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Increased Mating Signals End of Wintering Season

By late February, mating attempts and successes have increased dramatically. In the next month, a relatively short time, these butterflies will vacate their winter haunt and begin the re-colonization of North America. An ambitious project, to be certain.

In the last week of February, for example, 1-5 mating pairs might be seen at one time in the trails below the colonies. Leading up to the monarchs' departure, we'll see three times as many mating pairs.

Copyright Dr. Lincoln Brower, Sweet Briar College

Because their lives will soon end, both males and females must attempt to pass their genes to the next generation. Their behavior suggests different strategies for males and females, and a form of deception is occurring during these early matings.

Mating proceeds like this: A male patrolling the canopy grasps the wings of a female, using the claws on his feet as he encounters her. The pair parachutes down to the ground. If he manages to attach to her (about 10% of the time), she folds her wings and he then flies off with her, if he can, to a safe perch where mating is completed. This is called the nuptial flight. The pair will stay coupled for many hours, including overnight.

Related Link
Dr. Dennis Frey and colleagues study nuptial flight
.

When mating, males deliver sperm to females in a package called a spermatophore. The females will probably not use the sperm from early matings to fertilize their eggs. Instead they may use the spermatophore for nutrition. One theory holds that during these early matings, females are collecting nutrients to make egg-yolk. The spermatophores delivered by males to females contain nitrogen--an important ingredient in egg yolk--which cannot be obtained from nectar or from the female's fat reserves.

So, despite the male's effort to reproduce, these early matings probably do not produce offspring for the male. Instead they serve to increase the female's fitness.


National Science Education Standards

Life Science
Each plant or animal has different structures that serve different functions in growth, survival, reproduction. (K-4)

Plants and animals have life cycles that include being born, developing into adults, reproducing, and eventually dying. The details of this life cycle are different for different organisms. (K-4)

In many species, including humans, females produce eggs and males produce sperm. An egg and sperm unite to begin development of a new individual. (5-8)

Every organism requires a set of instructions for specifying its traits. Heredity is the passage of these instructions from one generation to another. (5-8)

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