Monarch Size & Condition: Comparisons at Different Times and Locations

Note: In order to compare monarchs measured at different times, you need to use statistical tests. It’s not enough to just compare average values. For an introduction to basic statistics, check out our Basic Stats section.

Migratory Generation  |  Overwintering Monarchs  |  Breeding Monarchs

Migratory Generation

David Astin, Jayme Eggum, and Pete Jamrogiewiez*
Jane Borland, Trey Crumpton and Wendy Allen**
Carol Johnson, Markisha Thomas and Isaac Jemal°
Sonia Altizer, Karen Oberhauser, Michelle Prysby, Liz Goehring and Michelle Solensky°°.

*Wayzata HS in Wayzata, MN
**Lamar HS in Arlington, TX
°John Jay HS in San Antonio, TX
°°University of Minnesota, St Paul MN

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 were 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? 

continue to Overwintering Monarchs