A few cardinals have been documented to be a special kind of hermaphrodite called a “bilateral gynandromorph”

 Specifically, a few cardinals have been documented to be a special kind of hermaphrodite called a “bilateral gynandromorph.” The genders of these birds are literally split down the middle: One side of the body is male and has red feathers, the other side is female and has tan-brown feathers. This unique condition has been documented in other bird species, as well as in butterflies and crustaceans.

Gynandromorphic Northern Cardinal. Photo by Shirley Caldwell.

Biologists have come up with a few theories to explain its occurrence. One possibility is that a bilateral gynandromorph starts out as two separate embryos, one female and one male, that fuse during development. Another is that a female inadvertently produces an egg carrying copies of both of her sex chromosomes, instead of just one, which is then fertilized by two separate sperm. (Female birds have two different sex chromosomes, labeled Z and W.) The split down the middle is just a byproduct of the symmetrical way that vertebrates develop.

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