Eastern Tiger Swallowtail (Papilio glaucus) A study in protective mimcry

May 2017 Nature Notes contributed by BRNN Membership Director Charlotte Caplan

We’ve all seen this iconic butterfly. Its striking forewing markings – black stripes over a yellow ground – shriek “look at me!”. You can see the males on any spring or summer day cruising along woodland trails and other forest openings looking for the shyer females. Protective mimicry – NOT!

Well not in the adult males & most females, but in all other phases of its life cycle this species shows remarkable variety in its repertoire of defensive mimicry.

Let’s look at Papilio glaucus in each stage of life.

Egg (x 7) Wikipedia.org

Eggs
You have to be dedicated to find these. At 1mm they are actually large for a butterfly egg, but they are laid singly on the leaves of a wide variety of deciduous trees and are, well, leaf-colored. Looks like a rain-drop doesn’t it? Or maybe a pimply leaf gall. Laying single eggs takes more time and energy than clusters, so fewer are laid, but the advantages in avoiding predation during the 4-10 days before hatching are obvious.

Larva (caterpillar)
This is the longest and therefore most vulnerable life stage, lasting up to 2 months. During this time the larva grows from 1/8” to 2” long, with 5 stages separated by molting. In the early stages the tiny caterpillar is brown with a prominent white saddle. Predators don’t go for bird-droppings, much.

Final stage caterpillar – approx. life size Marylandinsects.com

As the larva grows, the brown and white “bird-dropping” coloration is less useful. By the 5th stage it has turned leaf green and developed a pair of fake “eyes” on the thorax. Snake-like eyes are a bird-deterrent adopted by many different species. But swallowtails add another defensive structure, a strange mouth appendage, called an osmetrium, normally hidden but protruded when the caterpillar is alarmed. The similarity to a snake’s tongue is impossible to ignore. The extended osmetrium also emits a foul odor.

One defensive strategy not used by Tiger Swallowtails is poison. The caterpillars consume a wide variety of plant & tree foliage and, unlike some other species, do not accumulate poisonous chemicals from their diet. But more on this later ….

Pupa (chrysalis)

Do you really want to eat me?     Papilio caterpillar with extended osmetrium (x 5) phys.org

Pupation can last as little as 10 days or the whole winter. Hiding in plain view is the strategy. The mottled brown chrysalis looks like a broken twig or curled dead leaf.

phys.org

Adult

Now we’ve come full circle back to those decidedly visible yellow and black butterflies that look only like themselves
– or do they?

I said that most females are black and yellow, similar to the males but with more blue on the hind wings. But some of them are quite different. So-called dark morph females completely lack the black and yellow coloring. Instead they closely resemble another species: the Pipevine Swallowtail (Battus philenor)

Pipevine Swallowtails’ distribution overlaps with the Tiger Swallowtails, extending further south into central America but less far north and west. Their host plants include pipevines (Aristolochia species), from which they accumulate poisons that protect both the caterpillars and adults from predation. Dark morph Tiger females (and several other butterfly species) borrow this protection through mimicry, without actually being poisonous.

There are no dark male Tiger Swallowtails; mimicry is reserved for the females through a sex-linked chromosome. Most species can tolerate a higher death rate in males than females, so linking protective mimicry to female gender benefits the species more than linking to male – and so does maintaining two color forms within the female population. Mimicry becomes ineffective if the mimics are more numerous than the poisonous models. The frequency of dark females varies geographically and is largely absent outside the range of the Pipevine Swallowtail, but dark females are always the minority. Studies have shown that the dark females have better survival rates than the tiger-striped females, but less breeding success, as males tend to prefer the yellow females. These opposing selective forces ensure a balance between the two morphs that is highly sensitive to location.

It is tempting to ascribe The Tiger Swallowtail’s success as a species mainly to its use of adaptive mimicry. But many species that have evolved equally elaborate survival strategies remain rare. There are always other factors. Perhaps the Tiger Swallowtail’s mating success has been enhanced by human activity providing openings in the forest canopy.