Male vs. Female Monarch Behavior
Based on their need to reproduce, how do you think the
behavior of male and female monarchs might be different?
Thanks to Dr. Karen Oberhauser for help in summarizing some of the differences
listed here. Karen has studied the reproductive behavior of monarchs since
1985.
Female Behavior
Finding a male isn't really an issue for females, since there are so many
males around looking for females! After a female mates once, she can lay
fertile eggs for the rest of her life. A single copulation can lead to
hundreds of offspring. (Dr. Oberhauser's record in a captive female is
1,207 eggs!) The sperm are stored in an organ called the spermatheca,
where they remain viable for at least several weeks.
However, most females do mate more than once. Even though they get enough
sperm from one male, they do get nutrients from the spermatophore that
they can use in egg production. When females mate multiple times, most
(but not all) of the eggs they lay are fertilized by sperm from the LAST
male with which they mated.
Because females lay hundreds eggs, they must find hundreds of milkweed
plants. This is because they usually lay only one egg on each plant. (Females
actually visit even more milkweed plants than they lay eggs upon. They
seem to reject most milkweed plants they see. Females will fly around
in a field full of milkweed and land on several plants before laying an
egg.)
Male Behavior
A male must find and mate with many females in order to increase his chances
of producing offspring. (As mentioned above, the LAST male to mate is
most likely the one to fertilize the eggs.) Thus, a male's "reproductive
success" is maximized by mating many times.
Because males don't lay eggs, they don't need to travel in constant pursuit
of milkweed plants they way females do. However, because milkweed habitat
is the best place to find females, this may explain the following behavior
Karen Oberhauser and others have observed. In her Mark, Release, Recapture
studies, she notes that males tend to patrol an area, and remain in one
place longer than females do:
"We rarely recapture females in the same locations in which we release
them. However, we recapture about 15-20% of the males if we go back to
the same area one to four days later. This suggests that females are leaving
the areas, while many males remain longer in the same location. Males
tend to patrol an area--leading to their being recaptured more frequently
in the same locale. In contrast, female flight appears to be more directional.
Females are captured less frequently because they do not remain in the
same area as long. Rather, they cover more area in their lifetime, probably
in pursuit of milkweed."
Try This!
If you had to find some 500 milkweed plants, as a female monarch does
in order to lay one egg on each, how far would you have to travel? The
next time you're on a walk, count the number of milkweed plants you see.
Then imagine how much farther you'd have to walk to find 500 plants!
National Science Education Standards
Science
as Inquiry
Scientists use different kinds of investigations depending on the questions
they are trying to answer. (K-4)
Different kinds of questions suggest different kinds of scientific investigations. Some involve observing and describing objects, organisms, or events. (5-8)
Life Science
Reproduction is a characteristic of all living systems; because no individual
organism lives forever, reproduction is essential to the continuation
of every species. Some organisms reproduce asexually. Other organisms
reproduce sexually. (5-8)
In many species, including humans, females produce eggs and males produce sperm. (5-8)
Behavior is one kind of response an organism can make to an internal or environmental stimulus. Behavioral response is determined in part by heredity and in part from experience. (5-8)
Biological evolution accounts for the diversity of species through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment. (5-8)
National Math Standards
Number
and Operations
Understand numbers, ways of representing numbers, relationships among
numbers, and number systems.
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