Vital Statistics and Measuring
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 (1998) 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 and send us your results! A sample datasheet is provided at the bottom of the page, and here's a step-by-step how to!
How to measure monarchs
Sex: Is the monarch male or female?
Male and female monarchs can be distinguished easily. Males have a black spot (indicated by red arrow) on a vein on each hind wing that is not present on the female. The ends of the abdomens are also shaped differently in males and females, and females often look darker than males and have wider veins on their wings.
Wing length is interesting because it doesn't change from the time that the butterfly emerges, and is thus determined by the size of the larva when it pupated. So it actually gives us information about the larval stage: Did the monarch get enough to eat when it was a larva?
We measure forewing length from where it attaches to the thorax to the tip, or apex, of the wing. If you have them, calipers are the most accurate way to do this, but it is fine to use a small clear ruler that measures in millimeters. Average monarch forewings are about 50 mm long.
While the mass of a newly-emerged adult is determined by its life as a larva or pupa, the mass of older butterflies can change over the course of a day, as they do things like fly, eat, and mate. Mass will also change over the course of the adult life as butterflies use up the lipid reserves built up as larvae. Thus the mass of a butterfly, unlike its winglength, can provide information about what has happened to it as an adult.
To weigh live monarchs, you will need a balance that weighs things to the nearest 0.01 gram, or preferably, 0.001 gram. We use glassine envelopes, available from biological supply companies, to hold the butterflies as we weigh them. It is also possible to use a folded piece of paper as an envelope. Weigh (tare) the empty envelope, then place the butterfly inside and weigh them both together. If you don't have a balance that automatically does this, you will need to subtract the mass of the envelope from the total mass to get the mass of the butterfly. Adult monarchs weigh, on average, about 500 mg, or 0.5 g.
If you don't have a balance available, you can estimate how fat the butterfly is. We use a scale based on the appearance of the abdomen. The abdomens of normal butterflies look convex when you look at them from the bottom (see butterfly on left below). Thin butterflies look more concave.
All Lepidoptera lose scales throughout their lives, and if you touch the wings of many butterflies or moths, you will be able to see a fine patch of these scales on your fingers. Even though monarchs are tougher than many other species, and don't lose many scales when you touch them, they do lose scales as they fly, attempt to mate, and brush against plants. It is thus possible to get a roughestimate of a monarch's age by looking at how many scales it has lost.
We look carefully at the inside of the butterfly's wings, and assess how bright they look, and whether scales are missing. We then assign the butterfly a wing condition score from 1 to 5.
|Condition 1||a newly emerged butterfly, with wings in perfect condition|
|Condition 2||in very good condition, with very few scales lost|
|Condition 3||a few patches of missing scales, wings are slightly dull|
|Condition 4||large patches of missing scales, wings look quite dull compared to a new monarch|
|Condition 5||more than a third of the scales missing, wings look very dull and even transparent in spots|
In some cases, monarchs may lose pieces of their wings when birds try to eat them, when they get caught in something, or when they bump into something. Since these pieces tend to be removed all at once, rather than wearing off gradually like scales, we measure wing damage separately from wing wear. We hold the butterfly with all of its wings spread out, and look at the outer margins of each wing. We then assign it a wing damage score from 0 to 4.
|Damage 0||Excellent condition, no damage|
|Damage 1||Little damage that causes little immediate reduction in flying efficiency|
|Damage 2||Moderate damage, significant enough to likely cause some reduction in flying efficiency|
|Damage 3||Significant damage causing labored flight|
|Damage 4||Major damage, flight extremely labored|
Damage scores adapted from: McCord, J. W., and Andre K. Davis. 2012. Characteristics of monarch butterflies (Danaus plexippus) that stopover at a site in coastal South Carolina during fall migration. Journal of Research on the Lepidoptera. 45: 1-8.
In a long-term study of the incidence of a protozoan parasite (Ophryocystis elektroscirrha) that infects monarch butterflies, we have found very interesting patterns in the degree to which different monarch populations are infected (Parasite Study).
We assess whether monarchs are infected with this disease in a way that is relatively easy and inexpensive, and most importantly, does not harm the monarch. To do this, first touch a 1 cm x 1 cm square of CLEAR scotch tape to the butterfly’s abdomen, removing a small patch of scales. It is easiest to this holding the tape with a tweezers. Place the tape on either a clear glass slide (for viewing under a compound microscope) or a white index card (for viewing under a dissecting microscope). Be sure to write on the slide or the card which butterfly the scales came from. View the tape under 12 to 40 X magnification. If the butterfly is infected with this parasite, you will see small football-shaped spots among the much larger butterfly scales. These are spores of the parasite. Count (or approximate) the number of spores on the tape, and assign the butterfly a score:
|Score||# of spores seen|
|Zero||no spores seen|
|5||over 1000 spores|
Very few (less than 5%) of butterflies in the eastern migratory population are infected with this parasite. More (about 60%) are infected in the western migratory population, and almost all of the monarchs in southern Florida are infected (except in the fall, when migratory butterflies join the population). Much of this information has come from citizen scientists involved in Project Monarch Health. Monarch Health is a citizen science project that tracks OE in monarch butterflies and informs scientists about how animal migrations effect infectious disease.
We record if the butterfly was flying in a straight line (and if so, in what direction), nectaring, laying eggs, mating, roosting in a tree with other butterflies, flying in a nondirectional manner, or chasing other butterflies just before we caught it. Collecting this information in different places and at different times can tell us a great deal about the yearly migratory cycle of monarchs.
- Vital statistics datasheet - Sample datasheet that can be used for collecting measurement data on monarchs or other butterflies.