Monarch Butterfly Migration A Miracle

By Scott Shalaway
WHEN THE SUBJECT of migration comes up, most people imagine birds winging their way north or south.

But Monarch Butterflies make an equally impressive journey to the mountains of central Mexico each year.

This is shaping up to be a great Monarch season–I’ve seen more of these familiar butterflies this summer than I have in years, and I’ve gotten similar reports from many readers.

Western Monarchs winter along the southern California coast, while those east of the Rockies migrate to the Gulf Coast or central Mexico. Mark and recapture studies have shown that Monarchs travel as far as 1,800 miles in just four months. They move only by day at a leisurely pace of 5 to 18 mph.

Much of what we know about Monarch migration comes from a University of Kansas citizen science program called Monarch Watch. Participants can purchase self-adhesive tags to place of the hind wings of captured Monarchs. During the winter, in Mexico help biologists understand the complexities of Monarch migration.

Not only do Monarchs travel great distances, they do so with unerring accuracy. Year after year they return to the same winter areas, even the same trees. So reliable are these migratory aggregations that they have become major tourist attractions in Mexico and southern California.

What makes the Monarch migration even more amazing is that each butterfly makes the trip only once. Monarchs that leave the wintering grounds lay eggs on their way north, then die. Consequently, several generations of Monarchs separate those that return in the spring from those that head south in the fall. Yet somehow each fall, inexperienced Monarchs return to their ancestors’ traditional wintering areas. Some researchers suggest that genetic olfactory cues could provide the guidance system.

On the winter grounds, Monarchs are sluggish and inactive. They congregate on tree trunks by the tens of millions. During the winter, Monarchs consume very little of their fat reserves.

When February rolls around, they still have plenty of stored energy for the trip north. Mating occurs before migration begins and females lay eggs as they move northward. This insures a new generation of Monarchs into northern Mexico and south Texas. Subsequent generations lay eggs as they work their way northward until monarchs return as far north as Canada.

Monarchs occur throughout temperate North America. Their life cycle is a textbook example of complete insect metamorphosis. Females lay clusters of pinhead-sized eggs on the undersides of leaves of milkweed plants. In three to 12 days the eggs hatch.

Tiny, ravenous caterpillars emerge. Bold black, white, and yellow rings encircle the body and identify the fleshy larval stage. Within 14 days they devour enough milkweed leaves to weigh more than 2,000 times their hatching weight.

Each caterpillar then finds a protected perch and molts into a cocoon-like pupal case called a chrysalis. In the next two to three weeks, the contents of the pale-green, gold-flecked chrysalis transforms from a lowly caterpillar into the beautiful burnt-orange and black adult butterfly. This entire process can be observed by placing caterpillars and a supply of fresh milkweed leaves in a large jar or terrarium.

Beautiful as they may be, however, most Monarch larva and adults taste terrible to anything that eats them. In laboratory experiments, blue jays vomit within minutes of eating a Monarch because monarchs are what they eat.

Many species of milkweed are highly toxic. Though Monarchs are unaffected by the toxin, they incorporate the poison into their own body tissues. The poison is retained through metamorphosis so even adults have high concentrations of the foul tasting chemicals.

Curiously, the toxin is more highly concentrated in the wings and exoskeleton than in the body. Thus, a predator that nips even a piece of wing discovers that Monarchs are distasteful. As an added protection, a Monarch’s abdomen — and that of many other distasteful species — is tough and leathery, difficult for a predator to bite.

These adaptations enable predators to learn that Monarchs taste badly without necessarily killing the butterfly. This also explains why we often see Monarchs with badly battered wings — battle scars from the ongoing struggle between predator and prey. –Pittsburgh Post Gazette