Interspecific and intraspecific relationships between tooth size and jaw size in primates

The association between mandibular robusticity, postcanine megadontia, and canine reduction in hominins has led to speculation that large and robust jaws might be required to spatially accommodate large canine and molar teeth in hominins and other primates. If so, then variations in mandibular form that are generally regarded as biomechanical adaptations to masticatory demands might instead be incidental effects of functional requirements of tooth support. While the association between large teeth and deep, robust jaws in hominins is well known, the relationship between tooth size and jaw size has not been systematically evaluated in a comparative sample of primates. We evaluate the relationships between molar tooth size, canine tooth size, and mandibular corpus and symphyseal dimensions in a sample of adult anthropoids in interspecific (n=84 species) and intraspecific (n=36 species) contexts. For intraspecific comparisons, tooth size and jaw size are correlated, but for a majority of species this is a function of sexual size dimorphism. Interspecific comparisons lend little direct support to the hypothesis that jaw breadth directly covaries with molar tooth breadth, but they do support the hypothesis that mandibular depth is associated with canine tooth size in males. The latter observation suggests that if there is a causal association between canine size and mandibular depth, it is subject to a threshold effect. In contrast, neither corpus nor symphyseal robusticity, measured as a shape index of breadth/height, are correlated with tooth size. Our results suggest that further studies of the relationship between tooth size and corpus morphology should focus on tooth root size and corpus bony architecture, and that species-specific factors should have a strong impact on such relationships.     

Body size, locomotion, and long bone cross-sectional geometry in indriid primates.

The geometry of the midshaft cross-sections of the femur and humerus of five indriid species was analysed. Internal (marrow cavity) and external diameters were measured on X-rays in the anteroposterior (a-p) and mediolateral (m-l) planes; cross-sectional areas, second moments of area, and section moduli were calculated using formulae for a hollow ellipse. Cortical thickness, robusticity indices (relating external diameters to the length of the bones), and a-p/m-l shape variables were also calculated. Model II regression was supplemented by analyses of correlation between size and shape. Indriids are saltatory, i.e., their locomotion is dominated by the hind limbs. Accordingly, the femur is more rigid than the humerus, and it shows a consistent difference between the a-p and m-l planes in measures related to bending strength. Cortical thickness varies considerably both within and across species. The type specimen of the new species Propithecus tattersalli is virtually indistinguishable from P. verreauxi on the basis of its long bone cross-sectional geometry. Femoral robusticity is uncorrelated with size, but humeral robusticity decreases significantly with increasing size. Femoral shape variables (a-p/m-l) are all negatively correlated with body size, indicating that m-l dimensions of the femur increase at a faster rate than do a-p dimensions. The highly loaded plane of movement seems to be more reinforced in the smaller species. Contrary to static biomechanical scaling predictions of positive allometry, all cross-sectional parameters scale relatively close to isometry. It is concluded that either changes in locomotor performance must compensate for the weight-related increase in forces and moments or that the larger-bodied animals operate appreciably closer to the limits of their safety margins.     

Postcanine tooth size in female primates

The results of many allometric studies of postcanine tooth size in mammals have not corresponded to expectations of tooth size based on energy requirements and dental function. The purpose of this study is to investigate the relationship between postcanine occlusal surface area, body size, and the metabolic demands of pregnancy and lactation in female primates. Tooth and body sizes from 38 primate species were taken from the literature to test two hypotheses: 1) females should have relatively larger teeth than males in order to masticate additional food for the energetic costs of reproduction; 2) taxa with the largest neonatal size (a measure of average metabolic costs of pregnancy and lactation) should have females with a greater degree of relative dental enlargement. The results show that relatively large female teeth are not found consistently in primate species. Females have less occlusal surface area than expected on the basis of the male tooth and body size regression in 21% of the species, and there is no correlation between relative female tooth size and relative newborn size across higher primate taxa. The degree of female dental enlargement is most closely related to degree of sexual dimorphism in body weight. The correlation between degree of body weight dimorphism and relatively larger postcanine teeth in females than in males is 0.87 in the 38 species. Species that are monomorphic in weight tend to be monomorphic in tooth size even though females apparently require more food than males.(ABSTRACT TRUNCATED AT 250 WORDS)     

