Sexual dimorphism in chin shape: implications for adaptive hypotheses

Area of Science:

• Human Anatomy
• Evolutionary Biology
• Anthropometry

Background:

• The human chin (mentum osseum) is a unique anatomical feature.
• Existing hypotheses for its adaptive value include mechanical stress and sexual selection.
• Sexual selection predicts chin shape dimorphism, while biomechanical hypotheses do not.

Purpose of the Study:

• To develop a novel method for quantifying chin shape.
• To investigate the presence and extent of sexual dimorphism in human chin shape.
• To differentiate between adaptive hypotheses for chin evolution.

Main Methods:

• Quantification of mentum osseum contours using elliptical Fourier function analysis (EFFA).
• Analysis of Fourier coefficients via principal components analysis (PCA).
• Assessment of sexual dimorphism using principal component loadings in a diverse human skeletal sample.

Main Results:

• Statistically significant differences in chin shape were identified between sexes.
• Elliptical Fourier function analysis (EFFA) proved effective for quantifying complex shapes.
• Principal components analysis (PCA) revealed distinct patterns of variation related to sex.

Conclusions:

• The study provides the first quantitative, morphologically based evidence of sexual dimorphism in human chin shape.
• Findings support adaptive hypotheses predicting dimorphism, particularly sexual selection.
• The developed methodology offers a new tool for analyzing craniofacial variation.