tiger swallowtails

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These two eastern tiger swallowtails — a bright yellow male and a dark-phase female — could easily be taken for different species.

The coloration of animals is an interesting study. One of the stranger aspects is the fact that creatures which are similar and lead similar lifestyles can be remarkably different in color and pattern and yet seem to be equally successful.

Butterflies are a good example. In Florida, we have several species of swallowtails, which are as a rule large and showy. Eastern tiger swallowtails (Papilio glaucus) are bright butter yellow with black stripes. But there are other species such as the black swallowtail, which is mostly as the name says: Black wings with yellow spots on the edges, and a lovely deep blue iridescence on the hind wings. Some female tiger swallowtails occur in a black color phase, which looks very much like a black swallowtail.

But why though? According to the laws of natural selection, all organisms are shaped by their environments. Those that are a poor fit are less likely to survive long enough to reproduce. So what is it that has caused these butterflies to adopt such strikingly different color patterns? As adults, they share the same general habitat, the same predators, the same food sources.

As it turns out, you have to look at other species to make sense of the puzzle. There is another butterfly, the pipevine swallowtail, that is also black with blue iridescence. Pipevine swallowtails are named for the food they eat as caterpillars — toxic plants in the Aristolochia genus. These toxins remain in their bodies even as adults, making them distasteful to birds and many other predators.

Now it makes sense. Black swallowtails, black phase tiger swallowtails, and a handful of other related species all benefit from a resemblance to their poisonous cousin. This is similar to the more familiar situation in which the monarch butterfly, which gets its toxicity from milkweeds, is imitated by the perfectly edible queen and viceroy butterflies. This type of imitation is called Batesian mimicry, and there are lots of Batesian species complexes out there.

OK, mystery solved! Except — if imitating a toxic relative is such an effective strategy, why aren’t all tiger swallowtails black? In some areas, most females are black, but not all. And males always have those easily seen bright yellow wings. Hmmm …

There’s another force at work here: Sexual selection. If you outfox your predators but can’t find a mate, then your genes don’t get passed on to the next generation. And that’s what happens with some of the black phase tiger swallowtails. Even though dark females still act just like other tiger swallowtails, and even though they produce the same pheromones, males just aren’t as interested in them. These guys are apparently traditionalists and prefer the good ol’ yellow and black that their species is known for.

You can see this for yourself. All you need is an area where tiger swallowtails frequent. My field lab was the front yard of my childhood home. There were many acres of open grassland, scrubby forest, palmetto uplands and orange grove surrounding the place, so we had all sorts of animals around — including dozens of butterfly species. Tiger swallowtails were fairly common, and dark-phase females made up perhaps 10 percent of the population.

When swallowtails are pairing up to mate, the male and female will fly frantically around each other. Some territorial butterflies will perform similar maneuvers as males try to chase each other off, but the mating dance is different. Territory battles move quickly and are over quickly. Male-female interactions last longer.

Yet male tiger swallowtails encountering a black-phase female would usually react as they would to another male. A few quick circles and they were done. Occasionally I’d see a dark female and yellow male pair off, but more often than not the males were clearly not interested.

As for why this color occurs only in females, apparently the mutation that leads to a high melanin level and therefore darker coloration is sex-linked. That’s not at all uncommon in the animal kingdom, where sexual dimorphism (visible differences between male and female such as coloration or size) is the norm. There are also traits that are common in one gender but not the other (such as colorblindness in humans, which affects 8 percent of men and 0.5 percent of women).

So what we’re seeing here is an animal in a weird evolutionary flux. Natural selection is trying hard to push them into mimicking a toxic species, which would keep them from being eaten as often. But sexual selection is preventing what is clearly a beneficial genetic trait from becoming fully established.

If you’re the type who looks for life lessons, there’s one here: Be careful to avoid getting in the way of your own success by making dumb decisions. If those picky males could just get over themselves, it would ultimately be for the better of their kind.

However, nature is a lot like us, in that it often makes bad decisions when good ones would be so simple. If there’s a moral here, I think that’s it: We all do dumb things sometimes, no matter how much experience we have (or think we have).

Capt. Josh Olive is a fifth-generation native Florida Cracker and a Florida Master Naturalist, and has been fascinated by all sorts of wild things and places since he was able to walk. If you have questions about living with wildlife, contact him at Publisher@WaterLineWeekly.com or 941-276-9657. You can also follow him on Instagram @florida_is_wild.

Capt. Josh Olive is a fifth-generation native Florida Cracker and a Florida Master Naturalist, and has been fascinated by all sorts of wild things and places since he was able to walk. If you have questions about living with wildlife, contact him at Publisher@
WaterLineWeekly.com or 941-276-9657. You can also follow him on Instagram @florida_is_wild.

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