Occipital Bone Morphology and Activity Patterns

Do subsistence practices shape the occipital bone beyond biological sex?

bioarchaeology | skeletal biology | sexual dimorphism | biomechanics | ANOVA | discriminant function analysis | R
Author

Kara C. Hoover

Published

February 18, 2026

Overview

We examined metric and nonmetric traits of the occipital bone across nine skeletal collections representing hunter-gatherers, horticulturalists, and agriculturalists to test whether subsistence-based biomechanical loading shapes occipital morphology beyond biological sex. All five outcome variables showed significant sex-based differences, but only bicondylar breadth and lambda-inion distance also varied by subsistence practice — suggesting a possible biomechanical signal that warrants further investigation with larger, occupation-specific datasets. Foramen magnum area showed clear sexual dimorphism but yielded only 71% sex estimation accuracy, insufficient for forensic use.

Note

Published Work

Hoover, K. C., & Thomas, G. P. (2022). Sexual dimorphism and biomechanical loading in occipital bone morphological variation. American Journal of Human Biology, e23792. https://doi.org/10.1002/ajhb.23792

This repository contains the published paper, original data, mined data, and revised R scripts.

Research Questions

  1. Does foramen magnum area vary by biological sex and/or subsistence practice?
  2. Does bicondylar breadth vary by sex and/or subsistence?
  3. Do ectocranial traits (lambda-inion distance, occipital form index, nuchal index) vary by sex and/or subsistence?
  4. How accurately can foramen magnum area predict biological sex, alone and combined with bicondylar breadth?

Study Design

Occipital bone research has been biased toward evolutionary and forensic questions, often with unexplained variance. We were interested in whether behavioral differences — specifically gender-based divisions of daily activity and variation in subsistence workload — account for some of that variation.

We used traits fixed early in development and under stronger genetic control (foramen magnum length and breadth) alongside traits influenced by muscle use and development (bicondylar breadth, lambda-inion distance, external occipital protuberance depth, nuchal crest presence, nuchal line count, general EOP form). We expected all traits to show sexual dimorphism and further expected ectocranial traits and bicondylar breadth to show subsistence-based variation.

Two datasets were analyzed:

Original Dataset (n = 98): Collected from Florida skeletal collections by the authors.

Mined Dataset (n = 565): Published thesis data on Plains Arikara and large modern study collections.

These were combined where variables overlapped (foramen magnum area and bicondylar breadth) to create a Combined Dataset (n = 663).

Table 1. Sample composition by site, dataset, subsistence category, and sex.
Site / Collection Dataset Subsistence Date Range n F M
Windover (8BR246) Original Hunter-Gatherer 8522–7421 BP 47 24 23
Hutchinson Island (8MT37) Original Hunter-Gatherer 2000–1000 BP 16 8 8
FSU Teaching Collection Original Agriculture 20th century 22 11 11
St. Mark’s Military (8WA108) Original Agriculture 19th century 13 0 13
Larson (39WW2) Mined Horticultural 1679–1733 115 57 58
Leavenworth (39CO9) Mined Horticultural 1802–1832 30 14 16
Mobridge (39WW1) Mined Horticultural 1600–1700 56 31 25
Hamann-Todd Collection Mined Agriculture 1912–1938 107 54 53
Terry Collection Mined Agriculture 1899–1941 257 129 128

What We Found

Foramen Magnum Area

Sex: Males significantly larger in both datasets (Original: F = 9.27, p = .003; Combined: F = 129.26, p < .002). Difference of approximately 10 cm2 in the combined dataset.

Subsistence: No significant difference (Original: F = 0.00, p = .952; Combined: F = 2.41, p = .09). Consistent with the foramen magnum being developmentally fixed by age 7 with no adult remodeling capacity.

Interpretation: Foramen magnum area reflects developmental trajectory, not adult activity patterns.

Bicondylar Breadth

Sex: Males significantly larger in both datasets (Original: F = 14.12, p < .002; Combined: F = 70.9, p < .002).

