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Related Experiment Video

Updated: Jun 29, 2026

Who is Who? Non-invasive Methods to Individually Sex and Mark Altricial Chicks
08:14

Who is Who? Non-invasive Methods to Individually Sex and Mark Altricial Chicks

Published on: May 24, 2014

Digit ratio in birds.

Michael P Lombardo1, Patrick A Thorpe, Barbara M Brown

  • 1Department of Biology, Grand Valley State University, Allendale, Michigan 49401-9403, USA. lombardm@gvsu.edu

Anatomical Record (Hoboken, N.J. : 2007)
|October 4, 2008
PubMed
Summary
This summary is machine-generated.

Digit ratio (2D:4D) in birds is not consistently sexually dimorphic, but pooled data show males have larger ratios. This challenges hormone-based predictions for digit development in vertebrates.

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Area of Science:

  • Developmental Biology
  • Evolutionary Biology
  • Comparative Anatomy

Background:

  • Homeobox (Hox) genes are crucial for tetrapod digit development.
  • Sex steroids (estrogens and androgens) may influence Hox gene expression and digit growth.
  • Manning's hypothesis predicts sexually dimorphic digit ratio (2D:4D) due to differential prenatal hormone exposure.

Purpose of the Study:

  • To test Manning's prediction of sexually dimorphic digit ratio (2D:4D) in birds.
  • To investigate the relationship between digit ratio and other sexually dimorphic traits in avian species.

Main Methods:

  • Measured digit lengths (2D and 4D) and other morphological traits in four bird species: house sparrow, tree swallow, budgerigar, and chicken.
  • Calculated the 2D:4D ratio for each individual.
  • Analyzed data for sexual dimorphism within species and pooled data across species after averaging and z-transforming ratios.

Main Results:

  • No significant sexual dimorphism in 2D:4D was found within any of the four bird species studied.
  • No association was observed between 2D:4D and other sexually dimorphic traits.
  • Pooled analysis of all species revealed significantly larger 2D:4D ratios in males compared to females.

Conclusions:

  • Tetrapods exhibit varied sexual dimorphism in 2D:4D (larger in males in some birds/reptiles, smaller in males in some mammals).
  • Natural selection for perching feet in early birds may have influenced digit ratio development, potentially overriding hormonal effects.
  • Caution is advised when inferring prenatal hormone exposure from digit ratio in birds.