WE are seeing an irruption of painted lady butterflies just now.
The insects hail from north Africa, where rains in the Atlas Mountains have led to a butterfly explosion. A single Moroccan field had 150,000 caterpillars. Pressure of numbers forced the insects to move, en masse, into southern Europe, where they bred. Their offspring, in turn, have come north, and painted-ladies are laying their eggs on Irish thistles. The generation they will produce here is doomed. All will perish during our cold Irish winter, a depressing thought. Hibernation, at the egg or adult stage, is the norm for our native butterflies, but such behaviour is unknown to tropical species; they don’t have winters.
Nature is seldom wasteful, so why does she allow butterflies to behave like this? Irruptions are a response to desperate situations. Dramatic fluctuations in animal numbers are rare in the tropics, but they’re the norm on the tundra and taigas of northern latitudes.
There, the populations of voles, shrews, and the predators that target them, fluctuate wildly from year to year. Even hare numbers rise and fall in Nordic countries, while those further south remain stable. The reasons why these cycles occur are well-known, but how animal numbers become synchronised over large areas is much debated. The Norway lemming lives in Scandinavia and Finland. It’s an endearing little rodent, yellow and black with a very short tail. Lemmings don’t hibernate; they remain active all winter under the snow. Thin on the ground, with about three animals to the hectare, they are seldom seen, but, every few years or so, something extraordinary happens. Like our painted-lady visitors, there are suddenly huge numbers of lemmings – up to 300 per hectare.
People thought that the animals crystallised high in the sky and fell to the ground in heavy rain, and even some hard-headed scientists agreed. Ole Worm, the celebrated Danish physician of the 17th century, who made significant contributions to embryology (‘Wormian’ bones link the plates of the skull), believed that the lemmings were carried on strong winds from areas in which they were more abundant.
The truth is more mundane; their numbers rise because these animals don’t believe in family planning. They can breed six times a year, with up to 13 babies in a litter. The fortunes of the young depend on the availability of grass and moss under the snow, the timing of spring, and the number of predators waiting to greet the animals after the thaw. If spring is early and new shoots have appeared, many animals survive. Mild conditions may be a mixed blessing; foxes, owls and hawks might be more numerous if the winter has been mild. The factors governing the lemming cycle are complex, but, every few years or so, they conspire to make the rodent population explode. What happens then has given rise to the most famous of the lemming myths.
The little animals, it is claimed, gather in hordes in search of a cliff or a promontory overlooking a river or a lake. There, they hurl themselves down to a watery grave. So persistent is this belief, that even humans are said to behave "like lemmings". Why would any animal do such a thing? The instinct to survive is deeply ingrained in all creatures; natural selection would have removed any propensity for self-destruction.
The suicide myth is a mix of sense and nonsense. Lemming exoduses do take place, but there is nothing odd about irruptive behaviour. Waxwings invade Ireland every few years, when food becomes scarce back home.
We even irrupt ourselves; Celts, Norse, and Normans are among the peoples who came to Ireland in the past.
The sea-crossing deterred some would-be colonisers, notably the Romans. Irrupting lemmings also encounter water obstacles. They are forced to swim across rivers in their quest for salvation. Some dither on the banks, until they are pushed in by the multitude pressing from behind them.
Not all manage to make the crossing; carcasses of drowned animals are found from time to time, but a verdict of "misadventure," rather than "suicide," is returned by the coroner.
a d v e r t i s e m e n t
This appeared in the printed version of the Irish Examiner Monday, June 15, 2009
