The Aurelian was bumping around on the back seat of a minibus when the conversation turned to the topic of migration. Yes, many butterflies migrate, that much is in escapable, but why do they bother?
Considering the well-known examples, like the multi-generational migration of the Monarch in the USA, there seems a reasonable case for finding fresh nutritional larval foodstuffs, returning to warmer climes to see out the winter. There also seems to be a culling effect, where those weaker individuals, or those affected by parasites, fail to contribute to a complete cycle, and are thus eliminated from further reproductive activities.
The Australian Bogong moth is known to migrate to higher altitudes to avoid the heat of the summer, and many Indian species migrate from east to west, and back again, to coincide with the appearance of host plants and monsoonal weather.
The important point in these examples is that there is a return migration. The insect, or its progeny, comes back again to complete the circle, and be part of the restart the following season.
But consider the Painted Lady, probably the most migratory of European species. It is the only butterfly found in Iceland and Spitsbergen, and the journey from its overwintering ground in Africa probably takes multiple generations. All well and good, but then what? The fourth or fifth generation makes it over six thousand kilometres from where the great-great-grandmother set off, flies for a few days, then as the weather deteriorates, dies. There is no genetic or evolutionary benefit to this particular species to have spawned such adventurous progeny, to have them die uselessly at the conclusion of this magnificent odyssey.
At a stretch, one could perhaps suggest that a migratory species is better equipped to take advantage of a changing climate, being able to establish itself in fresh environments as they become suitable faster than more sedentary insects. But on balance, it would seem that in evolutionary terms, migration of this kind is literally a dead end strategy.
In the back of the minibus, The Aurelian persisted with his argument – that this made no sense at all, and evolutionary pressures would select for butterflies that stayed at home, rather than those that wasted their precious energy reserves on futile travel.
So, it was no surprise and something of a vindication when, in 2010, it was observed for the first time that there was indeed a partial high-altitude return migration of Vanessa cardui to Africa from temperate Europe. The return of butterflies that would parent the next generation of migrants.
Of course there was a return migration. Logic and science said there had to be. But given that this migration was taking place at what were considered to be absurd heights for butterflies, well over ten thousand feet, the return had never been previously observed.
Now we know better.