Why do traveling mice migrate in groups?

In northern Europe, Norway, Sweden and other places, there is a strange traveling mouse. This kind of mouse gives birth seven or eight times a year. Within six weeks, young mice can develop into adult mice and reproduce the next generation, so this kind of mice can reproduce to an amazing extent within a few years. Usually live in groups, dig holes deep in the mountains and eat roots, turf, lichens and so on. The strangest thing is that every three or four years, these mice will leave several companions, and the remaining hundreds of thousands or even millions will migrate in large groups.

The great migration of lemmings is mainly carried out at night, eating and resting during the day, wading through mountains and rivers at night, and never looking back when encountering a river ditch on the road. A large number of mice were drowned, and their bodies were filled into the river ditch. The mice behind them would continue to squeeze around on their companions' bodies. Sometimes, giant lemmings will die due to diseases and bad weather during migration, or be preyed by other animals, and the number will gradually decrease. Some mice will go to the coast, still fearless in the face of the vast sea, and continue to March forward bravely until the whole army is wiped out.

Why do traveling mice take this way of traveling towards death? Is it because of over-breeding, breeding crisis and natural elimination, or because of bad temper? Scientists have not solved the mystery so far. In nature, many animals migrate in groups. In order to feed, locusts migrate in groups, and they can fly 10 kilometers a day, and crops are destroyed wherever they go. There is a dragonfly that will fly in groups at a certain time and flood into the city in large numbers; In addition, frogs, snakes and other animals have traveled in droves, which is puzzling.