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Biological Sex Is Binary and Rooted in Anisogamy.
Tim Janicke1, Göran Arnqvist2, Pierre-André Crochet1
1CEFE, Univ Montpellier, CNRS, EPHE, IRD, Montpellier, France.
Ecology Letters
|July 10, 2026
Summary
Biological sex is defined as a binary classification based on gamete size, rooted in anisogamy. This scientific framework separates biological sex from human gender, accommodating diverse sexual phenotypes.
Area of Science:
- Evolutionary Biology
- Reproductive Biology
- Sexology
Background:
- Societal debates increasingly conflate biological sex with human gender diversity.
- Conceptual clarity is needed to distinguish between biological principles and social constructs.
- Existing definitions of biological sex lack universal applicability across diverse species.
Purpose of the Study:
- To propose a precise and universally applicable definition of biological sex.
- To differentiate biological sex from the concept of human gender.
- To leverage evolutionary theory for a robust framework of biological sex.
Main Methods:
- Analysis of evolutionary theory regarding reproductive strategies.
- Examination of empirical evidence on gamete production and sexual systems.
- Comparative study across species with varying sexual phenotypes and reproductive strategies.
Main Results:
- Biological sex is defined as a binary classification based on anisogamy (production of two discrete gamete types of different sizes).
- Gamete size is presented as the ultimate criterion for biological sex, applicable across all sexual systems.
- Evolutionary insights explain associations between biological sex and sex-specific selection, dimorphism, and parental care.
Conclusions:
- The definition of biological sex based on gamete size provides a coherent scientific framework.
- This definition is compatible with the diversity of sexual phenotypes in anisogamous organisms.
- The proposed definition clearly distinguishes biological sex from the concept of human gender.
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