About 5% of common Australian wild birds including kookaburras and lorikeets could have undergone a “sex reversal” where their genetic sex does not match their reproductive organs, according to a new study.
The study is thought to be the first to find widespread sex reversal across multiple wild bird species, but the cause of the phenomenon is not yet known.
The results suggest sex reversal is more common in wild birds than previously thought, and have raised concerns about the potential impact of chemicals that can disrupt hormones in animals.
Researchers tested 480 birds across five common species that had died after being admitted to wildlife hospitals in south-east Queensland.
Researchers first used a DNA test to determine a bird’s genetic sex; in birds, males have a pair of Z chromosomes and females have one Z and one W.
But after dissecting the birds, they found a mismatch between the DNA test and the reproductive organs of 24 of the birds.
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Associate Prof Dominique Potvin, a co-author of the research at the University of the Sunshine Coast, said the team were deeply sceptical when the results first came in.
“I was thinking, is this right?” she said. “So we rechecked, and rechecked and rechecked. And then we were thinking, ‘oh my God’.”
Potvin said she had revealed the results to ornithologist friends. “They were mind-blown,” she said.
Concern sex reversal may skew data
Almost all of the “sex discordant” birds were genetically female but had male reproductive organs, the research, published in the Royal Society journal Biology Letters found.
In one case, a kookaburra that was genetically male had a stretched oviduct – the passageway for an egg – that suggested “recent egg production”, Potvin said.
Two genetically female crested pigeons had both testicular and ovarian reproductive structures, the research found.
Other birds tested were rainbow lorikeets, scaly-breasted lorikeets and Australian magpies. The lowest levels of sex reversal was 3%, found in Australian magpies, and the highest was 6.3% in crested pigeons.
Dr Clancy Hall, the lead author of the research, also at the University of the Sunshine Coast, said sex-reversed birds could affect reproductive success that should raise concerns about the impact on threatened species.
She said: “This can lead to skewed sex ratios, reduced population sizes, altered mate preferences, and even population decline.
“The ability to unequivocally identify the sex and reproductive status of individuals is crucial across many fields of study.”
Experts say chemicals may be to blame
The causes of the sex reversals were unclear, but one factor could be contact with chemicals in the environment.
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Sex reversal is known to occur in some molluscs, fish, amphibians and reptiles and can occur naturally or be influenced by chemicals that can affect an animal’s hormones – known as endocrine disrupting chemicals (EDCs).
Prof Kate Buchanan, who studies the evolutionary biology of wild birds at Deakin University but was not involved in the study, said because the default sex of birds was female, it was not surprising that most of the sex reversal affects were in the direction of female to male.
She said: “The most likely explanation of the masculinisation is some environmental stimulation, probably anthropogenic chemicals.”
Buchanan has been part of research that has found EDCs in insects that develop in sewage treatment works and are food for some birds, as well as a study that found male European starlings exposed to the chemicals developed longer and more complex songs, but had a damaged immune system.
She said even if the masculinisation of affected birds was reversible in their lifetime, “it would probably knock them out of being reproductive”.
Dr Clare Holleley is the head of vertebrate collections at the Australian government science agency the CSIRO and has studied sex reversal in lizards.
“What’s doing this is now the big question,” she said.
While a cause could be natural – for example, sex reversal in lizards can be triggered by temperature changes – Holleley said it was likely “something else is going on”.
“If sex determination gets disrupted then something has to push you off track. The most likely [cause] is endocrine disrupting chemicals.”
Dr Golo Maurer, the director of conservation strategy at BirdLife Australia, said the research was likely to cause a stir in the ornithological field.
He said the presence of EDCs and the potential impacts was a “huge concern”, given the other crises facing birds, from climate change to habitat clearance and plastic pollution.
Some experts were cautious about extrapolating the results to the wider population of wild birds, because the birds were not a random sample but had been admitted to hospitals.
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