Monarch Viceroy Puzzle
At one time, the relationship between the Monarch butterfly (Danaus plexippus) and the Viceroy butterfly (Limenitis archippus) was considered a clear textbook example of the evolutionary strategy of Batesian mimicry.
Batesian Mimicry?
In 1862, Henry Bates, an English naturalist and explorer of the Amazon, suggested that non-toxic species might evolve over time to look like toxic species. The non-toxic species would benefit because predators would avoid it as well as the toxic species.
During the mid-twentieth century, Batesian mimicry was widely accepted as an explanation for the very close resemblance between the Monarch and the Viceroy. That was principally due to the studies of Jane Van Zandt Brower, published in Evolution in 1958.(1) The Monarch was assumed to be toxic, due to the fact that the caterpillars eat milkweed plants (Asclepiadaceae) and absorb the toxic cardiac glycosides. The Viceroy was assumed to be non-toxic. Its caterpillar feeds on willow family members.
For example, the United States Geological Survey gave this information in the summer of 2003: “The Viceroy butterfly (Limenitis archippus) is edible, but mimics the poisonous Monarch in order to gain protection from predators.”(2) Similarly, the University of Kentucky Agriculture Department’s Entomology Web page stated in the same month that “By looking like a monarch butterfly (which tastes terrible to predators and makes them sick!), the viceroy (which is a tasty prey) is avoided by predators who have had a bad experience with monarchs in the past.”(3)
However, more recent research has cast doubt on any such simple relationship between the two butterflies—leaving a large mystery in the wake. In 1991, David Ritland and Lincoln Brouwer fed only the abdomens of Monarchs, Viceroys, and Queens to red-winged blackbirds, a frequent butterfly predator. (The Queen (Danaus gilippus) is a southern relative of the Monarch, with a darker wing shade.)
Presented with abdomens only, the birds would not be able to identify butterflies by their wing patterns, and would have to rely on smell and taste alone. The birds found Monarchs and Viceroys about equally unpalatable (40% eaten), and Queens more palatable (70%). They ate 98% of the non-toxic controls.(4) Clearly, the Viceroy does not reduce its chance of being eaten by looking like the Monarch. Predators do not like either species much. Batesian mimicry was considered to be disconfirmed, although many popular sources continue to offer it as an explanation.
A Comparison of the Two Butterflies
There are detectible differences between the Monarch and the Viceroy. The Viceroy has a black band running horizontally across the hind wings, which the Monarch lacks. Also, the Viceroy flutters, whereas the Monarch, usually a much bigger butterfly, flaps and then glides. There are some slight differences on the undersides as well. (In southern regions, the Viceroy is closer in color to the Queen.)
The resemblance is not the result of a close genetic relationship. The Monarch (Danaus) and the Viceroy (Archippus) belong to different subfamilies of the order Lepidoptera (butterflies and moths).(5) Their habits are also very different. The Monarch migrates over three to five generations between summer sites in the northern United States and Canada, and overwintering sites in Mexico. The Viceroy, which is territorial, overwinters in its home environment at the first and second growth stages (instars) of the caterpillar, in a form of hibernation (diapause).
The caterpillars are not at all similar. The Viceroy caterpillar resembles a bird dropping; the Monarch is striped orange, yellow, and black, with horns on both the head and tail.
Mullerian Mimicry?
Researchers have proposed another explanation for the resemblance of the adult Monarch and Viceroy: Mullerian mimicry. In 1878, German zoologist Fritz Muller, an Amazon naturalist like Bates, suggested that a number of toxic species may adopt the same warning pattern (an aposematic pattern). The predators would learn faster if they had to learn only one signal to avoid, so, overall, fewer of all the prey species in the group (mimicry complex) would be eaten.
However, both the feeding habits and the toxicity of the Monarch are more complex than was originally supposed. While the caterpillars feed only on milkweed plants, the adults drink nectar from members of the composite flower family (Asteraceae), producing pyrrolizidine alkaloids. Also, in the overwintering sites, major predators such as birds and mice are not affected by the poisons. Some predators eat only the less toxic parts, others do not absorb the toxic substances well, or are simply immune to them. In any event, northern milkweeds are less toxic than southern ones, and older butterflies are less toxic than younger ones.(6)
Ritland, based at Erskine College in South Carolina, has shown that Viceroys in the Southeast vary little in how tasty they are to predators, but Monarchs and Queens vary between 6% and 85%.(7) It may well be asked, if the Monarch displays warning coloration when many Monarchs are not poisonous, does the predator learn to avoid—or seek out—Monarchs? The kill rate of Monarchs is very high, especially in the overwintering sites. Why mimic such an ambiguous signal?
While studying another butterfly mimicry complex, Akira Saito remarked in Forma in 2002 that “during the 100 years following discovery of mimicry, not only has little been discovered about mechanisms of mimicry, but little evidence has been found to even verify the function of mimicry. ”(8)
An Open Question
Another complicating factor is that in Canada, the Viceroy and other members of Limenitidinae, commonly called Admirals, hybridize to some extent.(9) The other members found in Canada are Red and White Admirals, and Red-spotted Purples, none of which resemble the Viceroy in wing pattern.
How can natural selection produce such similar wing patterns as those of the Monarch and Viceroy by a slow process of adaptation when the regional situation varies, the toxicity level is constantly changing—making the signal ambiguous or dangerous—and the Viceroy species does not appear to have strayed very far genetically in the process of acquiring the similarity?
– Denyse O’Leary, oleary@sympatico.ca
References
1. Brower, J.Vz. (1958) Experimental studies of mimicry in some North American butterflies. Evolution 35:32–47.
2. United States Geological Survey, http://www.npsc.nbs.gov/resource/distr/lepid/bflyusa/wa/92.htm. Accessed August 14, 2003. The information is under “Comments.”
3. The site is http://www.uky.edu/Ag/Entomology/ythfacts/mystery/mystry50.htm, accessed August 14, 2003.
4. David Ritland and Lincoln Brouwer, “The viceroy butterfly is not a batesian mimic,” Nature 350:497–498 (1991).
5. Both Danaus and Archippus belong to the Nymphalidae family, and but Danaus is a member of the Danainae subfamily and Archippus is a member of the Limenitidinae subfamily.
6. Kim A. Pike, “Antipredator Adaptations by Monarch Butterflies”, http://www.colostate.edu/Depts/Entomology/courses/en507/papers_1999/pike.htm
7. David B. Ritland, Erskine College, Department of Biology, “More about Viceroy Mimicry” http://www.erskine.edu/academics/biology/biology.mimicry.html.
8. Akira Sato, Mimicry in Butterflies: Microscopic Structure Forma, 17, 133-139, 2002. The complex studied was Hypolimnas anomala and Euploea mulciber. The female of the former species imitates the male of the latter.
9. “Family Nymphalidae,” Canadian Biodiversity Information Facility, www.cbif.gc.ca/spp_pages/butterflies/families/nymphalidae_e.php
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