Interpreting sex differences in enamel hypoplasia in human and non-human primates: Developmental, environmental, and cultural considerations

Abstract

The purpose of this review is to provide a synoptic, critical evaluation of the evidence of, and potential etiological factors contributing to, sex differences in the expression of enamel hypoplasia (EH). Specifically, this review considers theoretical expectations and empirical evidence bearing on two central issues. The first of these is the impact of a theorized inherent male vulnerability to physiological stress on sex differences in EH. The second issue is the potential contribution to sex differences in EH of intrinsic differences in male and female enamel composition and development. To address this first issue, EH frequencies by sex are examined in samples subject to a high degree of physiological stress. Based on the concept of inherent male vulnerability (or female buffering), males in stressful environments would be expected to exhibit higher EH frequencies than females. This expectation is evaluated in light of cultural practices of sex-biased investment that mediate the relationship between environmental stress and EH expression. Defects forming prenatally afford an opportunity to study this relationship without the confounding effects of sex-biased postnatal investment. Data bearing on this issue derive from previously conducted studies of EH in permanent and deciduous teeth in both modern and archaeological samples as well as from new data on Indian schoolchildren. To address the second issue, fundamental male-female enamel differences are evaluated for their potential impact on EH expression. A large sex difference in the duration of canine crown formation in non-human primates suggests that male canines may have greater opportunity to record stress events than those of females. This expectation is examined in great apes, whose canines often record multiple episodes of stress and are sexually dimorphic in crown formation times. With respect to the first issue, in most studies, sex differences in EH prevalence are statistically nonsignificant. However, when sex differences are significant, there is a slight trend for them to be greater in males than in females, suggesting a weak influence of greater male vulnerability. Cultural practices of sex-biased investment in children appear to have greater impact on EH expression than does male vulnerability/female buffering. With respect to the second issue, sex differences in the composition and development of enamel were reviewed and determined to have limited or unknown impact on EH expression. Of these factors, only the duration of crown formation was expected to affect EH expression by sex within the great apes. The data support an association between higher defect counts in the canines of great ape males relative to those of females that may be the result of longer crown formation times in the canines of great ape males. This review concludes with an assessment of the nature of the evidence currently available to examine these issues and suggests future avenues for research focused on elucidating them.

Inference of emergent cattle pastoralism in the southern Sahara desert based on localized hypoplasia of the primary canine

“…LHPC are focal, plane-form defects (Guatelli-Steinberg and Lukacs, 1999;Hillson and Bond, 1997) that manifest as pits of varying sizes in the enamel with occasional dentine involvement. In contrast to linear enamel hypoplasia (LEH), LHPC are non-linear, form on the facial aspect of the deciduous canines almost exclusively, and typically show no statistically significant differences between males and females within populations (Guatelli-Steinberg and Lukacs, 1999). These characteristics suggest that the etiology of LHPC is distinct from that of LEH.…”

“…One form of enamel defect, in particular, is well-suited for reconstructing prehistoric health experience because of its ease of observation, early age of formation, and linkages to clinical contexts dealing with modern health disparities; these are called localized hypoplasia of the primary canines (LHPC). LHPC are focal, plane-form defects (Guatelli-Steinberg and Lukacs, 1999;Hillson and Bond, 1997) that manifest as pits of varying sizes in the enamel with occasional dentine involvement. In contrast to linear enamel hypoplasia (LEH), LHPC are non-linear, form on the facial aspect of the deciduous canines almost exclusively, and typically show no statistically significant differences between males and females within populations (Guatelli-Steinberg and Lukacs, 1999).…”

Sex differences in linear enamel hypoplasia (LEH) in early modern Japan

“…Studies of living children have shown that differential treatment of males and females in childhood can predict differences in levels of LEH in adulthood (May et al, 1993). Moreover, from the investigation of enamel hypoplasia in deciduous and permanent dentition of human and nonhuman primates, Guatelli-Steinberg and Lukacs (1999) insisted that though females are better buffered against environmental stress, a weak influence of male vulnerability on the expression of enamel hypoplasia is most likely to be detected in samples of very large size (>1000 individuals), and that evidence of higher enamel hypoplasia prevalence in girls might therefore be used as a biological marker of preferential investment in sons.…”

Stress near the start of life? Localised enamel hypoplasia of the primary canine in late prehistoric mainland Southeast Asia

“…It is argued that deciduous dental enamel defects are good indicators of permutations in amelogenesis from the second trimester to the end of the first year of life (Goodman and Rose, 1990a). The importance of looking at deciduous teeth and the defects specific to these teeth is becoming recognised both in archaeological and modern human and non-human primate studies (e.g., Guatelli-Steinberg and Lukacs, 1999;Lukacs, 1999;Skinner and Newell, 2003), but still few bioarchaeological studies collect these data or report on these defects, compared with the vast number that investigate LEH in permanent teeth.…”