Batesian Mimicry

Take a look at the first two images below. Both butterflies are beautiful. The one with the white banding is called a white admiral, the other a red-spotted purple. Clearly different species. Or are they?

White admiral butterfly. Photo by Don Scallen.

Red-spotted purple butterfly sipping dogbane nectar. Photo by Don Scallen.

Science calls the white admiral Limenitis arthemis arthemis and the red-spotted purple Limenitis arthemis astyanax. The specific name arthemis tells us that these two different looking butterflies are, in fact, the same species, separated only on the subspecies level.

They can mate and often do, producing young that are intermediate in colour and patterning. What gives?

Bear with me a moment. The red-spotted purple is a Batesian mimic while the white admiral isn’t. A Batesian mimic is an animal that benefits from a resemblance to another animal that is toxic or poisonous. With the red-spotted purple this animal is the pipevine swallowtail.

John Flannery from Richmond County, North Carolina, USA, CC BY-SA 2.0, via Wikimedia Commons

Pipevine swallowtails pack a toxic wallop because they ingest poisons from their food plant, Dutchman’s pipe. Like monarch butterflies, pipevine swallowtails make birds vomit.

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Looking like a pipevine swallowtail is a survival enhancer for several species of lepidoptera including red-spotted purples, black swallowtails and promethea moths.

So why didn’t the white admirals adopt the same warning garb? Geography provides the answer.  Pipevine swallowtails are a southern species because pipevine, their food plant, does not grow in Canada.

At least it doesn’t grow in the wild. Some homeowners here grow pipevine on trellises and porches. These domesticated pipevines do sometimes lure pipevine swallowtails north of the Great Lakes.

Black swallowtail, another pipevine swallowtail mimic. Photo by Don Scallen.

Spicebush swallowtail, another pipevine swallowtail mimic. Photo by Don Scallen.

Getting back to the white admirals, they generally live north of the pipevine swallowtails’ range. Hence there is less benefit to be gained from looking like them. Red-spotted purples are generally distributed southward, so they share most of their range with pipevine swallowtails.

Evolution has been hard at work for millennia, tweaking the attributes of plants and animals to boost their survival prospects. The results are often astonishing.