraspecific maxillofacial allometry in the chacma baboon

Adult static intraspecific allometry of jaw size and tooth area was evaluated in a sample of 104 Papio ursinus crania (52 male, 52 female). Tooth areas were calculated from mesiodistal and buccolingual measurements of all the teeth in both arcades and were scaled to four viscerocranial measurements: bimaxillary width, maxillo-alveolar length, mandibular length and bigonial width. Craniodental allometric analyses indicate that larger animals will tend to have proportionately shorter and narrower lower jaws. From the log-transformed interspecific analyses between P. ursinus and C. aethiops we conclude that males and females within each species share a common exponential value for jaw length. Hence increased sexual dimorphism for muzzle length in P. ursinus is attributable to increased divergence between the male and female slopes. Post-canine area was found to be significantly correlated to maxillary length and to canine size only in females, with exponential values similar to those reported for the same bivariate regressions in C. aethiops. A hypothesis of nutritional equivalence is advanced to account for these observations. Canine base area and the area of P3 were the only tooth areas that scaled in a positively allometric fashion to jaw size--but only in males. Hence the existence of a canine complex is confirmed in the male Chacma baboon, the size of which is related to jaw length.     

Reduction of maxillary molars in Homo sapiens sapiens: a different perspective.

Crown and cusp areas, and buccolingual and mesiodistal diameters of maxillary molars of complete upper tooth rows (30 males, 30 females) were analysed in order to quantify changes in size and shape from the first to the third molar. Uni- and multivariate analyses revealed the mesial cusps, in particular the protocone (mesiolingual cusp), to be more stable than the other cusps. Although there is a gradient in size from the first to third molar, shape changes were found to be marked. Overall, the findings are in keeping with the field theory and the hypotheses of environmental constraints on later developing teeth. However, not all of the results could be entirely explained by these concepts. Functional aspects seem to account for the relative stability of the protocone and the buccolingual crown diameter. It appears that this functional complex is relatively stable despite the overall reduction of tooth size, which is probably secondary to processes occurring in the jaws and the cranium. This finding may have implications for studies on tooth reduction between populations of different time periods.     

Static intraspecific allometry of jaws and teeth in Cercopithecus aethiops

Adult static intraspecific allometry of jaw size and tooth area was evaluated in a sample of 100 Cercopithecus aethiops crania (50 male, 50 female). Tooth areas were calculated from mesiodistal and buccolingual measurements of all the teeth in both arcades and were scaled to four viscero-cranial measurements: bimaxillary breadth, maxillo-alveolar length, mandibular length and bigonial width. Allometric coefficients calculated for jaw dimensions alone indicate tighter viscerocranial integration in females than in males. A finding of note was that half of the variation in maxillo-alveolar length may be accounted for by variation in mandibular length: females are isometric, males negatively allometric.
A similar degree of allometric mosaicism was found when maxillary incisor size was scaled to maxillary length and width. In females, the relationship was negatively allometric, whilst incisor size in males was found to be unrelated to either. Negative allometry characterized the relationship of canine base area to jaw length in both sexes, with males additionally being positively allometric to mandibular width.
The scaling of postcanine tooth areas to jaw length was characterized by a dichotomous pattern: males showed significant mandibular integration whilst females showed only significant maxillary integration. Compensatory tooth size interaction between maxillary canine base area and the summed incisor and postcanine areas was suggested by the significant negative allometric relation between them.     

Decreased odontometric sex difference in individuals with dental agenesis

Partial Dental Agenesis has been shown related to decreased mesiodistal tooth size. The sexual difference in permanent tooth size of 104 normal individuals with congenital absence of one or more secondary teeth has been studied. Males are observed to have more mesiodistal size reduction than females. In only two of 14 tooth types examined was a statistical sex difference in mesiodistal size shown. It is of interest that each of these was in the canine field area. A sex difference was slightly more apparent when studying the buccolingual dimension; however, several difficulties were encountered in obtaining measurements in this diameter.     

