The curious case of the gynandromorph

Graphic by Kinsey Ratzman ’21

Graphic by Kinsey Ratzman ’21


A cardinal spotted in January by a resident of Erie, Pennsylvania was different than most. According to an article published by the New York Times titled “A Rare Bird Indeed: A Cardinal That’s Half Male, Half Female,” this bird, which displayed both male and female sex characteristics, is known as a bilateral gynandromorph. Its left side appears to be the tawny brown of a female, while its right side displays the vivid scarlet of a male cardinal.

In order to confirm that this cardinal is actually a gynandromorph, or an organism that displays both male and female characteristics, a blood test would have to be conducted, said Daniel Hooper, an evolutionary biologist at the Cornell University Lab of Ornithology. However, the drastic color difference in the bird’s feathers is a fairly clear indicator.

Like many species, female and male cardinals look and behave differently. Male cardinals are bright red and sing more, while female cardinals are more muted and less flashy. Owing to the half male, half female brain of the gynandromorph, it’s unclear how an individual gynandromorph would behave. This particular bird was spotted several times with a male bird, but Hooper is unsure as to whether it is a potential mate or the bird’s father.

Hooper believes that all bird species have the possibility of gynandromorphism, but it’s only visually apparent in species with sexual dimorphism, or when the male and female look distinctly different. This particular bird is so exciting because it, unlike most gynandromorphs, may be able to reproduce. This is because, in birds, only the left ovary is functional, and this cardinal appears to be female on the left side.

Scientists don’t know for sure how gynandromorphs come to be. They could possibly be created through the fusion of two separately fertilized developing embryos. It is also possible that they develop when an egg, which contains both Z and W chromosomes, is fertilized by sperm that contains two Z-chromosomes. Female birds carry the ZW chromosome and male birds carry the ZZ chromosome, as opposed to XX and XY respectively in humans. Because vertebrates develop in a bilaterally similar way, the split runs down the center of the body, though there may be some mixing of the cells from the two embryos dispersed throughout the body.

“Gynandromorphs[…]are not as rare as most people imagine,” said Mount Holyoke Assistant Professor of Biological Sciences Patricia Brennan. “There are reports of gynandromorph chickens, and zebra finches as well as cardinals. Insects and crustaceans can also sometimes be gynandromorphs,” she said. Gynandromorphs can occur in several species. According to the National Geographic, a research team from the 1980s that found five gynandromorphic butterflies out of 300,000 raised. Another instance was a gynandromorph lobster, which was half-orange and half-brown. However, this is far less typical in mammals than in other types of animals. This is due to the fact that sex hormones are more important than the DNA of individual cells in a mammal’s sexual expression. Many mammals can be born with the primary and secondary sex characteristics of multiple sexes, but their gynandromorphism may not always be easily identifiable.

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