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Sex determination unexpectedly complicated in birds [1]

['Rob Beschizza']

Date: 2025-08-15

A study of wild Australian birds found that an invidiual's gonads and appearance may not align with its genetic or chromosonal sex determination. The phenomenon is more common than expected and "may challenge the reliability of traditional sexing methods" reliant on morphology.

The ability to unequivocally identify the sex and reproductive status of individuals is crucial across many fields of study. Recent evidence indicates that avian sex determination is more flexible than once believed, with sex-reversed individuals identified in domestic bird populations—that is, individuals exhibiting gonadal and morphological traits of one sex while possessing the chromosomal make-up of the opposite sex. The presence of these individuals can challenge the reliability of traditional sexing methods that rely solely on external morphology, internal anatomy or genetic markers. These methods, when used in isolation, fail to identify sex-reversed or intersex individuals, potentially overlooking their impact on population dynamics. In this pioneering study, we investigated the prevalence of sex-reversed individuals in five common free-living avian species in Queensland, Australia. By comparing internal and external morphological characteristics with polymerase chain reaction results from sex-linked molecular markers, we identified sex-reversed individuals in all five species, with rates ranging from 3 to 6%. Our findings suggest that sex reversal is a common and potentially widespread phenomenon in avian species.

Science reviews the report.

Judith Mank, a zoologist at the University of British Columbia, notes it's the genes carried on those chromosomes—not the chromosomes—that are the main players. The SRY gene on the Y chromosome, for example, kick-starts male development in mammals. Anyone missing this key gene will end up developing as female, even if they have XY chromosomes. "We think of sex chromosomes as being sex determining," says Mank, who also wasn't involved in the new research. "That's not true."

The phenomenon at hand is sex reversal, where "the pathway directed towards the already determined-sex fate is flipped towards the opposite sex, creating a discordance between the primary sex fate and the sex phenotype expressed."

I recall that this sort of thing comprised the plot of a successful movie: "life will find a way."

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[1] Url: https://boingboing.net/2025/08/15/sex-determination-unexpectedly-complicated-in-birds.html

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