Patterning the size and number of tooth and its cusps

Mice and rats, two species of rodents, show some dental similarities such as tooth number and cusp number, and differences such as tooth size and cusp size. In this study, the tooth size, tooth number, cusp size and cusp number, which are four major factors of the tooth patterning, were investigated by the heterospecific recombinations of tissues from the molar tooth germs of mice and rats. Our results suggest that the dental epithelium and mesenchyme determine the cusp size and tooth size respectively and the cusp number is co-regulated by the tooth size and cusp size. It is also suggested that the mesenchymal cell number regulates not the tooth size but the tooth number. The relationships among these factors in tooth patterning including micropatterning (cusp size and cusp number) and macropatterning (tooth size and tooth number) were analyzed in a reaction diffusion mechanism. Key molecules determining the patterning of teeth remains to be elucidated for controlling the tooth size and cusp size of bioengineered tooth.     

Jaw and long bone marrow derived osteoclasts differ in shape and their response to bone and dentin

Increasing evidence suggests the existence of osteoclast diversity. Here we investigated whether precursors obtained from marrow of the mandibula or long bone could give rise to phenotypically different osteoclasts. Formation of multinucleated cells was assessed after culturing mouse marrow cells of the two bone types with macrophage colony stimulating factor (M-CSF) and receptor activator of NFκB ligand (RANKL) for up to 10 days on plastic, bone or dentin. Two times more osteoclasts formed from long bone marrow cells on bone compared to dentin, whereas higher numbers of jaw osteoclasts formed on dentin. Resorption of dentin or bone was similar for osteoclasts formed from both types of precursors. In contrast to jaw marrow derived osteoclasts, long bone osteoclasts predominantly had a multi-compartmented shape, with at least two nuclei containing compartments per cell. Osteoclasts on bone contained two times more actin rings than osteoclasts on dentin, regardless of their precursor origin. However, the area per osteoclast covered by actin rings was similar (20%) for both substrates. This study suggests that marrow cells obtained from different bones give rise to different osteoclasts. The substrate on which the osteoclasts are generated plays a role in steering their formation rather than their resorption.     

Differential long bone growth of children between two months and eleven years of age

Diaphyseal lengths of the humerus, radius, femur and tibia of upper-middle class White children between two months and 11 years of age show positive allometric growth, indicating substantial shape or proportional change. The segments of the lower extremity display greater allometric increase than the humerus and radius; variation in relative growth within each extremity is small and inconsistent. Sex differences are consistent, with slightly greater proportional increases demonstrated for boys. The results suggest that absolute intralimb variation in growth, following a disto-proximo growth or maturity gradient, is due to initial differences in size or scale and not to differences in patterns of growth. Developmental variation between extremities is due to scaling plus variation in relative growth patterns.     

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.     

Heritability of deciduous tooth size in Australian aboriginals

The contributions of genetic and evironmental influences to observed variability of deciduous tooth size were quantified in a group of Australian aboriginals. Phenotypic variability was partitioned into four components; between sides, between fathers, between mothers, and between offspring. Results suggested that about 58% of deciduous tooth-size variability was due to additive genetic variance and 15% to common environmental variance. It appears that additive genetic variance is similar in both deciduous and permanent dentitions, but that common maternal effects are more important in determining deciduous tooth-size variability.     

