Migration: Unraveling Nature’s Blueprint for Mouvement
Migration is one of the most extraordinary feats in the animal kingdom and is a crucial adaptation for survival undertaken by birds and insects alike. However, the mechanisms that drive different animals to migration are far from being the same. To test this, researchers have compared the Eurasian blackcap’s migration with that of the North American monarch.
In non-migratory conditions, a bird’s day consists of a 24 hour long activity period, during which they get short bouts of sleep, only a few minutes at a time. Research from the Max Planck institute for ornithology has shown that lower levels of light do not affect the length of this, meaning that this is regulated internally: by the Circadian Clock. When the time comes for the bird to migrate however, the clock slows down, the activity lengthening to last 27 to 28 hours; their clock is running slower. They also experience two distinct activity periods instead of one: during the day and at night: Zugunruhe, also known as migratory restlessness. The same researchers have also shown that there are separate circadian clocks for day and night. As the days go by, the nocturnal clock shifts day by day until it coincides with the daytime clock. When this occurs, Zugunruhe is suppressed. By the time the migratory period comes again, the nocturnal clock will have shifted back into the night.
The Circadian clock is genetically encoded as are other aspects of migratory behaviour, including migratory pathways and yearly timing. There is variation in these different aspects of migratory phenotypes even in single species. In Europe, there are 2 main populations of blackcaps: a southwesterly population migrating to Spain and Algeria in winter and a southeasterly population migrating to Jordan. Interbreeding these 2 different populations has created ‘intermediate’ birds migrating south. This proves blackcap migration is genetically encoded. Miriam Liedvogel, from the Institute of Avian Research, Germany, believes that bird migration is only coded for by a few genes, given evidence so far.
In contrast, the migration of monarch butterflies is regulated in a very different way. Unlike blackcaps, in which individuals will complete one or more migratory cycles over their lifetimes, the shorter lifespan of monarchs means that different generations will be doing different parts of the migratory cycle. It appears that epigenetic changes, triggered by environmental factors such as temperature change and photoperiod, determine which migratory behaviour each generation exhibits. Every autumn, as the days shorten, monarchs leave their breeding areas in the north and migrate to Mexico, where they spend the winter in a state of reproductive inactivity, called diapause. In spring, as temperatures rise and days grow longer, they become active, mate, and migrate north to the southern U.S. There, females lay their eggs on milkweed plants, the only food source for their larvae. Unlike birds, these monarchs do not return to their original breeding sites. Instead, it takes multiple generations of butterflies, with each generation moving further north as milkweed becomes available. By late summer, the last generation of monarchs of the year emerges in the north, and these butterflies are then "re-programmed" to migrate south for the autumn journey, restarting the cycle. This generation has a longer lifespan as a consequence.
As Liedvogel suggests, more research, in particular on bat or fish migration is needed to better understand the genetics and epigenetics of migration. Genomic studies such as whole-genome sequencing could help reveal the full range of migratory behaviours. The different lifecycles and ecologies of different species necessitates different migratory strategies and control mechanisms.
There is still so much more to say about migration.
