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  • Dr. Brower With His Monarch for Symbolic Migration

    How Far North do Monarchs from Mexico Travel?
    Dr. Lincoln Brower's Recolonization Theory

    Dr. Lincoln Brower, a renown monarch butterfly biologist from the Univeristy of Florida, theorizes that it takes two monarchs generations to repopulate eastern North America.

    He demonstrated this in a very clever way, based on these key facts:

    1. Different species of milkweed occur in southern and northern parts of North America.
    2. Unique chemicals called "cardenolides" are found in each unique milkweed species.
    3. Monarch caterpillars eat only milkweed.
    4. The adult monarch's bodies contain the unique cardenolides from the milkweed species it fed upon as a caterpillar.

    Dr. Brower collected butterflies from different geographic regions and took their "fingerprints". He did this by measuring the cardenolides in each monarch. He found that they did indeed contain chemicals from the exact geographical regions he suspected they would:

    USFWS
    • Monarchs that over-wintered in the Mexican colonies contained chemicals from milkweed species of the northern U.S. states. (That is, the monarchs that migrated to Mexico fed upon milkweed in the north during the previous summer.)
    • Similarly monarchs that returned to the northern U.S. States & Canada the following spring contained chemicals found in milkweed species of the southern U.S. states! (These northern butterflies would be the offspring of the butterflies that wintered in Mexico. They flew to southern states, ate southern milkweed species, developed into adults and flew to the north.)

    The results of his research suggest that the over-wintering generation travels to the U.S. Gulf states, reproduces, and dies. It is their offspring that continue the migration to points further north.