Rainbow lorikeet. Credit: Andrew Allen (CC BY, https://creativecommons.org/licenses/by-nc-nd/4.0/)
A study of nearly 500 Australian birds revealed that up to 6% had undergone ‘sex reversal’ – developing the physical features of one sex while having the sex chromosomes of the other.
Co-author of the study Dominique Potvin, an associate professor at the University of the Sunshine Coast (USC), Australia, says the new findings indicate that “sex determination in wild birds is more fluid than we thought – and can persist into adulthood”.
The research, published today in Biology Letters, has implications for the reproduction of threatened species and the ways in which scientists study birds.
The researchers examined birds admitted to wildlife hospitals in southeast Queensland, which died naturally or were euthanised due to illness or injury, to determine sex-reversal rates in wild birds.
The animals belonged to 5 species found in the area: Australian magpie, laughing kookaburra, crested pigeon, rainbow lorikeet and scaly breasted lorikeet.
The team examined each bird’s reproductive organs and tested its DNA to determine its genetic sex.
Potvin says, “92% of sex-reversed birds were genetically female but had male reproductive organs.”
“We also discovered a genetically male kookaburra who was reproductively active with large follicles and a distended oviduct, indicating recent egg production.”
Laughing kookaburra. Credit: Teale Britstra (CC BY-NC-ND, https://creativecommons.org/licenses/by-nc-nd/4.0/)
Sex reversal is known to occur in a range of animals, like gastropods (molluscs), fish, amphibians and reptiles, which have fluid sex-determination processes where environmental cues such as temperature can influence the outcome.
“In vertebrates, there is a range of sex-determination processes ranging from strict genetic control at one extreme, to environmentally determined sex (e.g. temperature dependent) at the other,” the authors explain in the study.
Sex reversal happens less in birds and mammals, where sex determination is more rigidly controlled by genetics.
In birds, genetic sex is determined by the sex chromosomes Z and W: males have ZZ, while females contain ZW.
Crested pigeons. Credit Robin Gwen Agarwal (CC BY-NC, https://creativecommons.org/licenses/by-nc/4.0/)
“Although the genetic sex does not change after embryogenesis, an individual’s phenotype [physical characteristics] may be redirected if there is a disruption to the initiated genetic pathway or there is a failure to repress the opposite pathway,” the authors write.
“Genetic males may thus present with a complete and reproductively functional feminine phenotype, or (more commonly in birds) genetic females develop a complete masculine phenotype.
“A partial sex reversal may also result in ovotestes, where an individual develops a combination of both testicular and ovarian characteristics … The term ‘sex reversed’ is used here to describe birds in which at least the gonads but possibly other sexually dimorphic characteristics appear discordant with the genetic sex.”
Potvin says, “Understanding how and why sex reversal occurs is vital for conservation and for improving the accuracy of bird research.”
This is because traditional methods of identifying bird sex – such as genetic markers, plumage, or behaviour – may not be reliable when identifying sex reversed birds.
Australian magpies. Credit: Darcy Whittaker (CC BY-NC, https://creativecommons.org/licenses/by-nc/4.0/)
“The ability to unequivocally identify the sex and reproductive status of individuals is crucial across many fields of study,” says lead author Dr Clancy Hall, also from USC.
“Bird researchers often take small DNA samples from blood or feathers and make decisions based on the results showing male or female, but this actually could be wrong up to 6% of the time.”
She adds that the presence of sex-reversed individuals could affect reproductive success in wild populations.
“This can lead to skewed sex ratios, reduced population sizes, altered mate preferences, and even population decline,” says Hall. In combination with other stressors, the authors suggest sex reversed birds may undermine conservation efforts.
The study provides a baseline of sex-reversal rates in wild birds, which the authors suggest will “improve population modelling methods that currently use either visual or genetic sexing data”.
The causes of sex reversal in birds remain unclear. The researchers say more studies are needed to better understand its environmental triggers and potential impacts.
Scaly breasted lorikeet. Credit: Silvia Alexander (CC BY-NC, https://creativecommons.org/licenses/by-nc/4.0/)