Migratory and Overwintering Monarchs | Breeding Monarchs
| Monarch Size & Condition
In 1997, we measured many monarchs throughout the course of the summer in the eastern
(collected in WI and MN) migratory population. These samples were different than
those from the migration and the overwintering period because they included monarchs
from several different generations.
Does the size of different summer breeding generations of monarchs vary?
We captured monarchs weekly. Most of our sampling efforts took place at our
regular field site in west-central Wisconsin, although a small proportion
of the data reported here were collected at a farm about 20 miles east of the site
in east-central Minnesota. All butterflies were released after we measured them.
Figure 4 summarizes winglength data from this study. Monarchs
increased in size throughout the course of the summer. Those we measured in June
were, on average, smallest, and those we measured in September were largest. In
addition, males were larger than females. We did a statistical analysis called a
multiple linear regression on these data. Both the collection month (p < 0.04)
and sex (p < 0.01) had significant effects on winglength.
We have not studied factors that might be responsible for changes in winglength
over time in the eastern population. Since winglength is determined before the adult
emerges from the pupa, these factors must be acting during the larval stage. It
may be that temperature changes (see study by
Liz Larkin), or changes in the hostplant are responsible for the
differences we observed. It is also possible that a combination of several factors
is responsible for these differences. We do know that larvae fed leaves from old
Asclepias syriaca plants are larger, on average, than those fed young A. syriaca
(see study by Liz Goehring). The milkweed species that the
larvae eat could also be important. The monarchs we measured in WI and MN in June
probably migrated north from southern or central states, where they probably fed
on other milkweed species (generation
1 and 2 in
Yearly Life Cycle).
Discussion of winglength studies
Continue to Sex Ratios