Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [89]
Monarch butterfly.
To keep the spectacular migration spectacle alive requires protecting the cool forests that promote the torpor and extend the insects, energy supplies. It is a sobering thought that most of the population of eastern North America could be wiped out by an irresponsible woodcutter with a chain saw. Continual survival is only as secure as the weakest link, of the innumerable links—thousands—to the existence of any species. In the long term, however, it is not only the roosting groves that are critical. Given environmental change, such as the global warming that is now melting the glaciers all over the world at a phenomenal rate, the alternative and as yet unused potential roosting sites will become important in the future.
The larger the aggregation, the less the individual butterflies in it risk predation. However, a main reason the butterflies use specific overwintering sites is that they can maintain the low body temperatures there required to maintain energy balance while at rest for three months of virtually no feeding (Masters, Malcolm, and Brower 1988). On average the butterflies’ body fat reserves are such that upon entering their hibernation site, they should last about ninety days at 15°C (ibid.). On the other hand, if the body temperature of resting butterflies were 30°C, then their rate of resting metabolism would be high enough to exhaust their fat reserves in fewer than ten days. In addition, they would likely dehydrate.
It is also important to note that the migratory behavior—restlessness, flight direction, and maybe duration and destination—has evolved. The butterflies won’t just stay at the first cool spot they hit, because it could become hot, or too cold. They rely on the genetic, or long-term, experience of the race. Hence the importance of specific overwintering sites that have proven to be safe for their ancestors.
By February as the days get longer and the monarchs’ critical photoperiod of 11.3 hours is passed, the hibernating butterflies can again become reproductively active. There is, of course, nothing magical about 11.3 hours of daylight per day as such for reproduction, except for monarchs. Like overwintering sites, that specific time represents an evolved memory of the race. It is the photoperiod that in their evolutionary history has proven to be the best, for them to be prepared to be active. After this photoperiod is experienced, the butterflies simply wait for the next cue—temperature. Rising temperatures trigger a massive mating response in the colony (Brower et al. 1977). After mating (another cue), the females then migrate northward and eastward back into the United States and southern Canada. Along the way, they respond to the scent of milkweed, a cue for laying their green eggs on the newly emerging plants.
For a long time there was controversy on whether the monarchs colonized their entire eastern range with the first spring generation leaving the Mexican overwintering sites, or whether the northward march was achieved in steps, by successive generations. Thanks to the fact that milkweed contains cardenolides (to us nauseating chemicals that are also heart poisons), this issue has now been resolved: It requires up to four generations for the monarchs to reach their northernmost breeding grounds.
The method to generating the above answer exploited the fact that different milkweed populations contain specific chemically distinct arrays of cardenolides. The monarchs ingest these poisons as caterpillars, store them in the pupa and transfer them into the adult butterflies where they serve as a chemical defense against predators. By extracting these poisons from a butterfly, one can get a chemical “fingerprint” to match that found in the milkweed of different