TERATOLOGY: A gynandromorph cardinal: one half male, the other half female (Via Max Blake)

Male cardinals are bright red while females are a dull brownish color to provide camoflauge. This particular bird sports the colorations of both birds and is literall split down the middle.

Here’s photographer’s description of how he saw the bird:

"As you can see, the left side is male and the right female. For two winters the bird appeared at the feeder of a retired high school biology teacher. I was able to observe it on several occasions, and noticed that it didn’t associate with other cardinals, nor did I hear it produce any vocalizations. We attempted to capture it with mist nets so that Rob Fleischer and I could get blood samples for further study, but we caught every bird in the neighborhood except this one! Alas, it never returned the third winter.

This, by the way, is a very graphic demonstration of the differences between males and females, with the brighter color of the male almost certainly reflecting sexual selection (with bright colors presumably advantageous in males because they attract females, but disadvantageous in females because they attract predators).

Now how does this happen? I used to see this sometimes in my Drosophila flies, and we’ve long known how a half-male, half-female fly forms. In flies the sex is determined by the ratio of X chromosome to autosomes. Flies, like all diploid species, have two copies of every autosome. If you also have two X chromosomes, you’re a female because the ratio of autosomes to Xs is 1:1. If you have one X chromosome and one Y chromosome, your ratio is 2:1 and you’re male. The Y doesn’t matter here: if you lose a Y chromosome, and hence are XO, you still look like a male, although you’re sterile (the Y carries genes for making sperm).

So to get gynandromorphs in flies, all that has to happen is that one X chromosome gets lost in one cell when the initial cell in a female (XX) zygotes divides in two. One half of the fly then becomes XX, the other XO, and the fly is split neatly down the middle, looking like the one below. But gynandromorphs don’t have to be “half and halfs”. X chromosomes can get lost at almost any stage at development, so flies can be a quarter male, have irregular patches of maleness, have just a few male cells, or even a male patch as small as a single bristle.

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