Contours of maxillary molars studied in Australian aboriginals

Distances from the central pit to the perimeter of the crown of permanent upper molars were measured on standardized occlusal photographs of dental casts representing 210 male and 181 female Aboriginals from Yuendumu in the Northern Territory of Australia. In both males and females the first molar was the largest tooth but it showed least variability. Variabilities of the distances tended to be greater for radii constructed in the buccolingual direction than for the transverse mesiodistal radii. The most marked size reduction in the molar series from first to third related to the distolingual part of the crown, which was also the most variable region. Size differences between molars in the mesial contour radii were not marked. Sexual dimorphism was evident in most crown radii, being most marked for the second molar.     

Alternative dental measurements: proposals and relationships with other measurements

Most archaeological and fossil teeth are heavily worn, and this greatly limits the usefulness of tooth crown diameter measurements, as they are usually defined at the widest points of the crown. There are alternatives, particularly measurements at the cervix of the tooth, where the crown joints the root, and measurements along a diagonal axis in molars, that are much less affected by wear. These would allow a wider range of specimens to be included, e.g., in the study of dental reduction in Upper Palaeolithic and Mesolithic Homo sapiens. In addition, they would allow the little-worn teeth of children to be compared directly with well-worn teeth in adults. These alternatives, however, have been little used, and as yet there have not been any studies of the repeatability with which they can be measured, or of the extent to which they are related to the more usual crown diameters. The present study is based on a group of unworn teeth, where direct comparisons could be made between the alternative measurements, which are not much affected by wear, with the usual crown diameters, which are very much affected. In an interobserver-error study of this material, cervical and diagonal measurements could be recorded as reliably as the usual crown diameters. The buccolingual cervical measurement was strongly correlated with the normal bucclingual crown diameter in all teeth, whereas the mesiodistal cervical measurement was highly correlated with the normal mesiodistal crown diameter in incisors and canines, but less so in premolars and molars. The molar diagonal measurements showed high correlations with all other measurements. Crown areas (robustness index) calculated from the usual diameters were strongly correlated with crown areas calculated from cervical measurements, and crown areas calculated from molar diagonals were strongly correlated with both other areas. Despite the long usage of the more usual maximum crown diameters, the alternative dental measurements could be measured just as reliably, could record similar information about tooth crown size, and would be better measures for the worn dentitions seen in archaeological and fossil material.     

Sexual dimorphism in crown units of mandibular deciduous and permanent molars in Australian Aborigines.

Sexual differences in the crown units of mandibular molars were investigated in Australian Aborigines. The first and second deciduous molars (dm1 and dm2), and first to third permanent molars (M1, M2 and M3) were measured on dental casts using a sliding caliper. Measurements of tooth crowns included overall mesiodistal and buccolingual diameters, as well as the mesiodistal and buccolingual diameters of the trigonid and talonid. Percentage dimorphism values were greater in the talonid dimensions than the trigonid, indicating that sex differences tend to be larger in the later-developing crown units. Sex differences in mesiodistal diameters increased from dm1 to M2 but decreased for M3, the tooth that showed the least dimorphism of all the molars. This result seems to be due to the marked variability in size of the M3 between individuals.     

Carabelli's trait and tooth size of human maxillary first molars

Carabelli's trait is a morphological feature that can occur on the protocone of human maxillary molars. This study tests the hypothesis that Carabelli's trait is correlated statistically with the dimensions of the crown's four principal cusps or whether, as a cingular feature, the trait truly accretes onto an otherwise unaffected crown. Computer-assisted image analysis was used to measure the 6 intercusp distances and 12 angular relationships among cusp tips on the permanent first molar of 300 young adult American whites. Carabelli's complex was scored using an 8-grade ordinal scheme. Crown size was quantified in three ways, namely as 1) maximum mesiodistal and buccolingual diameters, 2) the 6 intercusp distances, and 3) the 12 angular cusp arrangements. There was no sex difference in the morphological expression of Carabelli's trait in this sample. Overall crown size and intercusp distances were significantly and progressively larger in molars with larger Carabelli's trait expressions. There are graded size responses between crown size (mesiodistal and buccolingual diameters), sizes of the four principal cusps, and morphological stage of Carabelli's complex, though the statistical relationships are appreciably stronger in males than females. Carabelli's trait occurs preferentially in larger molars. In contrast, angular (shape) relationships among cusp tips are not discernibly affected by trait size in either sex. There is the situation, then, that Carabelli's trait is developmentally correlated with crown size, but with no apparent alteration of cusp arrangements, suggesting that the increases are isometric across the occlusal table. Why the association is much weaker in females remains speculative, but these data provide yet another line of evidence that, within a population, tooth size is associated in a positive fashion with crown complexity.     

