6 days ago
Why did a male bird lay an egg?
For wild birds, sex reversal may be more common than ornithologists originally believed. A new study of roughly 500 birds from five species found that up to six percent had the physical features of one biological sex, but the genetic make up of the other. In one instance, a genetically male kookaburra had a distended oviduct. The scientists' only possible conclusion? The male bird had recently laid an egg.
'This indicates that sex determination in wild birds is more fluid than we thought—and can persist into adulthood,' said Dominique Potvin, a behavioral ecologist, ornithologist, and evolutionary biologist from the University of the Sunshine Coast in Queensland, Australia. Potvin is also a co-author on the study published August 13 in the journal Biology Letters.
Not so straightforward
Sex reversal is well known in fish, amphibians, and reptiles. However, it is rarely documented in wild birds and mammals. In humans, those born with XX chromosomes generally develop as females. Those with XY chromosomes are usually male. However, it is the genes carried on those chromosomes (not the chromosomes themselves) that are the main players at determining biological sex. For example, the SRY gene on the Y chromosome, is what will typically kickstart male development in mammals.
How these genes are expressed cell-by-cell also matters. The individual cells in chickens, zebrafish, and fruit flies all have their own sexual identity. This identity is based on the genes the cells contain or express, instead of being influenced by overall hormone levels. If different cells have different sets of chromosomes, this can result in animals considered gynandromorphs, which show both male and female characteristics.
In humans, the term intersex describes a body that falls outside of the strict male-female binary. Generally, it is someone born with biological sex characteristics that are not typically male or female, or who develop them naturally during puberty. A person who is intersex is born with sex chromosomes, external genitalia, or an internal reproductive system that is not considered typical for a female or a male. The Intersex Society of America estimates that 1 in 1500 to 1 in 2000 births are considered intersex, but it is a difficult statistic to calculate.
[ Related: Rare bird with half male and half female plumage photographed in Colombia. ]
DNA doesn't always tell a complete story
The study looked at five species: the Australian magpie (Gymnorhina tibicen), laughing kookaburra (Dacelo novaeguineae), crested pigeon (Ocyphaps lophotes), rainbow lorikeet (Trichoglossus moluccanus), and the scaly breasted lorikeet (Trichoglossus chlorolepidotus). The birds had previously been treated for injury or illnesses at wildlife hospitals in southeast Queensland and were all examined after they had died. The team identified their reproductive organs and then tested their DNA to determine their genetic sex.
The sex-reversed birds were all classified in three ways: genetic males that have a complete feminine phenotype (i.e. all an organism's observable characteristics), genetic females with a complete masculine phenotype, and birds showing a combination of both testicular and ovarian characteristics.
'One of the key findings was that 92% of sex-reversed birds were genetically female but had male reproductive organs,' said Potvin. 'We also discovered a genetically male kookaburra who was reproductively active with large follicles and a distended oviduct, indicating recent egg production'—our male, egg-laying bird friend.
Additionally, two genetically female crested pigeons had both testicular and ovarian reproductive structures.
The presence of sex-reversed individuals in a population could affect a bird's reproductive success. One bad mating season can spell serious trouble for threatened species, such as Antarctica's Emperor penguins.
'This can lead to skewed sex ratios, reduced population sizes, altered mate preferences, and even population decline,' study co-author and biologist and ornithologist Clancy Hall added.
Sexually ambiguous birds also challenge the traditional methods of identifying bird sex. These include genetic markers, plumage (such as male peacocks having more elaborate plumage to attract a mate), or behavior.
'The ability to unequivocally identify the sex and reproductive status of individuals is crucial across many fields of study,' said Hall. 'We've now established that DNA evidence does not always reflect the suspected sex. 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.'
Searching for clues
According to the team, the causes of sex reversal in these birds is unclear. More studies are needed to better understand environmental triggers. These types of factors are not unheard of in nature. Temperature can affect the sex of turtle hatchlings buried in the sand, with cooler incubation temperatures creating more male hatchlings and warmer temperatures generating more females.
For birds, it is possible that high stress hormones or endocrine-disrupting chemicals in their environment may be influencing sex development in birds. Earlier studies have found endocrine-distrupting chemicals in insects that develop near sewage treatment plants that are eaten by birds. Another study found that male European starlings exposed to these chemicals develop longer and more complex songs, but have damaged immune systems.
The reproductively active, genetically male kookaburra in the study was found in a zone between an urban and more rural area, where endocrine-disrupting chemicals may accumulate.
Despite the unknown cause of sex reversal in birds, the study provides a baseline for understanding the phenomenon in Australia's many birds. More accurate data on the numbers of male birds versus female birds and the role sex reversal plays in the population should also aid conservation research for the most at-risk species.
'Understanding how and why sex reversal occurs is vital for conservation and for improving the accuracy of bird research,' Potvin said.
Solve the daily Crossword