Subsistence: Significant in both datasets (Original: F = 9.93, p = .003; Combined: F = 19.6, p < .002). Post-hoc Tukey HSD confirmed horticulturalists and hunter-gatherers both exceeded agriculturalists; the horticulturalist vs. hunter-gatherer difference was negligible (p = .95).

Interpretation: Bicondylar breadth reflects both sexual dimorphism and possible activity-based condylar remodeling in populations with more physically demanding subsistence strategies.

Lambda-Inion Distance

Sex: Males significantly longer (F = 11.93, p = .001), averaging approximately 4.8 mm more than females.

Subsistence: Significant in the two-way model controlling for sex (F = 5.95, p = .017). Hunter-gatherers averaged approximately 3.5 mm longer than agriculturalists.

Note: Plot p-values reflect unadjusted one-way comparisons for visualization. The two-way model controlling for sex is the reported result.

Interpretation: Lambda-inion distance may be sensitive to nuchal ligament development linked to physically demanding activity, though it may also partly reflect EOP morphology.

Occipital Form Index and Nuchal Index

Sex: Both significantly higher in males (Form Index: F = 29.66, p < .002; Nuchal Index: F = 23.18, p < .002), reflecting deeper protuberances and greater nuchal muscle marking.

Subsistence: Neither index differed significantly by subsistence (Form Index: p = .670; Nuchal Index: p = .390).

Interpretation: Ectocranial morphology of the nuchal region reflects sexual dimorphism but not subsistence-based biomechanical loading in these datasets.

ANOVA Results

Table 2. Type II SS two-way ANOVA results. * white.adjust = TRUE used for heteroscedastic models.
Variable Dataset Predictor F p Sig.
Foramen Magnum Area Original Subsistence 0.00 .952
Sex 9.27 .003 **
Foramen Magnum Area Combined* Subsistence 2.41 .090 .
Sex 129.26 <.002 ***
Bicondylar Breadth Original Subsistence 9.93 .003 **
Sex 14.12 <.002 ***
Bicondylar Breadth Combined Subsistence 19.6 <.002 ***
Sex 70.9 <.002 ***
Lambda-Inion Original Subsistence 5.95 .017 *
Sex 11.93 .001 ***
Occipital Form Index Original* Subsistence 0.18 .670
Sex 29.66 <.002 ***
Nuchal Index Original Subsistence 0.73 .390
Sex 23.18 <.002 ***

Significance: . p < .10 | * p < .05 | ** p < .01 | *** p < .001

Discriminant Function Analysis

Foramen magnum area alone predicted biological sex at 71% accuracy (combined dataset, 70/30 train/test split, LDA coefficient = 1.108). Adding bicondylar breadth as a second predictor reduced accuracy to 66%, consistent with biomechanical loading introducing noise into condylar-based sex estimation. The high overlap in discriminant function plots by both sex and subsistence confirms these metrics are useful as supporting evidence in bioarchaeological contexts but cannot reliably classify sex on their own in forensic settings.

Figure 1A–B. Foramen magnum area by sex (A) and subsistence (B). Significant sex dimorphism; no subsistence effect.

Figure 2A–B. Bicondylar breadth by sex (A) and subsistence (B). Significant effects for both predictors.

Figure 3A–B. Lambda-inion distance by sex (A) and subsistence (B). Plot p-values are unadjusted one-way comparisons; see Table 2 for two-way model result.

Figure 4A–B. Occipital form index by sex (A) and subsistence (B). Sex effect only.

Figure 5A–B. Nuchal index by sex (A) and subsistence (B). Sex effect only.

Methods

Sample: 663 individuals from nine skeletal collections spanning Florida Archaic hunter-gatherers, South Dakota Arikara horticulturalists (historic contact period), 19th-century military remains, and 20th-century modern study collections.

Data collection: Three measurement trials for five metric traits using an electronic sliding caliper; two observation trials for three nonmetric traits. Intra-observer agreement confirmed via kappa (irr). Measurement error assessed via Levene’s test across trials (all <0.51% of trait variance).

