There are times when the world of nature can fill the soul with delight. This set of relationships does that for me. It involves several players: a plant, two butterflies and a bunch of birds. It demonstrates several ecological principles: camouflage, warning coloration and mimicry.
The story starts with the plant. It's called Butterfly Milkweed (Asclepias tuberosa, for those of you who are into such things). Butterfly milkweed grows as a smallish, mound shaped shrub. Its flowers are beautifully orange, and grow in typical milkweed umbels (a cluster of several flowers all attached to the end of the same branch).
Butterfly milkweed, like the rest of the milkweeds, produces chemicals in the alkaloid family (the milky substance in the sap which gives this family of plants its common name). Alkaloids are poisonous to most animals, including the majority of birds, so birds learn to stay away from milkweeds when they are looking for a snack, since eating these bright orange beauties will make them sick (or worse).
The first of our butterflies is the Monarch (Danaus plexippus). This is one of the loveliest of butterflies, with its bright orange and black wings. Monarchs are very fond of butterfly milkweed, and can often be found sitting on the plant's flower clusters, against which they are beautifully camouflaged.
In fact, the Monarch lays its eggs on the Butterfly Milkweed, so its caterpillars grow up using the plant as their food source. (They aren't bothered by the alkaloids, obviously). When the caterpillars metamorphose into Monarch butterflies, the tissues of the butterfly contain the alkaloids from the plants that the caterpillars have been eating.
The Monarch coloration is camouflage while they are sitting on the milkweeds, but as soon as they leave the plant, that striking orange and black coloration makes them stand out wherever they go. Butterflies are high on the useful snack list of many species of birds, and the Monarch colors advertise their presence. So why don't all of the Manarchs get eaten by birds? Guess what happens to a bird who is na´ve enough to eat a Monarch butterfly? The butterfly is full of those alkaloids from the milkweed, so the same thing happens to the bird that would happen if it ate the plant. So birds learn to stay away from the flamboyantly colored Monarchs. This is called warning coloration where the animal is brightly colored to advertise that it is not a smart choice for dinner. A lot of poisonous snakes show warning coloration. So do skunks.
This would be a pretty great story even if this were all there was to it but there's more. There's a second species of butterfly called the Viceroy (Limenitis [Basilarchia] archippus). Viceroys are also brightly colored. The Viceroy's caterpillars don't grow up eating milkweed, so they don't bequeath alkaloid poisons to their adult forms.
Yet birds stay away from Viceroys when they are looking for a snack. Why? Because Viceroys look a lot like Monarchs. Even though some of those bright orange and black butterflies would be perfectly OK to eat, it doesn't pay off for the bird to take a risk. This is mimicry. The Viceroy mimics the Monarch, and is thus protected by the same system which protects the Monarch.
A couple of important points need to be kept in mind about this lovely story. First, the Viceroys don't "plan" to look like Monarchs, and no one had to plan it for them. The most likely reason for their mimicry is that, many generations ago, there were some Viceroy ancestors among a varied population who looked kind of like Monarchs. They were less likely to get eaten than the ones which didn't. Many generations later, the entire species' range of coloration has shifted closer and closer to the Monarch pattern. This is natural selection, and it can be a very powerful force in nature.
Secondly, note that for this system to work, there must be a lot more Monarchs in the ecosystem than there are Viceroys, since otherwise the payoff for the birds wouldn't be certain enough. If 50% of the orange and black butterflies around are perfectly good snacks, the birds will never learn the lesson.
And finally, this system only works if the predator species has enough brains to be able to learn to stay away from potential prey with a certain appearance. There are good reasons that, in general, predatory species have larger brain to body size ratios than their prey do. They really do have to be smarter.
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Updated 25 September 2004