Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [90]
Every fall I now eagerly and admiringly watch monarchs, our most conspicuous insect migrant. Day after day in October, the handsome orange-and-black-striped butterflies flap and sail lazily over the sunny fields, the woods, and water, all flying individually yet all heading in a southerly direction. In the evening they stop and gather on the purple New England asters to sip nectar, and in the morning they bask and shiver, rise into the air, and resume their journeys.
Individual butterflies tagged in Canada in the fall have been recovered thousands of miles south. By winter most of the eastern population has settled into their winter mountain retreat near Mexico City where their great-great-grandparents had been before them. It is a destination they seek out with incredible energy expense, never having been there, nor knowing where they are going. By the tons, in a spectacular shimmering orange display, they festoon the trees.
The million-dollar question is: Why do monarchs bother? Why don’t they all hibernate and stay north, as do most butterflies? Like most questions that relate to history, this one does not have a simple answer: because history, especially evolutionary history, is never just one thing acting in isolation of everything else.
The monarch butterfly is a member of the family called the Daneidae, a tropical group. Monarch’s relatives live in the tropical lowlands of New Guinea, and they are prominent in the American tropics as well. The ancestors of the present-day monarch butterfly, Danaus plexippus, were presumably also adapted to a tropical climate. Like most butterflies, they are predisposed to disperse when the larval food runs out. Not being able to survive freezing temperatures, there was then strong selective pressure for dispersion in a specific rather than a random direction; many dispersed in the wrong direction but all of those individuals left no offspring to pass on their trait. Only those who lucked out by flying southward survived; thus an evolutionary direction and destination was born and then grew.
At the same time they were evolving directional dispersal, monarchs had to surmount another problem. On their annual southern treks they had to cross into the hot, arid environments of the Mexican deserts. Here there was little chance for these strong fliers to refuel, and facing a long interlude until feeding was again possible, they must have experienced intense selective pressure to conserve energy. Partly by chance, some individuals probably ended up in the mountains, perhaps blown there by tropical updrafts. I once saw aggregations of insects on Mount Meru in Tanzania, and all of them were torpid. The cool mountain air had reduced the insects’ rate of energy expenditure to such an ebb that they could not even fly. They were now in cold storage and along with that went a reprieve from needing to feed, until they were again warmed.
It seems probable that, given an insect’s high reproductive rate, those few monarchs that took a flight path that somehow landed them in cold storage for the duration when no food was available would have a huge selective advantage over those that would have exhausted their energy supplies by staying in the heat in an environment with little food. By sifting for survivors, evolution selected where the population would overwinter. Generalities such as these should, given the right circumstances, also apply to other animals, including Lugong moths