Sunday, August 15, 2010

Monarch Butterfly

Yesterday I finally caught up with a Monarch Butterfly.  Reading about Monarchs in Wikipedia, I was surprised by two statements.

I thought that Monarchs make their marvelous migrations in a single flight. "But no single individual makes the entire round trip. Female monarchs deposit eggs for the next generation during these migrations...The length of these journeys exceeds the normal lifespan of most monarchs, which is less than two months for butterflies born in early summer. The last generation of the summer enters into a non-reproductive phase known as diapause and may live seven months or more. During diapause, butterflies fly to one of many overwintering sites...It is thought that the overwinter population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. It is the second, third and fourth generations that return to their northern locations in the United States and Canada in the spring. How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research; the flight patterns appear to be inherited, based on a combination of the position of the sun in the sky."

Next Wikipedia cited the similar-looking Viceroy Butterfly as an example of Mullerian mimicry, which is when two poisonous creatures look similar, thereby not taxing predators' learning curves.  I always thought that the mimicry here was Batesian, wherein a palatable species (the Viceroy) masquerades as the poisonous one (the Monarch).  The Viceroy is less common than the Monarch, which is essential for Batesian mimicry to work.  But Wikipedia claims that both species store heart-stopping cardenolide aglycones in their bodies, ingested when the caterpillars feed on milkweed.

(A third type of mimicry, Aggressive, occurs when predators appear to be "safe" species.  Zone-tailed Hawks have a flight silhouette similar to Turkey Vultures.  The hawk soars with the vultures, and surprise their unsuspecting prey when the hawk swoops from the vulture flock.)

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