Migratory and Overwintering Monarchs
Migratory and Overwintering Monarchs | Breeding Monarchs
| Monarch Size & Condition
By David Astin, Jayme Eggum, and Pete Jamrogiewiez
Wayzata HS in Wayzata, MN
By Jane Borland, Trey Crumpton, and Wendy Allen
Lamar HS in Arlington, TX
By Carol Johnson, Markisha Thomas, and Isaac Jemal
John Jay HS in San Antonio, TX
By Sonia Altizer, Karen Oberhauser, Michelle Prysby, Liz Goehring, and Michelle
Solensky
University of Minnesota, St Paul, MN
Introduction
We compared the size and physical condition of monarchs during the migration by
capturing monarchs in Minnesota and Texas during the migration and in the overwintering
colonies in Mexico. All of these monarchs were presumably from the same generation.
From the timing of our collections and information on how migratory monarchs were
moving through the US, we assume that our MN sample represented some of the first
monarchs to leave the upper Midwest and our TX sample represented the first wave
of monarchs moving into TX from northern states. The Mexico sample was taken after
the monarchs had been at the overwintering sites for approximately three and a half
months.
Sample Questions
- Are larger butterflies more likely to migrate successfully?
- Does migration or overwintering cause wing wear and tear? When does most damage
occur?
Methods
We captured monarchs on August 20 and 21, 1998 in Hennepin and Goodhue counties,
MN; October 6-20, 1998 in the Hill Country of South TX near Austin and San Antonio,
and February 24-27, 1998 in the Sierra Chincua sanctuary in the state of Michoacan,
Mexico. We recorded sex,
forewing length, wing condition,
and wing damage for each monarch.
We released all butterflies after measuring them.
Results
Butterfly size: Butterflies collected at different locations were not significantly
different in winglength (Figure 2a). Females tended to be smaller
than males, on average, but the only sample for which this difference was statistically
significant was in Texas (t-test, p < 0.025).
Wing Condition: Monarchs’ wings, as measured by scale loss (wing wear), were
in the best condition in Minnesota at the beginning of the migration, and the worst
condition in Mexico (figure 2b). These differences are all
statistically significant for males (p < 0.05), but not for females (p > 0.05).
The amount of wing damage (as measured by the number of wings with pieces broken
off) also increased over the course of the migration (figure 2c).
Monarchs in Texas had slightly more damaged wings than in Minnesota (this difference
is only statistically significant for males, p < 0.01), and there was much more
wing damage in Mexico for both sexes. In Mexico only, males’ wings had significantly
more damage than females’ (p < 0.001).
Sample sizes for data in Figures 1 and 2: MN males = 63, MN females = 24, TX males
= 130, TX females = 73, MX males = 405, MX females = 620.
Discussion
If larger monarchs are better able to migrate, we would expect butterflies collected
early in the migration to be smaller, on average, than those collected later. This
would happen if small butterflies were less able to make the flight, and were thus
not a part of the population we measured later. However, there were no significant
size differences between monarchs measured in different places, suggesting that
small butterflies are just as likely as large ones to make the entire migratory
flight.
Monarch wing condition did get worse during the migration, with those in Minnesota
having wings in the best condition, those in Mexico in the worst condition, and
those in Texas being intermediate. It appears that monarch wings are more likely
to be damaged by activities that take place during overwintering than during migration,
since there is a bigger change in the amount of wing damage between October and
February than between August and October. However, in order to test this hypothesis,
we would need to measure monarchs as they arrive at the overwintering colonies.
If activities that take place during overwintering are harder on monarch wings than
migration, we would expect wing damage in arriving monarchs to be more similar to
those measured in Texas than those measured in Mexico in February.
What else could we learn by comparing monarchs throughout
their migration?
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