Question

6. Monarch butterflies migrate thousands of miles every fall from Canada and the United States down to Mexico. There they go into a state of diapause for the winter months. Diapause is a period of dormancy in which no growth occurs. When monarchs emerge from this state in the spring, they head north to southern parts of the United States, where they breed. After producing offspring, this generation of butterflies dies. Over the spring and summer, several more generations come and go before the final generation that must make the journey south to Mexico in the fall. In this way, some monarchs never make the migration journey, and those that do must make it without any guidance from other monarchs.

Explain how both environmental and genetic factors must contribute to the timing and directional movement of monarchs in their migration behavior.

Answer 1

In monarch butterflies environmental factors and genetics plays an important role monarch butterflies migrate to warmer climates to escape from the upcoming cold weather and the food shortage that will result from the temperature fall. But how do the butterflies know whether it's cold or hot. What the monarch butterflies sense is the changing amount of light present and the variability of day and nighttime temperatures. With the change of seasons from fall to winter comes the inevitable shortening of the days, longer nights, and also colder night time temperatures. Once these characteristics show up, the monarchs leave for their overwintering sites.
Why do the monarchs follow such a particular  thing is based on three principle factors: (1) monarch larvae feed exclusively on species of milkweed; (2) the migratory pattern is from northeast to southwest; and (3) there is a long history, extending over eons of time, of the distribution of the milkweed species.

The conclusion is that, " the northwesterly-southwesterly migrations are correlated with the changing distribution of the species Asclepias resulting from changes in the North American land mass over millions of years." (Urquhart 1987) In plain terms, the migrational pattern presently observed originated in the distant past when the monarchs were following the milkweed species which were spreading westward. This east-west movement was eventually incorporated into the monarch's genetic code to produce a cyclical migration related to some as yet unknown response to seasonal changes on the planet.

Monarchs have no control over what happens to their environment, they can only respond to what changes occur, which usually means either surviving or dying. Humans are the ones who have the most control over what will happen to the monarch butterfly population and the biggest problem that the monarchs face is the loss of habitat.

Monarch populations are particularly vulnerable in their overwintering sites in the high-altitude fir forests of the Transvolcanic Range of Mexico; only two of the eleven known roosting sites are well protected from logging (Brower and Malcolm 1991). The oyamel trees on which the monarchs cluster are valuable lumber sources, and local people need additional sources of income (Snook1993). If the roost sites are destroyed, monarch populations are likely to decline precipitously. Protection of the roost sites will be difficult since preservation of the sites and the monarch butterfly, will conflict with the increasing needs and changing priorities of a growing Mexican population.

Milkweed, the host plant of the monarch, is also a concern. In Canada, milkweed has been declared a noxious weed. This means that the plant is considered illegal and cannot be allowed to grow on private or public lands in Canada. Although not labeled noxious in the states, farmers consider the plant a nuisance to crops and often use herbicides to control it along with other weeds. More and more roadsides are being planted in grass instead of being allowed to overgrow with wildflowers and weeds. The result is that butterflies have fewer places in the wild to find nectar and lay eggs.

And when we come to genetics that influence the migration of the butterflies scientists claim that 40 genes are responsible for this Scientists studying Eastern North American monarch butterflies (Danaus plexippus) have uncovered a suite of genes that may be involved in driving the butterflies to migrate towards Mexico for the winter. Their research describes 40 genes that are linked to the butterflies' compulsion to orientate themselves by an internal 'sun compass' and begin the 4000km journey southwards.

The references are made from www.sciendaily.com

www.ncbi.com