Date of Graduation

5-2022

Document Type

Thesis

Degree Name

Master of Arts in Anthropology (MA)

Degree Level

Graduate

Department

Anthropology

Advisor/Mentor

Kathleen S. Paul

Committee Member

Joseph M. Plavcan

Second Committee Member

Lucas Delezene

Keywords

deciduous, morphology, permanent, teeth, Testosterone, twins

Abstract

The twin testosterone transfer (TTT) hypothesis posits that females gestated with male co-twins develop more masculine phenotypes due to in-utero androgen exposure. Research has shown co-gestational effects to be associated with increased deciduous and permanent tooth size in females belonging to opposite-sex dizygotic twin pairs (OSF) as compared with females belonging to same-sex monozygotic (MZF) and dizygotic (SSF) twin pairs and female siblings. This study evaluates whether the TTT hypothesis explains patterns of dental morphological variation, namely differences between OSF and other females (SSF, MZF, female siblings) in a contemporary sample that includes both deciduous and permanent data. This work probes the underpinnings of crown morphology expression, which is assumed to be sexually monomorphic (i.e., male/female data pooled) in applied anthropological research. Resampling statistics and Mann-Whitney U tests were used to compare crown morphology scored using the Arizona State University Dental Anthropology System (ASUDAS), with a focus on canine and molar traits. In the deciduous dentition, significant overexpression in OSF was found for maxillary second molar (m2) Carabelli’s trait, mandibular second molar (m2) cusp number, and mandibular second molar (m2) cusp 6, even with application of a Bonferroni correction to account for potential Type I error. In the permanent dentition, highly significant differences in expression between OSF and all other females were noted for two first molar (M1) traits (M1 hypocone and M1 cusp 7), even after application of a Bonferroni correction. Again, OSF, on average, showed elevated expression for these traits. These results suggest in-utero hormone exposure leads to elevated expression for some but not all canine and molar traits; the inconsistent results may be due to varying levels of testosterone exposure at critical times during morphogenesis. As such, this thesis lends partial support to the TTT hypothesis. Of note, only a few traits showed strong overexpression in males across this population, not all of which differed between OSF and other females. This suggests that Y-chromosome effects may be more important than androgen exposure in driving population-level sexual dimorphism in crown morphology.

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