Canine tooth size and fitness in male mandrills (Mandrillus sphinx)

Sexual selection theory explains the evolution of exaggerated male morphologies and weaponry, but the fitness consequences of developmental and age-related changes in these features remain poorly understood. This long-term study of mandrill monkeys (Mandrillus sphinx) demonstrates how age-related changes in canine tooth weaponry and adult canine size correlate closely with male lifetime reproductive success. Combining long-term demographic and morphometric data reveals that male fitness covaries simply and directly with canine ontogeny, adult maximum size, and wear. However, fitness is largely independent of other somatometrics. Male mandrills sire offspring almost exclusively when their canines exceed approximately 30 mm, or two-thirds of average adult value (45 mm). Moreover, sires have larger canines than nonsires. The tooth diminishes through wear as animals age, corresponding with, and perhaps influencing, reproductive senescence. These factors combine to constrain male reproductive opportunities to a brief timespan, defined by the period of maximum canine length. Sexually-selected weaponry, especially when it is nonrenewable like the primate canine tooth, is intimately tied to the male life course. Our analyses of this extremely dimorphic species indicate that sexual selection is closely intertwined with growth, development, and aging, pointing to new directions for sexual selection theory. Moreover, the primate canine tooth has potential as a simple mammalian system for testing genetically-based models of aging. Finally, the tooth may record details of life histories in fossil primates, especially when sexual selection has played a role in the evolution of dimorphism.     

The incidence of agenesis and polygenesis in the primate dentition

The incidence of dental agenesis and polygenesis was demonstrated in 978 Old World monkeys, 390 great apes, 194 lesser apes and 5,000 humans of Caucasoid and Negroid stock. The frequency of agenesis was highest in the human sample, particularly in the Negroids, that of polygenesis was highest in the great apes. Within both groups, the polymorphism was commonest in the molar (predominantly third molar) region. Despite numerous difficulties in the interpretation of such data, on the basis of the differences between man, apes and monkeys, it is suggested that the high incidence of agenesis seen in man is a phylogenetic accompaniment of the shortening of the maxillomandibular skeleton, which is such a marked feature of human evolution.     

The contribution of cancellous bone to long bone strength and rigidity

A recent article (Burr and Piotrowski, 1982) suggested that structural analyses of long bone cross-sectional geometry will be inaccurate and should be considered inappropriate when cancellous bone accounts for 10-15% or more of the cross-sectional area. Consideration of material property differences between compact and cancellous bone, however, indicates that even significant proportions of cancellous bone (10-40% of total cross-sectional area) will very likely have negligible effects on bone strength and rigidity, and can be effectively ignored in geometrical analyses of diaphyseal sections. In metaphyseal and epiphyseal regions, however, geometric analyses of section properties such as area moments of inertia are inappropriate, both because of significant trabecular bone effects, and because of the inherent constraints of mechanical beam models.     

Reduced tooth size in 45,X (Turner syndrome) females

Mean values and variances of deciduous and permanent tooth dimensions were compared between 121 45,X (Turner syndrome) females and 171 control subjects to clarify the role of the X chromosome on dental development. Although deciduous molars tended to be smaller than normal in 45,X females, there was no evidence of a reduction in tooth size for deciduous anterior teeth. In the permanent dentition, all mesiodistal dimensions were significantly smaller in 45,X females but only some of the buccolingual dimensions were smaller. The findings for deciduous tooth-size may reflect a sampling effect related to the extremely high frequency of spontaneous abortion in 45,X individuals. Results for permanent teeth are consistent with the concept of a decrease in enamel thickness in 45,X females.     

Family studies of tooth size factors in the permanent dentition.

Correlations between dimensions of the permanent teeth in Australian Aboriginals were studied by factor analysis to disclose the main sources of shared variability. Findings indicated that in both males and females most of the common variability in the tooth dimensions could be accounted for by factors representing mesiodistal size of anterior teeth, buccolingual size of anterior teeth, generalized size of the premolars and generalized size of the molars. Factor scores derived from the analysis were used to calculate intraclass correlations among brothers and among sisters. These correlations tended to be higher for the factors contributing most to the common variability indicating that the factors might represent fields under direct genetic control. There was no trend for intraclass correlations among siblings derived from multivariate scores to be consistently higher than those based on observed tooth dimensions. The main advantage to the user of factor analysis is the ability to interpret associations between interrelated variables more objectively than is possible by conventional correlation methods.     

Correlation of tooth size and body size in living hominoid primates, with a note on relative brain size in Aegyptopithecus and Proconsul.

