Imitation is often described as the sincerest form of flattery, but for the viceroy and monarch butterflies, whose patterns of orange and black wing coloration are remarkably similar, it is a form of survival. Long considered a classic example of Batesian mimicry—when a harmless organism, for its own protection, resembles a poisonous or otherwise dangerous organism—the relationship between the viceroy and monarch was challenged in the early 1990s, when zoologists David B. Ritland and Lincoln P. Brower proposed a new theory, one based on Müllerian mimicry—when two unrelated noxious organisms resemble one another, with each mimetic benefiting.

Ritland and Brower's research, which was published in 1991 in the journal Nature, suggested that the viceroy, like the monarch, was unappetizing to its predators and that its bright coloration warned its predators of this. Moreover, the study indicated that the mimetic relationship between the viceroy and the monarch was extraordinarily complex, far more so than was widely believed.

The monarch (Danaus plexippus) is loaded with toxic substances known as cardiac glycosides, which it acquires as a result of feasting on milkweed plants as a caterpillar. These substances render it unpalatable to many of its predators, and its brightly colored wings serve as a warning sign of its toxicity. For many years, it was thought that the viceroy (Limenitis archippus), which shares many of the same predators as the unpalatable monarch, mimicked the coloration of the latter to gain protection against predators.

This notion was reinforced in 1958, with the publication of a study in the journal Evolution that had been conducted by zoologist Jane Van Zandt Brower, who tested the palatability of monarchs and viceroys in a predator, the Florida scrub jay (Aphelocoma coerulescens). Caged birds that were fed monarchs discovered the butterfly's unappetizing quality and quickly learned to avoid it, and when these birds were later fed monarchs and the similar-looking viceroys, they avoided both butterflies. In contrast, birds that were fed only viceroys ate the viceroys.

But there was another finding that emerged from the 1958 study—the viceroy, though more palatable than the monarch, was still less palatable compared with non-mimetic butterflies. And in fact, as caterpillars, viceroys feed on the leaves of willows and poplars, species that produce noxious chemicals to deter herbivores. Among the chemicals these trees produce is salicylic acid—the same, bitter-tasting compound from which the active ingredient in aspirin was derived.

But the actual palatability (or the lack thereof) of the viceroy had never really been tested directly, until Ritland and Brower's study. They decided to compare the palatability of the viceroy and monarch by feeding birds only the insects' wingless abdomens, which prevented the birds from determining palatability based on the butterflies’ coloration. The researchers found that neither butterfly appealed to the avian palate.

Ritland and Brower’s thinking about the relationship between the viceroy and the monarch was revolutionary, and their work gained support from subsequent research on the toxic compounds stored in the bodies of the monarch and viceroy. In fact, recent studies have revealed that when stressed the viceroy releases volatile phenolic glycosides, which deter predator attack.

Still, many questions remain concerning the mimetic relationship between the viceroy and monarch. An area of particular interest is predator immunity. Indeed, certain predators of the monarch, such as the black-eared mouse (Peromyscus melanotis), appear to be unaffected by the insect's glycosides.
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Kara Rogers is the senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is also a member of the National Association of Science Writers and a contributor to the Britannica Blog, where she runs a series called Science Up Front. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.

This post also appears on the Britannica Blog.