Tooth size and arcadal length correlates in man

Intra-arcadal mesiodistal and buccolingual tooth size correlations were evaluated in a sample of 125 caucasoids with ideal occlusion. Dental dimensions were corrected for arcade mength (as a measure of jaw size) by a series of regression analyses of each mesiodistal dimension on the sum of the mesiodistal dimensions within each arcade. Regression coefficients of tooth dimension on arcade length were calculated to gain an insight into the dimensional sensitivity of individual teeth to arcade length variation. The data presented here suggest a strong association between arcadal length (jaw size) dependence, and the dimensional stability of individual teeth. When corrected for arcade length, a definite pattern of tooth size correlation emerges: postcanine maxillary and mandibular teeth are negatively correlated to the anterior teeth and are positively correlated to one another. The hypothesis is developed that anterior and postcanine teeth should be viewed as two separate and negatively size-correlated units, beyond the boundaries of the four morphological tooth classes. Recognition of this basic dichotomous size arrangement within each jaw allows for a reassessment of some of the problems associated with hominid dental evolution.     

Molar size sequence in Australian Aboriginals

Percentage frequencies for molar size sequence of first and second molars were calculated in a group of contemporary Australian Aboriginals using mesiodistal and buccolingual dimensions, as well as crown areas. Comparisons were made between sexes, arches, and dimensions within the Aboriginal groups and also between Aboriginal data and those published for other populations. The frequencies of the M2 > M1 molar size sequence in the Aboriginals fell within the range of frequencies reported for other contemporary populations. Differences in the frequencies of the M2 > M1 sequence between the sexes and between arches, together with the relatively high frequency of asymmetry in molar size sequence within Aboriginals, supported the notion that local environmental conditions acting during odontogenesis, together with differential responses to other environmental influences, play an important role in determining observed patterns of molar tooth size.     

Individual variation in enamel structure of human mandibular first premolars

Nine human mandibular first premolars were examined to assess variation in external morphology and enamel structural organization within a tooth type. The relationship of enamel ultrastructure to gross dental morphology was also studied. The teeth were cut in the mesiodistal direction just lingual to the buccal cusp, and etched. Montages were constructed of the cut enamel surface photographed in the scanning electron microscope at 100 X magnification. Parameters were measured and correlation coefficients were calculated for the comparison of various odontometric features. The mesiodistal and buccolingual dimensions were highly correlated and the occlusal thickness of enamel was significantly correlated to crown height but not crown width. Hunter-Schreger bands were less pronounced in fossa areas than at lateral aspects, cusps, or ridges; these bands were directly related to the geometry of the tooth. It was concluded that within this tooth type, there is a large amount of individual variation not only in gross morphology but also in enamel ultrastructure. This result underscores the fact that interspecific comparisons must be made with care.     

Associations between Carabelli trait and cusp areas in human permanent maxillary first molars

Few dental anthropological studies have investigated the associations between tooth crown size and crown traits in humans using quantitative methods. We tested several hypotheses about overall crown size, individual cusp areas, and expression of Carabelli cusps in human permanent first molars by obtaining data from standardized occlusal photographs of 308 Australians of European descent (171 males and 137 females). Specifically, we aimed to calculate the areas of the four main molar cusps, and also Carabelli cusp, and to compare the relative variability of cusp areas in relation to timing of development. We also aimed to compare cusp areas between males and females and to describe how Carabelli cusp interacted with other molar cusps. Measurements included maximum crown diameters (mesiodistal and buccolingual crown diameters), the areas of the four main cusps, and the area of Carabelli cusp. The pattern of relative variability in absolute areas of molar cusps corresponded with their order of formation, the first-forming paracone displaying the least variation, and the last-forming Carabelli cusp showing the greatest. Overall crown size and areas of individual cusps all showed sexual dimorphism, with values in males exceeding those in females. Sexual dimorphism was smallest for paracone area and greatest for Carabelli cusp area. Overall crown size and cusp areas were larger in individuals displaying a Carabelli cusp, especially the hypocone area. Although the combined area of the protocone and a Carabelli cusp was greater in cuspal forms than noncuspal forms, protocone area alone was significantly smaller in the former. Our findings lead us to propose that, in individuals with the genotype for Carabelli trait expression, larger molar crowns are more likely to display Carabelli cusps, whereas molars with smaller crowns are more likely to display reduced forms of expression of the trait. We suggest that the pattern of folding of the internal enamel epithelium in developing molar crowns, particularly in the protocone region, can be modified by a developing Carabelli cusp.     

