For more than a hundred years, scientists around the world have been closely monitoring the phenomenal migrations of lemmings, more like mass "suicides". In recent years, biologists have agreed that there is no mystery in this, just as there is no suicide. But in explaining the reasons, they will not be completely determined …
Migration is called periodic (for example, fur seals feeding in the Sea of Japan in winter, and moulting and moulting in the summer on the islands of the North Pacific; migratory birds) or non-periodic (for example, the eviction of nutcrackers from for the lack of feed from the north of Siberia to its south) the movement of animals for an individual (nesting) habitat during a season, a year or a number of years.
Such relocations can be permanent or one-time (eg locust departures).
We are used to the nomadic migrations of birds, but sometimes animals leave their original habitats without returning. Most often, migrations occur in connection with a change in living conditions or with the passage of a development cycle. Animal migrations can be passive (larvae, eggs, adults carried away by water currents; for example, the Gulf Stream carries the larvae of the river eel from the Sargasso Sea to the shores of Europe at a distance of 7-8 thousand km) and active (locust emergence). They are also distinguished by forage migrations of animals - in search of food, resettlement (for example, settlement of young mammals) and other, more specific forms of migration.
There is nothing surprising in this, as the well-known proverb says, a fish seeks where it is deeper, and a person - where it is better. However, any animal tries to settle in a place convenient for it, with a sufficient amount of food. For active migration, an animal needs a biological sense of time and direction. And all species have this quality. Lemming migrations are one of the special cases of a common environmental problem that has been the subject of heated discussions for several decades.
This cute touching shaggy little animal belongs to the vole subfamily of the rodent order.
The body length of an individual usually reaches 15 cm and ends with a tiny two-centimeter tail. One can only draw this in children's books. There are 20 species of lemmings in the forests and tundra of Eurasia and North America. The largest populations are represented by three species: Norwegian (found in Norway and some regions of Russia); Siberian, or brown (lives in Russia, Alaska and Canada), and ungulates (very widespread throughout the Arctic, including Greenland). Lemmings are mostly brown in color, although the Norwegian has darker spots on the head and back. The hoofed lemming also differs from its counterparts in that it changes its skin from brown to white in winter, and this makes it invisible in the snow.
The views of zoologists on the nature of the periodic migrations of lemmings in the tundra zone were repeatedly presented in the pages of all kinds of magazines more than 30 years ago. But then the mechanism of this rather well-studied phenomenon remained a mystery. All the past years, small animals from the vole subfamily, which play an important role in the life of tundra biocenoses, continued to interest specialists in many countries.
Lemmings, if they were reasonable, could be proud of such close attention. Scientists have studied them "far and wide." For example, their food assortment is thoroughly known: sedges and green mosses, shrubs of various dryads, grasses and forbs, and the fact that they devour vegetation by 50-90%. And nevertheless, even after intensive grazing (in the years of peak reproduction of lemmings), sedges and grasses significantly increase their aboveground phytomass the next year. This indicates a high adaptability of the tundra vegetation to the press on the part of lemmings and, in particular, its ability to quickly restore its biomass, and therefore, to supply the food necessary for the animals.
The information available in the literature does not give an exact answer to the question to what extent the lack of food affects the survival and reproduction of lemmings. Nevertheless, the lack of food, leading to a disturbance in the energy balance of rodents, should be considered as a very real reason only for their winter mortality. But, while recognizing the participation of the food factor in the regulation of lemmings, most experts do not share the point of view of many ecologists about its leading role.
They are not able to significantly disturb the "peace" of lemmings and the predators feeding on them - middle and long-tailed skuas, snowy owl and arctic fox. There is no common point of view among ecologists regarding the role of predators as a regulator of the number of lemming populations. When assessing the participation of predators in the dynamics of the number of lemmings, scientists recognize that the Arctic species of birds and mammals, which have long been feeding on these rodents, have a significant impact on their cycles. However, the role of predators is reduced mainly to the extermination of a significant part of the breeding population of rodents in the peak phase, which are separated from each other by intervals of three to four years and occur synchronously in rodents of this species. And as it turns out, the life cycle of many polar animals directly depends on this little rodent.
The waves of life of lemmings in the relatively unproductive Arctic tundra are amazing. It has long been known that they are able to reproduce under the snow. These small rodents spend the winter under the snow, nestling in the gaps created by the steam rising from the warmer ground after being covered with cold snow. Where there are no gaps, lemmings dig their own tunnels and live and breed in this warm underworld.
Adult females are capable of producing at least five to six snowy litters. This is what leads to a significant increase in populations. In summer, in different areas of the tundra, adult females bring two or three broods. Young female underyearlings can produce their first litter at the age of only two to three months, so a female born in March can have grandchildren by September.
It is also interesting that the faster the population grows, the earlier the period of summer breeding ends. And besides, reproduction under the snow does not resume; in parallel, mortality increases in all age groups, as a result, the number of rodents drops sharply. A year or two after the "collapse", the breeding intensity of lemmings remains moderate, and the mortality rate is relatively high, and only then the population enters the growth phase again. In this regard, we can say that nature, in fact, laid in the breeding process of lemmings a natural barrier to excessive population growth.
An important factor in the collapse of lemming populations is tularemia epizootics, which can affect highly multiplied populations of Siberian and ungulate lemmings and cause their mass death. However, natural foci of tularemia were found not in all areas of the tundra, moreover, predators, destroying mainly sick and weakened animals, restrain the development of epizootics. So there are enough regulatory barriers to reproduction. All this suggested that lemmings, like many other voles of temperate latitudes, have adaptive mechanisms of autoregulation. And usually, lemmings are characterized by relatively short seasonal migrations, which do not significantly change the overall course of their development cycle.
