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Monarch Vital Statistics

Background | How to Measure Monarchs | Data Sheet | Sample Monarch Vital Statistics | Research Projects

If you observe adult monarchs in the wild, you will notice many differences between them. Some are larger than others, and some look very tattered and worn while others look fresh and new. In addition to differences in physical appearance, you may notice differences in behavior. Some monarchs fly quickly in one direction, while others seem to be flying in a less directed manner. Some chase other butterflies, some spend time drinking nectar from flowers, and others select milkweed plants on which to lay eggs.

A great deal can be learned by comparing monarch appearance and behavior in different places and at different times. We know that monarch wings become more tattered and worn with age; thus we can compare the relative ages of monarchs. If we see many monarchs, but none are laying eggs, we can guess that either all of them are males, all are too young to lay eggs, or all are in reproductive diapause. We can use observations such as these to learn more about aspects of monarch biology, such as migration and reproductive behavior. The studies described below used physcial appearance to understand monarch migratory patterns and mating behavior in the overwintering colonies.

Cockrell et al. (1993) compared wing conditions of the first monarchs to arrive in the spring in locations throughout the eastern US. Almost all of the butterflies they saw in southern states (farther south than Missouri, or 36° latitude) had very worn wings, suggesting that they were old. On the basis of this observation, and other evidence on the timing of the appearance of the monarchs, they concluded that these monarchs were part of the overwintering generation. On the other hand, almost all of the first butterflies observed north of 36° latitude had wings in good condition, suggesting that they were young. Cockrell et al. concluded that these monarchs were the offspring of the overwintering generation: the first new generation of the year.

Oberhauser and Frey (1999) and Van Hook (1993) compared the condition of males that were mating in the overwintering colonies to those that were roosting in trees. They found that mating males weighed less, had poorer wing condition and more wing damage, had smaller wings, and were more likely to be infected with a protozoan disease than roosting males. They concluded that these mating males were in such poor condition that they would be unlikely to survive the spring migration, and were thus mating early in order to have some chance to pass their genes on to the next generation.

It is interesting and useful to keep track of the physical characteristics and behavior of monarchs that you observe. At the University of Minnesota, we study the monarch breeding population throughout the summer, migrating monarchs in the spring and fall, and monarchs in the overwintering colonies in Mexico and California. We collect several measurements on these monarchs, and record what they were doing when we captured them.

If you would like to collect data on monarch butterflies in your area, please do so! You could record the physical appearance of the monarchs, their mass and wing length, their behavior, and whether they are parastized. To learn how to do this, check out our directions on How to Measure Monarchs. If you send your data to us, we will publish it on this site. Be sure to record the date that you caught the monarchs. A sample data sheet for you to copy follows the directions for measuring.

References

Cockrell, BJ., SB Malcolm and LP Brower. 1993. Time, temperature and latitudinal constraints on the annual recolonization of eastern North America by the Monarch Butterfly. In S. B. Malcolm and M. P. Zalucki (eds.), Biology and Conservation of the Monarch Butterfly. Natural History Museum of Los Angeles County, Los Angeles.

Oberhauser, K.S. and D. Frey. 1999. Coerced mating in monarch butterflies. In I. Pisanty, K. Oberhauser, L. Merino and S. Price (eds). Proceedings of the North American conference on the Monarch Butterfly.

Van Hook, T. 1993. Non-random mating in monarch butterflies overwintering in Mexico, pp. 49-60. In S. B. Malcolm and M. P. Zalucki (eds.), Biology and Conservation of the Monarch Butterfly. Natural History Museum of Los Angeles County, Los Angeles.