Dimension reduction: Foramen magnum length and breadth combined as an ellipse area index (pi x L x B). PCA on five ectocranial variables identified three vector clusters supporting two composite indexes: Occipital Form Index (EOP depth + general form) and Nuchal Index (nuchal crest + nuchal line count).

Analysis: Two-way ANOVA (Type II SS, car::Anova) with sex and subsistence as predictors, preceded by interaction testing (Type III). No interactions were detected. Heteroscedasticity-corrected models (white.adjust = TRUE) used for Index_Form (original) and Index_FM (combined). Post-hoc Tukey HSD (PMCMRplus) for combined dataset subsistence comparisons. DFA via MASS::lda.

Key Insights

Developmental Timing Determines Biomechanical Sensitivity

The foramen magnum, fixed by age 7, shows sexual dimorphism but no subsistence signal — consistent with its role as a genetically canalized trait. Traits subject to adult remodeling show more complex patterns.

  • Foramen magnum area: sex-only signal
  • Bicondylar breadth and lambda-inion: sex and subsistence signals
  • Nuchal region indices: sex-only despite adult biomechanical exposure

Biomechanical Signal Is Present but Partial

R2 values ranged 13–26% across models, leaving substantial unexplained variance. Subsistence captures only part of the activity-based signal; individual occupation data may explain more.

  • BCB and lambda-inion show the clearest subsistence signal
  • Horticulturalists and hunter-gatherers trend similarly vs. agriculturalists on BCB
  • Gender-based division of labor may contribute to within-subsistence variation

Occipital Metrics Are Insufficient for Forensic Sex Estimation Alone

71% accuracy with foramen magnum area is above chance but below forensic standards. Adding bicondylar breadth reduced rather than improved accuracy, suggesting biomechanical loading introduces noise into condylar metrics when used for sex estimation.

Significance

This research demonstrates that:

  1. Occipital bone traits show consistent sexual dimorphism but limited standalone forensic utility for sex estimation.
  2. Subsistence-based biomechanical activity contributes to variation in condylar and nuchal metrics, complicating their use as pure sex indicators.
  3. Developmentally fixed traits and remodeling-prone traits behave differently and should be interpreted accordingly.
  4. Larger datasets with known individual occupation data are needed to fully parse sex-based from activity-based variation in the occipital bone.

These findings contribute to understanding:

  • The reliability and limitations of occipital bone traits in bioarchaeological and forensic sex estimation
  • How gendered divisions of labor and subsistence intensity may shape skeletal morphology
  • The value of combining original data collection with published data mining to expand analytical scope

Data & Code

Repository: https://github.com/kchoover14/OccipitalActivityPatterns

Available:

  • Raw and cleaned data (CSV)
  • R scripts for data preparation, exploratory analysis, and statistical modeling
  • Diagnostic plots and final figures (PNG)

Reproducible workflow:

  1. revised-preparation-original.R — wrangling, reliability testing, measurement error, PCA, index creation for original Florida dataset
  2. revised-preparation-mined.R — standardization and merging of mined Arikara and study collection data
  3. revised-exploratory.R — assumption testing (normality, Levene’s, outliers) for original and combined datasets
  4. revised-analysis.R — two-way ANOVA, Tukey HSD, discriminant function analysis, and all plots

Tools & Technologies

  • R for all data management and statistical analysis
  • Packages: dplyr, tidyr, irr, car, psych, EnvStats, MASS, PMCMRplus, factoextra
  • Models: two-way ANOVA (Type II SS), heteroscedasticity-corrected ANOVA, linear discriminant function analysis, PCA
  • Visualization: ggplot2, cowplot, ggpubr, psych::scatterHist

Publication

Hoover, K. C., & Thomas, G. P. (2022). Sexual dimorphism and biomechanical loading in occipital bone morphological variation. American Journal of Human Biology, e23792. https://doi.org/10.1002/ajhb.23792