But the massive migrations of Norwegian lemmings brought wide popularity to these animals long before scientists became interested in them. There are numerous legends and traditions on this score. Indeed, the Norwegian lemmings, whose main biotopes are located in mountain tundra, in the peak phase in the second half of summer and in autumn, descend from vast plateaus, usually following river valleys into the forest zone below. Since most of the animals die in this case, such migrations naturally sharply reduce the population size. In addition, the number of lemmings, scientists say, is easily regulated thanks to tight control by their natural predators.
It is very difficult to understand what brings the population of Norwegian lemmings into a state of "stress". Perhaps the arctic tundra is still unable to support the colossal population of lemmings, and the tiny animals are forced to feverishly search for food. Sometimes they begin to eat even poisonous plants, and at times they become aggressive and even attack larger animals, finding death in their teeth. And as mentioned above, there are plenty of hunters to feast on lemmings, and the growth of populations of predatory animals directly depends on the "volume" of their populations, including the Arctic fox, ermine, white owl and other birds of prey. When the population of lemmings is small, these birds and animals have to look for other prey. The white owl doesn't even lay eggs if there aren't enough lemmings to feed the chicks,while gray foxes leave the tundra and go hunting in the endless forests to the south.
So, based on this, we can say that a decrease in the number of lemmings leads to a decrease in the number of predatory animals and birds, which, in turn, contributes to their subsequent rapid growth.
Perhaps this is when mass migrations take place. The scarcity of arctic and tundra vegetation, predators, and diseases limit the growth of the population, but once every three to four years, with an abundance of food, the growth of the lemmings population gives rise to an outbreak. It turned out that periodically the population of lemmings can increase dramatically 100 or even 1000 times of their original number. They are unable to find food for themselves. Desperate to find food, thousands of tiny rodents sweep in fluffy waves across the tundra in search of new territories. This dash for food sometimes ends tragically. As if the earth is covered with a fluffy carpet - these are lemmings uniting in one horde and amicably preparing to commit "suicide".
They see no obstacles on their way. Wolves, foxes and even fish swallow this easy prey, which does not even try to escape. For some reason, these hordes of lemmings rush along routes leading to the sea. They fill cities and towns; destroying crops, polluting the area and poisoning rivers and lakes. If the sea gets in the way of the lemmings, then a decent part of the flock will not even have time to notice how it will be there, since the animals look only at each other's tails and follow the leaders. Therefore, if the presenters also see the cliff, then even stopping abruptly, they will not be able to hold the entire mass behind the runners, who will simply push them and begin to fall themselves. But this does not mean that the whole flock without exception will drown and the animals that have fallen into the water will drown. They swim well and will be able to get out then to the shore, where they will gather again in a flock and continue their migration.
And some scientists believe that mass migrations of lemmings "towards death" directly depend on solar activity. Here is what the newspapers reported in 1970: “In the north of Scandinavia, the number of pied mice (lemmings) is increasing on an alarming scale, flooding all around in their non-stop death march. Hundreds of thousands of these black-reddish Arctic animals are moving in an endless stream to the south. On the way, they die by the thousands in lakes, rivers and, finally, in the sea …
Such a hike, similar to a suicide, the pestle makes almost regularly every few years. Usually timid, invisible creatures become extremely aggressive predators, destroying everything and everyone in their path. And this deadly march of them is unmatched in the animal kingdom.
The largest hikes of pestles were observed in 1918 and 1938. The current resettlement has caught the attention of alarmed Scandinavian authorities. The fact is that in November last year, during a similar campaign, pestles were crushed to death by cars on the roads, dogs were bitten. Piles of decaying animal corpses appeared everywhere, and the threat of epidemics arose."
Some scientists regard animal migration as insanity associated with imbalance in the neuroendocrine system. As you know, many metabolic processes in the body are controlled by the adrenal glands. It was found that in animals during the period of mass reproduction, as well as migrations (invasions), there are sharp changes in the adrenal glands. Hence the insanity that is caused by stress when extremely strong environmental stimuli act on the body.
So, for example, with the onset of very high solar activity in 1957, the adrenal glands degenerated and their enlargement were found in young deer. This defect affected approximately 80% of all animals. The result of this experiment was not slow to show itself: in the three months of the next year, about 30% of the reindeer died.
It should be said that not only flocks of lemmings make "lethal" throws. In this, innumerable hordes of maddened squirrels were noticed, which, for example, in 1956 (with very high solar activity), moved north, where they were awaited by cold, hunger and death. They swam across the flooded Amur, overcame high mountains and even tried to swim across the Tatar Strait! The animals' legs were bleeding, their fur was worn out, but they walked and walked in the same direction, not paying any attention to people and obstacles. Some villages passed up to 300 squirrels per hour, and they moved at about 30 km per day. And each squirrel carried hundreds of ticks infected with the encephalitis virus.
Locust migrations also have nothing to do with lack of food.
In the last century, these insects made nine invasions with a period of 11 years.
How can you understand the reason for the senseless migration of animals, which eventually ends in their death? This question arises for everyone who is familiar with the problem or at least heard about it. I would like to believe that the animals do not have enough food, and they are fleeing in panic. But facts contradict this …
V. Syadro, T. Iovleva, O. Ochkurova "100 famous mysteries of nature"