This past February, a very unusual Northern Cardinal was spotted in Erie, Pennsylvania. Its right side is brilliantly red, while its left side is a modest brown, with both sides perfectly split down the middle. The reason for the color split is remarkable; the left side is biologically female, while the right side is biologically male. Genetically, the two halves are as closely related as brother and sister.
The cardinal’s rare genetic mutation is known as bilateral gynandromorphism. Bilateral gynandromorphs have been recorded in crustaceans, insects, some reptiles, and many species of birds. All these animals share what is known as the ZW-sex determination system. Scientists are not entirely sure how gynandromorphs occur. Biologist Michael Clinton of the University of Edinburgh theorizes that there are two characteristics of the ZW-determination system that combine to make gynandromorphism possible:
The first characteristic is that the mother’s genes are responsible the sex of the offspring. Having ZW sex chromosomes causes an embryo to develop into a female, while having ZZ chromosomes causes an embryo to develop into a male. The mother contributes either a Z or W chromosome, producing either a male or female child, respectively. Clinton believes that female birds have a biological mechanism that causes them to overwhelmingly contribute either Z or W chromosomes in certain environmental conditions. This would explain the incredibly skewed gender ratios seen in some species; for example, it is not uncommon for some female parrots to hatch 20 male or 20 female offspring consecutively. If this biological pathway goes awry, a female bird might produce an egg cell with both Z and W chromosomes. This egg cell is then fertilized by two normal sperm cells, getting two more Z chromosomes. The fertilized egg now has full male and female genetic information, causing it to develop with both ZZ male and ZW female DNA.
The second characteristic is that animals with the ZW-sex determination system do not appear to rely on hormones to develop primary sex characteristics. This is the opposite of what is seen in animals with the XY-sex determination system, such as humans. Humans can be physiologically male and have XX chromosomes, or be physiologically female and have XY chromosomes. These genetic abnormalities are caused when certain hormones are not created or absorbed, or when certain hormones are created when they shouldn’t be. However, a bilateral gynandromorph is able to maintain perfect male and female halves, even though both its halves are soaked in male and female hormones manufactured by its mismatched testis and ovary. Sex differences in gynandromorphs even extend to their brains; in a gynandromorphic zebra finch, the male half of its brain contained the neural pathways to sing courtship songs, while the female half did not. (Sex-specific brain structures are not seen in humans, who do not have categorically male or female brains.)
Probably as a result of these brain differences, many gynandromorphs are unable to properly perform the social behavior of either gender. They are often are shunned or even attacked by their peers. Fortunately, the Erie, Pennsylvania cardinal is an exception; she/he is frequently in the company of another male bird. Researchers suspect that her/his left ovary might be viable, so she/he may even become a mother/father in the spring.
By Elise Brosnan
Robson, David “These Animals Are Male on One Side and Female on the Other.” BBC Earth. Last modified Sept. 22, 2015. http://www.bbc.com/earth/story/20150916-these-animals-are-male-on-one-side-and-female-on-the-other
Seaberg, Maureen “Rare Half-Male, Half-Female Cardinal Spotted in Pennsylvania.” National Geographic.Last modified Jan. 31, 2019. https://www.nationalgeographic.com/animals/2019/01/half-male-half-female-cardinal-pennsylvania/
Weintraub, Karen “A Rare Bird Indeed: A Cardinal That’s Half Male, Half Female.” The New York Times.Last modified Feb. 9, 2019. https://www.nytimes.com/2019/02/09/science/cardinal-sex-gender.html?action=click&module=Well&pgtype=Homepage§ion=Science