masses swarm migration biology move

Masses on the Move

When animals seek the company of lots of others

There is something ‘instinctive’ in humans following others: If people start running in one direction, chances are others will join them. In fact, we can’t help being fascinated by mass aggregations of organisms whether it is groups of humans, flocks of birds, herds of countless wildebeest or bison on their migration, schools of fish on their way to their spawning grounds, a plague of locusts, swarms of bees, or lemmings on the run.

An individual is often safer in a group than on its own, for it can hide amongst others and can rely on the others’ vigilance should an attack by a predator occur (even if a group is more visible). But predators, too, have an advantage over hunting singly, if they opt to cooperate with fellow individuals as with a pod of orcas, a pride of lions, or a pack of wolves, etc. The advantages of a group over an individual are certainly involved in the three fundamental reasons that bring huge numbers of individuals together: seasonal migrations to avoid winter or summer hardships; migrations for reproductive purposes, and migrations to feed and exploit a particular, often short-lived, food source.

Other reasons that can bring together lots of individuals are related to curiosity (an instinct known as neophilia) and to dangers like fire, floods, hunters etc. Finally there are the “taxes”, the directed movements from (=negative) or to (=positive) light (phototaxis), odours (chemotaxis), sound (phonotaxis), wind (anemotaxis), water current (rheotaxis), gravity (geotaxis), etc., which can lead to mass aggregations of individuals. When, however, thousands of individuals come together and none of the reasons listed above seem to apply, we are at a loss to explain the mass occurrence.

Such mass occurrence of about 600,000 millipedes in one and another 200,000 at another site plus uncounted thousands dead on the ground occurred on Hachijojima during the first autumn I spent on this Japanese island. Mating and reproduction were eliminated as reasons for the mass aggregation and so was food shortage (or sudden food abundance). Seeking hibernation sites can be ruled out, because the individuals climbed the outer walls of concrete bridges and pylons, exposing themselves to northern, southern, eastern, or western sides. Some migratory millipedes crawled up a slope, but heading downward was equally common. In other countries with mass migrating millipedes they often chose railway tracks as convenient marching surfaces to move along during the day, but Hachijojima’s tended to migrate only at night. Yet, all individuals were eyeless, which suggests that they detect light extra-ocularly, perhaps directly with brain cells.

Whichever reason and condition you look at: nothing seems to explain the mysterious wanderings of our mass-aggregating millipedes. When I discovered that all the individuals contained masses of roundworms in their guts and also harboured tiny mites on their bodies, the idea was born that they were perhaps ’driven’ by their parasites to do things they would not normally do. But even that is just an idea and no proof exists to back it up. It is one of those mysteries that continues to baffle me and others, who have seen millions of millipede legs on the march.

parasite manipulation biology science

For more reading about parasites : click for another story : “Parasitic manipulations”

© Dr V.B. Meyer-Rochow and, 2016.
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