Animals seem to use migration to escape the spread of deadly pathogens...so it's bad news that many animals have stopped migrating.
Billions of animals migrate across the world every year, with some taking months to get to their destination. Along the way, animals expose themselves to potentially deadly pathogens, encountering lots of different environments and making a few key stopover points that are shared by lots of different species. These areas can become hotbeds for disease. This mirrors how diseases spread in our modern civilization, as highly concentrated population centers can see massive outbreaks, and then a handful of people traveling elsewhere can take the disease with them.
But researchers at the University of Georgia have figured out the other side of migration, and why it actually can keep pathogens in check. Many parasites have transmission stages that require the host to stay in the same habitat. Speedy migration takes the infected host outside that habitat, effectively killing off the parasite before it has a chance to take hold. As a bonus, any parasites left behind in the now host-free habitat die off, cleaning up the entire area in time for the hosts' return from their travels.
There's also survival of the fittest at work here. It's pretty brutal stuff - migration forces the infected to take part in a long, strenuous activity that will either kill them or cure them. If, say, a sick bird can't make it through the day's long flight, it will peel off from the rest of the birds and die, sparing the rest of the flock from infection when they land for the night. And, again, there's a secondary effect at work here, as the dying hosts take the most serious strains of disease with them, leaving the surviving flock with the relatively mild pathogens. The migrating animals are essentially forcing evolution to work for them by promoting less virulent strains of disease.
Researcher Sonia Altizer focused on monarch butterflies from eastern North America, which can migrate all the way from Canada to central Mexico. These butterflies migrate much farther distances than their monarch relatives elsewhere, and some populations in always pleasant areas like Florida and Hawaii don't bother with migration at all.
Altizer and her colleagues found that infectious disease is lowest in the butterflies that travel the greatest distances, while the non-migratory monarchs are the most at risk of deadly pathogens. The parasite strains that did exist in the long-travelling butterflies were consistently weaker than their counterparts, all of which supports migration as a way of weeding out disease.
But monarch butterflies are just one of many species for which migration has become an endangered activity. Human activity has destroyed a lot of potential stopover sites and put up barriers like dams and fences that can block the way. This means once migratory animals are trapped in one habitat year-round, which places them at much greater risk of disease. Climate change is also a factor - the warmer it gets, the less likely animals are to bother to migrate to escape wintry conditions.
Altizer's colleague Barbara Han sums up the danger:
"Migration is a strategy that has evolved over millions of years in response to selection pressures driven by resources, predators and lethal parasitic infections-any changes to this strategy could translate to changes in disease dynamics."
These findings speak to how complex and innovative evolution can be, as an incredibly energy-intensive activity like migration, something that requires sophisticated navigational skills, has risen as a way to protect species from disease. Now it's just a question of whether migration is a good enough strategy to survive this changing world - a lot of species are depending on it.
[Read the full scientific article via Science]