Skull morphometry and sexual dimorphism in the Kyrgyz horse: A multivariate size-corrected analysis.

Objective Overview

• This research studied the skulls of Kyrgyz horses to understand the differences between males and females in terms of size and shape.
• The study found that male horses have significantly larger skulls overall, but the shapes of male and female skulls are very similar when accounting for size differences.

Introduction and Purpose

• The study focuses on sexual dimorphism, which refers to differences in size and shape between males and females of a species.
• Kyrgyz horses were selected as the subject due to their specific morphology and potentially distinct sexual differences.
• The researchers aimed to separate size effects from shape effects to see if male and female skulls differ only in scale or also in structure.

Methods

• Data Collection:
  • Measurements were taken from the skulls of 39 Kyrgyz horse specimens (both male and female).
  • 18 linear cranial measurements were used, focusing on diverse skull regions.
• Size and Shape Separation:
  • Mosimann log-shape ratios were applied to separate shape from size. This involves scaling measurements by the geometric mean and then log-transforming to isolate shape.
  • This method allows distinguishing differences caused purely by size from differences in the proportion or shape of the skull.
• Data Analysis:
  • Raw linear measurements were analyzed to detect size differences.
  • Size-corrected log-shape variables were examined to explore shape differences independent of size.
  • Multivariate statistics such as MANOVA (multivariate analysis of variance) and principal component analysis (PCA) were used to analyze variation patterns and sex effects.
  • False Discovery Rate (FDR) correction was applied to control for multiple statistical testing errors.

Key Findings

• Size Differences:
  • Males showed larger values across all raw cranial measurements compared to females, confirming sexual size dimorphism.
  • 14 out of 18 measurements remained statistically significant after correcting for multiple comparisons.
  • Measurements with greatest differences included total skull length, nasal length, length to the nasale landmark, breadth between the temporomandibular joint regions, and hard palate length.
  • Overall geometric mean size was greater in males, reinforcing strong size dimorphism in the skull.
• Shape Differences:
  • Multivariate analysis (MANOVA) revealed a significant sex effect on raw data mostly driven by size.
  • The first principal component (PC1) in raw data explained over 53% of variation and represented overall size differences, effectively separating males and females.
  • When focusing on size-corrected shape variables, no significant sex differences were detected after FDR correction, indicating weak evidence of shape dimorphism.
  • Residual shape variation, after accounting for size, was mainly seen in diastema length (space between teeth), occipital squama height, foramen magnum size, and nasal length.
  • However, these shape differences were not strongly linked to sex, implying they likely reflect individual variation rather than sexual dimorphism.
• Integration and Correlations:
  • Strong correlations were identified among rostrocaudal facial measurements (front-to-back dimensions) and midfacial breadth variables.
  • This integration suggests these skull regions scale together consistently.

Interpretation and Implications

• The main sexual dimorphism in Kyrgyz horse skulls is size-based rather than shape-based.
• Males have larger skulls that are essentially scaled-up versions of female skulls, rather than having distinctly different cranial proportions or features.
• This finding informs understanding of horse morphology, evolution, and may have implications for breeding or identifying sex in skeletal remains.
• The study demonstrates the importance of separating size and shape when evaluating sexual dimorphism to avoid conflating overall size differences with true shape variation.

Summary

• The study used detailed morphometric techniques to analyze Kyrgyz horse skulls.
• It confirmed strong sexual dimorphism in skull size but minimal differences in shape independent of size.
• This indicates males and females share similar skull shapes, with males simply having larger, proportionally similar skulls.