Dr. Karen Oberhauser
University of Minnesota
Department of Ecology, Evolution and Behavior
University of Minnesota Ecology graduate students Sonia Altizer and Liz Goehring and I just returned from the Sierra Chincua Monarch overwintering colony near Angangueo, Michoacan. We studied several aspects of Monarch biology during the week of February 22-29, 1996. After 11 years of working with Monarch butterflies in the northern part of their range, it was incredible to finally see the overwintering colonies in Mexico; it was truly an experience that will change my perspectives on an organism I thought I understood fairly well.
The Sierra Chincua colony is only open to researchers; it is near El Rosario, a colony open to the public. About 14 separate Monarch colonies form in the transvolcanic mountains west of Mexico City each fall. Last November, when the Monarchs first returned to Mexico, there were three congregations on Sierra Chincua, in places named Camino del Japones, Mojonera Alta, and Rincon Villalobos. All of these were near the top of the mountain. In January, just after the infamous snowstorm, the butterflies moved lower, to two spots called Barranca Honda and Llana del Toro. We worked in Barranca Honda, which contains the largest concentration. The butterflies were in an area approximately 100 meters in diameter, and covered the trunks and branches of all of the large trees. The trees were mostly oyamel fir, but there were also some pines and deciduous trees.
When we arrived at the colonies early each morning, before the sunlight warmed the butterflies, there was very little movement. But when the sun bathed the roosting butterflies, many of them poured off the trees, moving down the slope of the mountain toward an open area where they could drink nectar from flowers and dew that had collected on the ground. Our Mexican companions, Eneida Monteninos, Eduardo Rendon, and Manuel Riveria, all students at the University of Mexico, told us that they often fly up to a kilometer from the central roosting area to find nectar and water. At the end of the day, these butterflies returned to their roosts. Some Monarchs stayed on the roosts throughout the day, while still others flew around in the center of the colony.
Substantial evidence supports the hypothesis that nectaring and roosting Monarchs are in very different condition. Alfonso Alonso, at the University of Florida, and Eduardo Rendon are studying these differences, and our measurements confirmed their finding that nectaring butterflies tend to be in much worse condition; their wings are tattered and they are much thinner. These butterflies are running out of the lipid reserves that they stored up as larvae and during their southward flight last fall, and may not survive the winter. Roosting butterflies, on the other hand, are in beautiful condition - fat and with pristine wings. The butterflies flying around in the colony seemed to be mostly males that were attempting to mate. Mating occurs during the last three to four weeks of the overwintering period, and our observations confirmed those made earlier by Tonya Van Hook at the University of Florida that mating males tend to be in worse condition that non-mating males. These males may be mating because they will not live much longer, and are trying to assure that they will have at least a few progeny in the next generation. Males that wait longer to mate, or even fly north with the females, will probably be more successful, since the last male to mate with a female fertilizes most of her eggs. I will compare mating behavior in the colony with that I have observed in summer populations of butterflies.
We were amazed to observe the incredible number of dead butterflies littering the forest floor. Many of these have been killed by black-backed orioles and black-headed grosbeaks. The orioles slit open the abdomen and extract the insides, avoiding the cuticle which contains the toxic cardenolides from milkweed. Grosbeaks don't need to be so careful; they can tolerate the cardenolides and simply chomp of the abdomen and head. Eneida Monteninos is studying the numbers of Monarchs killed by each kind of bird. Other dead butterflies are called DWACS (dead without apparent cause), and researchers are trying to learn more about other causes of mortality in the colonies. Manuel Riveria is studying the number of Monarchs that were likely to have been killed in the December snowstorm, and Sonia Altizer tested hundreds of dead Monarchs for the presence of a protozoan parasite that causes high mortality in captive Monarchs.
We have brought back some butterflies to our lab, where Liz Goehring is studying the environmental cues that trigger the development of eggs in females. The work will add to her previous studies of the cues that trigger the onset of migration and reproductive diapause in fall Monarchs.
This visit made us aware of the vast number of unanswered questions concerning this amazing biological phenomenon: how do the Monarchs return to these mountains every fall, what causes the differences in condition and survival probability, what physiological changes occur during the winter, how do microclimatic features of the mountains affect the butterflies, are there preferred roosting sites within the colonies, what determines where the different colonies form, .... These and other questions could keep Monarch biologists busy for several decades!
However, the continued existence of the colonies for the decades needed to understand them is not assured. While the Sierra Chincua colony held millions of butterflies, and is in a relatively intact part of the forest, the oyamel trees are also valuable to humans for their lumber. Many of the landowners (ejidatarios) do not feel that they receive enough benefit from the presence of Monarchs, researchers, and tourists on their land. Sites of other colonies are in even more jeopardy. It is very important that humans find some way to work together to preserve the livelihood and lives of the people and the butterflies. We will write more later on this issue when we receive the transcript of a meeting we attended on Monarch conservation in Angangueo.