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Are there species with more than two sexes?
Executive summary
Yes — multiple documented lifeforms have more than two mating types or “sexes”: single-celled ciliates like Tetrahymena thermophila can have seven mating types [1] [2], some fungi (e.g., Schizophyllum commune) are estimated to have thousands of mating types [3], and researchers report animals and algae with three reproductive categories — Auanema nematodes with three sexes (ScienceDaily summary of Caltech work) and Pleodorina starrii algae with male, female and a bisexual third type [4] [5]. Coverage uses different terms (mating type, sex, hermaphrodite), and definitions vary among researchers [6] [3].
1. What scientists mean by “more than two sexes” — words matter
Biologists use several overlapping concepts — “sex,” “mating type,” and “hermaphrodite” — and these are not interchangeable; many reports that say “thousands of sexes” are describing genetically distinct mating types rather than discrete male/female body plans [3] [6]. The clearest examples of many categories come from fungi where genetic loci (A and B regions) have many variants, yielding very large numbers of mutually compatible mating types rather than different anatomical sexes [3].
2. Single-celled success stories: Tetrahymena and other protists
Protozoans such as Tetrahymena thermophila have multiple mating types — often described as seven in that species — where individuals can mate with any type except their own. Coverage in New Scientist and Live Science highlights that these are genuine biological systems with more than two alternatives for sexual pairing [2] [1].
3. Fungal extremes: thousands of mating types, not thousands of genders
Mushroom-forming fungi like Schizophyllum commune are estimated to possess tens of thousands of distinct mating-type combinations due to multiple polymorphic loci; journalists frame this as “thousands of sexes,” but the underlying mechanism is many allelic variants at mating loci rather than separate male/female morphologies [3]. The Science/education framing emphasizes the genetic mechanism and the evolutionary advantage for sessile organisms of having many compatible partners [3].
4. Algae and animals with three reproductive roles
Experimental and field work has documented three-role systems in multicellular organisms: the freshwater alga Pleodorina starrii has three sexes — male, female, and a bisexual third capable of producing both gamete types as a normal genotype — and this system mates in pairs [5]. ScienceDaily and Caltech writeups also report Auanema nematodes with three sexes, unusual life-history traits and extreme arsenic resistance in the Mono Lake species, described as having three reproductive categories [4].
5. Hermaphroditism and sequential sex change complicate a binary story
Many animals are hermaphrodites or sequentially change sex (e.g., some fish), so reproductive strategy diversity does not always map neatly to “how many sexes” a species has [6] [7]. Reviews and primers warn that using “sex” loosely risks conflating genetic compatibility systems, anatomical sexes, and behavioral roles — each is a different biological axis [6] [3].
6. Why multiple types evolve — ecological and genetic advantages
Authors explaining mating-type diversity argue that more mating types increase the chance of finding a compatible partner (advantageous for sessile or low-density organisms) and can arise through variation at multiple sex-determining loci [1] [3]. Lab and evolutionary commentary frame these systems as solutions to mate-finding and genetic-exchange problems, not as direct analogues to human social gender categories [1] [3].
7. Caveats, contested language, and reporting tendencies
Popular headlines (“thousands of sexes,” “three sexes discovered”) aim to grab attention but often gloss the technicalities: “mating type” vs. “sex,” and genetic vs. morphological distinctions. Scholarly reviews note this semantic muddle and caution readers that definitions differ across taxa [3] [6]. Where sources explicitly define their terms, reporting aligns with the underlying biology [5] [4].
8. Bottom line for readers interested in biology or social analogy
Biology clearly contains systems with more than two reproductive categories if you count mating types or distinct genetic compatibility classes [3] [1]. Multicellular organisms with three coexisting reproductive roles have been reported [4] [5]. However, available sources show that technical definitions matter: many “more-than-two” examples describe genetic mating systems rather than anatomically distinct third sexes [3] [6].
Limitations: available sources do not offer a single universal definition of “sex” across all life [6], and some popular articles simplify genetic mechanisms into punchy headlines [3] [1].