Second molar length and body weight are used to test the correlation between tooth size and body size in living Hominoidea. These variates are highly correlated (r= 0.942, p less than 0.001), indicating that tooth size can be used in dentally unspecialized fossil hominoids as one method of predicting the average body weight of species. Based on tooth size, the average body weight of Aegyptopithecus zeuxis is estimated to have been beteen 4.5 and 7.5 kg, which is corroborated by known cranial and postcranial elements. Using Radinsky's estimates of brain size, the encephalization quotient (EQ) for Aegyptopithecus was between 0.65 and 1.04. A similar analysis for Proconsul africanus yields a body weight between 16 and 34 kg, and an EQ between 1.19 and 1.96.     

Canadian Eskimo permanent tooth emergence timing

To identify the times of emergence of the permanent teeth of Canadian Eskimos (Inuit), 368 children and adolescents were examined. The presence or absence of all permanent teeth except the third molars was recorded and these data subjected to probit analysis. Female emergence times were advanced over males. Generally, the Inuit of both sexes showed statistically significant earlier emergence times than Montreal children, except for the incisors. The present results do not support hypotheses indicating that premature extraction of the deciduous teeth advances the emergence of their succedaneous counterparts. There is some indication the controls of deciduous tooth emergence continue to play some part in emergence of the permanent dentition, especially the first permanent teeth that emerge.     

Measuring the relationship between dietary quality and body size in primates

The feeding niche and the body size of any species are fundamental parameters that constrain the evolution of many other phenotypic characters. Moreover, previous work has shown that body size and diet are correlated, as a consequence of the negative allometry of metabolic rate. Unfortunately, the precise form of the association between body size and diet has never been specified, principally because no suitable cross-species measure of diet has been advanced. Here we develop a measure of diet that is sensitive over the whole spectrum of primate feeding niches, and use this measure to define the relationship between body size and diet for a sample of 72 primate species. Subsequently, we present several examples of how behavioral and ecological hypotheses can be tested by examining the extent to which particular species deviate from the general diet-body size pattern.     

Form and patterning of anterior tooth wear among aboriginal human groups

Form and severity of dental attrition was assessed in aboriginal human groups including hunter-gatherers (Eskimos, Australians) and those with dependence to a varying degree on food production (Southwest U.S. and Ohio American Indians). Wear on anterior teeth was both relatively and absolutely greater in the hunter-gatherers, as indicated by comparisons of wear on anterior and posterior teeth which come into occlusion at roughly the same time. Distinct differences in form of anterior wear were also apparent: The hunter-gatherers exhibited steadily increasing incidences of labially rounded wear with greater functional age, while the food-producing groups showed little or no rounding but instead high frequencies of heavily cupped wear (especially in those with premature loss of posterior teeth). These differences were attributed to nonmasticatory utilization of the front teeth in hunter-gatherers and to employment of the anterior teeth in masticatory (grinding) activities necessitated by large-scale molar loss in food producers.     

镶嵌式外固定架治疗长骨骨不连17例体会

目的：探讨长骨骨不连的一种治疗方法。方法：2007年1月至2009年8月,采用镶嵌式外固定架治疗17例长骨骨不连。本组17例,男11例,女6例,年龄16-64岁,平均31岁。2例为血源性骨髓炎病理性骨折后,股骨、胫骨各1例;6例为创伤性骨髓炎后骨折不愈,肱骨1例,股骨1例,胫骨4例;9例为手术后无感染性骨不连,肱骨2例,股骨2例,胫骨5例;7例有不同程度畸形,6例有1.5-8cm骨短缩,其中2例同时行骨痂延长术。结果：全部病人均获随访,随访时间9-20个月,以1975年天津全国骨科会议制定的骨折愈合标准为依据,本组17例病人均获得临床愈合,骨不连处平均愈合时间为4～9月（平均6.2月）,1例延长8cm,另1例延长6cm。结论：利用镶嵌式外固定架治疗长骨骨不连一种简单有效的方法。