The monarch butterfly, with its striking orange and black coloration, is familiar to even the most casual observer of the natural world. In the past few weeks, I have noticed quite a few of them and the very similar viceroy around the Rock River valley, but in the fall the monarchs will congregate in large number to start their annual migration.
The monarch is unique in that it is the only North American species of butterfly to make an annual migration to the south in the fall and a northward migration in the spring. Of course, many types of birds also make these trips.
For many years it was well known that the monarchs in the western part of the country chose that beautiful area of California known as the Monterey Peninsula to spend the winter, but where those monarchs living east of the Mississippi spent the winter was a mystery.
Dr. F.A. Urquhart, a Canadian entomologist, spent more than 40 years trying to solve this enigma by tracking the elusive eastern monarchs, until he finally found the secret revealed in the Sierra Madre Mountains in Mexico. He tagged eastern monarchs in Canada and the northern U.S. and found another type of treasure when he recovered them in the Sierra Madre.
During the monarchs winter vacation in Mexico, the eggs of the semi-hibernating females develop and are ready for fertilization in the spring. After mating, the butterflies head north. It is believed that most, if not all, of the males die on the way north. After depositing her eggs in America north of the Rio Grande or Gulf of Mexico, the female then dies, but the species is perpetuated. Those monarchs that head south again the next autumn are not the same ones that made the trip previously. The instinct to migrate is deeply embedded in the gene pool of the monarch.
I know an individual who is an assignment photographer for National Geographic magazine, and some years ago he was given the task of going to the Sierra Madre and photographing the monarchs during their winter sojourn. He found their numbers to be almost unbelievable. He observed trees and bushes for miles were literally covered with the insects, sometimes hanging 12-20 onto each other when no space remained.
As it is with most other species, only those famous or notorious can be expected to be mimicked. The monarch is in this category. The caterpillars, or larvae, feed extensively on plants of the milkweed family that contain bitter, poisonous substances (cardiac glycosides, similar to digitalis). A special enzyme in the insect detoxifies the milkweed juice. However, these materials are stored in the tissues and passed on to the adult butterfly when metamorphosis occurs, and though the monarch is immune to their effect, other animals are not. Consequently, a bird or other predator who selects a monarch for supper has a very unpleasant experience as the toxins severely irritate its mouth. Having been burned once, the bird is not apt to select a monarch as its next meal, giving monarchs a high degree of safety from predation
The appropriately named viceroy butterfly does not feed on milkweed and has no noxious substances in its body. However, the viceroy (the mimic) is so similar to the monarch (the model) in appearance few if any birds will risk a hot mouth, and the viceroy enjoys almost the same degree of protection as the monarch. Some years ago, Dr. Jane Brower of Oxford University performed definitive experiments that conclusively showed mimics that closely resembled models enjoyed almost the same degree of protection from predators. This form of co-evolution is called Batseian mimicry and is a very interesting aspect of evolutionary biology. It occurs in various groups of the Animal Kingdom, including reptiles and amphibians. In reptiles, the classical example of Batesian mimicry is the coevolution of the coral snake and the scarlet kingsnake. The coral snake is a colorfully banded, venomous species (the venom is similar to that of the cobra), and the scarlet or false coral kingsnake is not venomous but has very similar colored banding. Though the differences are subtle, it requires a very intelligent animal to determine the differences. The tip of the head of a coral snake is always black, whereas the same area in the king snake is red or pink. There is also a difference in the banding sequence. In the coral snake, the black bands are separated from the red by yellow, and in the scarlet king snake, the red bands are separated from the yellow by black. If this is confusing, just remember the little ditty, Red on yellow can kill a fellow. Fortunately for us, the coral snake occurs only in the Deep South.
Dr. Robert Hedeen is a former resident of Marylands eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.