Last week, I featured an unusual butterfly sent in by reader Sandy Sisk. This butterfly had two dramatically differently colored wings, which I at first took for a classic example of bilateral gynandromorphism, which is a natural phenomenon where an organism is split down the middle – half male and half female. But several eagle-eyed readers urged me to take a closer look at what was really going on, and I realized they were right. This photo was actually showing a butterfly who was entirely female, but displaying two different color forms, usually seen in different regions – known as a color mosaic. Feeling confused? Let me explain, but first take a look at the photo again.
Let me start by explaining that Eastern Tiger Swallowtail (Papilio glaucus) females display different coloration in different parts of the country. In some places, they rely on mimicry to defend themselves, and in those areas, the female Tiger will be nearly black, with only the faintest outline of tiger striping visible in bright sunlight. They are usually mimicking a cohabitant species like the Pipevine Swallowtail, which is toxic or at least distasteful to many predators. In areas where the Pipevine Swallowtail isn’t found, the female Tiger will display a similar yellow coloration to the males, but with a little more blue along the lower wing. A few pictures will help explain this best.
This first collage shows a dark form female Tiger Swallowtail on the left, and a light form on the right. Note that if you look carefully at the dark form, you can see the faint tiger striping on the wings.
This next collage shows a Pipevine Swallowtail (Battus philenor) on the left, compared with the same dark form Tiger Swallowtail on the right. While certainly not identical, the coloration is enough to fool many predators who’ve had a taste of a Pipevine Swallowtail and would rather not repeat the experience. Only females are known to display this dark form color, with males in all areas sticking to the light coloration.
Finally, compare a light form male on the left with a light form female on the right. Note the obvious blue markings on the female, and compare them to the right wing of Sandy’s photo.
In the photograph taken by Sandy, the butterfly shown is actually entirely female. If you look closely, you’ll note the giveaway blue coloration along the bottom of the right wing. I missed this the first time around, because here in Florida, our female Tigers are almost always the black color form, and I don’t usually have to look for the blue color to make the determination. Fortunately, some of our readers caught and corrected me on this, and I realized Sandy’s photo showed something a little different. As noted by the good folks at the What’s That Bug site, Sandy’s butterfly was actually what’s known as a color mosaic, meaning the butterfly is showing colors from several different forms. Tiger Swallowtails seem to be especially prone to this type of variation, with intersex examples like the true bilateral gynandromorph, or others like Sandy’s. Sometimes, the butterfly has two different color forms on the same side! If you’re interested in a rather scholarly work full of really great image examples, check out this paper by Dr. J. Mark Scriber.
Finally, a last word to those who were convinced Sandy had faked the whole thing with Photoshop – here are several photos of the same butterfly from another angle. I assure you, all of Sandy’s photos are genuine shots of a color mosaic butterfly, and I’m so glad she shared them. I’m also glad for readers who let us know when we’ve made mistakes, so we can correct them. The world is full of wonderful things to see, admire, and learn about – thanks for coming along on the journey with me!