Wildlife photographer snaps photo of transvestite bird

A strange bird showed up in Larry Ammann's backyard on Jan. 14. Clearly a cardinal, it had the bright red plumage of a male on its left side and gray, female feathers on its right.

"I had no clue how on Earth something like that could happen," said Ammann, a professor of statistics and a wildlife photographer who lives in a suburb of Dallas. "It was a learning experience."

Ammann and the biologists he consulted concluded the bird was most likely part female, part male. Creatures with this condition are called gynandromorphs. They are genetic anomalies: Some cells in their bodies carry the genetic instructions for a male, some for a female. While this gender-bending also occurs among insects, spiders and crustaceans, birds like this cardinal have raised questions about how sex identity is determined among some animals. [Gallery: Stunning Gynandromorphs of the Animal World]

As the breeding season began, other cardinals became more territorial, and the bird disappeared before it could be trapped and its feathers collected for genetic testing.

"The last view I got of it was two males chasing it away," Ammann said.

A question of hormones

In recent years, other he-she birds — a zebra finch and three funky chickens — have raised questions about how some animals develop male or female features.

For many animals, sex is determined by two chromosomes, called sex chromosomes. Among humans, for example, men have an X and a Y chromosome, while women have two Xs. Every cell in your body, except for sperm or egg cells, carries two sex chromosomes. But among mammals, like us, the Y chromosome carries a gene responsible for the development of testes, which release hormones that promote the development of male features. In women, the ovaries release different hormones that promote the traits we associate with being a female.

The hormones are key to sex identity in humans and other mammals. In fact the dominant hormones can overwrite an abnormality in the sex chromosomes — that is one reason we don't see gynandromorph humans. (Because of the influence of hormones, which diffuse throughout the body, one human can't end up with distinct male and female halves. A number of other disorders in sexual development can occur among humans, such as when external and internal sex organs don't match up or when someone possesses both male and female genitalia.)

Hormones were also assumed to be key to sex identity in other vertebrates, including birds. But studies of other birds with split identities — a zebra finch, in 2003, and three chickens, in 2010 — indicated otherwise.

An analysis of a dual-sex zebra finch revealed that, in spite of being exposed to the same hormones, the male and female sides of the bird's brain were different, and these differences appeared to have arisen because of the sex chromosomes carried by the brain cells. (This research appeared in the journal Proceedings of the National Academy of Sciences on April 15, 2003.)

Hormones, or some other chemical signal, were not completely out of the picture, however.

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