Development and size of the teeth of Macaca mulatta.

A cross-sectional sample of 151 skulls from Macaca mulatta of known age and similar rearing in U.S. Primate Centers was analyzed to determine age-related "norms" of stages of development and size of teeth. The stages of development from the follicle of a deciduous incisor in the fetus to completion of the root with apex closed of the permanent third molar were related to age. The age range observed for eruption of each tooth was noted and related to its stage of development. The crown of each erupted tooth was found to be completely developed, but growth of its root continued for a longer, indeterminate period. When a deciduous tooth was exfoliated, the crown of the permanent successor was found to be completed and root growth had begun. Measurements of both mesiodistal and faciolingual diameters and of crown length of the teeth in situ and of total length and root length on roentgenograms were examined for sexual dimorphism. The faciolingual diameter of the deciduous mandibular second incisor and of both second molars showed the greatest sexual dimorphism among both diameters of all deciduous teeth. The mesiodistal and faciolingual diameters of the mandibular premolars were found to be the best dimensions in discriminant functions for identifying sex in the absence of permanent canines.     

Genetic correlations between mandibular molar cusp areas in baboons

Primate evolutionary studies rely significantly on dental variation given the large role that teeth play in how an organism interacts with its environment (animal and plant) and conspecifics. Variation in cusp size has been shown to vary among primate taxa, although most studies to date focused on extant and extinct hominoids. Here we test the assumed hypothesis that a significant proportion of this variation in baboons is due to the additive effects of genes. We perform quantitative genetic analyses on variation in two-dimensional (2-D) mandibular molar cusp size in a captive pedigreed breeding population of baboons (Papio hamadryas) from the Southwest National Primate Research Center. These analyses show that variation in cusp size is heritable and sexually dimorphic. Additionally, we tested for genetic correlations between cusps on the same crown, between morphological homologues along the tooth row, and between cusp area and crown buccolingual width. We find that four of the six cusp pairs on the first molar have a genetic correlation of one, save for the metaconid-hypoconid and entoconid-hypoconid, which are not statistically different from zero. The second and third molars have lower genetic correlations, although the metaconid-hypoconid correlation is similarly estimated at zero and the entoconid-protoconid correlation is estimated to be one. This cross pattern of genetic and no genetic correlation does not immediately accord with the known pattern of development and/or calcification. We propose two explanative hypotheses.     

The effect of prenatal factors on crown dimensions

As shown in 870 white participants in the National Collaborative Perinatal Project (NCPP), maternal health status during pregnancy and birth size are systematically related to mesiodistal and buccolingual crown dimensions of I1, I2, dc, dm1, dm2 and M1. Maternal diabetes, maternal hypothyroidism and large size at birth are associated with larger maxillary and mandibular teeth in white children. Conversely, deciduous and permanent crown diameters are diminished in maternal hypertension, and in low birthweight and small birth-length conditions. These findings suggest that maternal and fetal (or gestational) determinants of both deciduous and permanent tooth crown dimensions may account for as much as half of crown-size variability with major implications to population comparisons and historical odontometric differences and trends.     

A comparative study of tooth and dental arch dimensions in Jewish children of different ethnic descent. I. Kurds and Yemenites

Dental casts of 99 Kurdish and 98 Yemenite Jewish children evenly distributed between the sexes, aged 12 years, were measured for mesio-distal and buccolingual tooth dimensions, and arch depth and width. Dental dimensions showed significant differences between the two groups in certain permanent teeth in mesiodistal length, while in the buccolingual diameter the differences did not reach statistical significance. Dental arch form in Kurdish children was more rounded due to significantly bigger arch width, while arch depth was not significantly different from Yemenites. Intragroup sex differences were found in both groups with reference to arch dimensions, while in the dental parameters they were more strongly expressed in Kurdish children.