How Animals Migrate

Animal migration, one of the most interesting but least understood biological phenomena, actually refers to the seasonal movement of all or part of an animal population to and from a given area. Familiar migrants include many birds; hoofed animals, especially in East Africa and in the Arctic tundra; bats; whales and porpoises; seals; and fishes, such as salmon.

Migration is some cases is obligatory, and in others facultative. Close and careful study of migratory animals indicates that migratory behavior varies in its adaptability to environmental changes. This variability in migratory behavior has led some scientists to question whether the concept of migration is valid. Certainly the description applied to some animals is difficult to reconcile with the description applied to others. For example, migratory behavior has been described as "traveling some distance in search of a new habitat" and as "a regular, annual movement between two habitats".

An example of facultative migration is the arctic fox. On the Aleutian Islands, which are located at the western extremity of the Alaskan Peninsula, the fox is sedentary and does not migrate, since there is a relatively stable and abundant food supply on those islands. But in some other parts of the Arctic, the food supply is seasonally limited, and arctic foxes may travel long distances in search of food. This type of movement between different habitats for the purpose of exploiting food resources may be termed facultative migration.

Another example of facultative migration is the large North American deer, or wapiti. In the Rocky Mountains, wapiti migrate seasonally from high - altitude summer ranges to the lower valleys in winter. However, when food is plentiful in the highlands, wapiti may not migrate at all or may migrate only short distances to nearby valleys. Thus, wapiti behavior varies in its adaptability to environmental changes.

The term "obligatory migration" implies that an animal must migrate. One example of obligatory migration is the Atlantic salmon. These fish are born in freshwater streams, and they spend the first few years of their lives in those streams. After that, they migrate to the ocean, where they live for several years. When they reach sexual maturity, they return to their natal streams to spawn. If they are prevented from migrating, they will not reproduce. Another example of obligatory migration is the monarch butterfly. These butterflies are born in North America, and they migrate south to Mexico for the winter. If they are prevented from migrating, they will die.

The reasons for migration are complex. Some animals migrate to find food, while others migrate to find a suitable place to breed. Still others migrate to avoid harsh weather conditions. In general, migration is a strategy that animals use to increase their chances of survival and reproduction.

动物迁徙是最有趣但了解最少的生物现象之一，实际上它指的是部分或全部动物种群在特定区域间的季节性移动。常见的迁徙动物包括很多鸟类；有蹄类动物，尤其是东非和北极苔原地区的；蝙蝠；鲸和鼠海豚；海豹；以及鱼类，如鲑鱼。

在某些情况下，迁徙是强制性的，而在另一些情况下则是兼性的。对迁徙动物细致深入的研究表明，迁徙行为对环境变化的适应性有所不同。这种迁徙行为的变异性使一些科学家质疑迁徙这个概念是否有效。当然，适用于某些动物的描述很难与适用于其他动物的描述相协调。例如，迁徙行为被描述为“长途跋涉寻找新的栖息地”以及“在两个栖息地间定期的年度性移动”。

兼性迁徙的一个例子是北极狐。在位于阿拉斯加半岛西端的阿留申群岛，北极狐是定居的，不迁徙，因为这些岛屿上有相对稳定和丰富的食物供应。但在北极的其他一些地区，食物供应有季节性的限制，北极狐可能会长途跋涉寻找食物。这种为了利用食物资源而在不同栖息地间进行的移动可被称为兼性迁徙。

兼性迁徙的另一个例子是北美大鹿，即马鹿。在落基山脉，马鹿季节性地从高海拔的夏季栖息地迁徙到冬季的低海拔山谷。然而，当高地食物充足时，马鹿可能根本不迁徙，或者只迁徙很短的距离到附近的山谷。因此，马鹿的行为在对环境变化的适应性方面有所不同。

“强制性迁徙”这个术语意味着动物必须迁徙。大西洋鲑鱼就是强制性迁徙的一个例子。这些鱼出生于淡水溪流，它们生命的最初几年是在这些溪流中度过的。之后，它们迁徙到海洋，在那里生活数年。当它们达到性成熟时，就返回出生的溪流产卵。如果它们被阻止迁徙，就不会繁殖。帝王蝶也是强制性迁徙的一个例子。这些蝴蝶出生于北美洲，它们向南迁徙到墨西哥过冬。如果它们被阻止迁徙，就会死亡。

迁徙的原因很复杂。有些动物迁徙是为了寻找食物，而有些动物迁徙是为了寻找合适的繁殖地。还有些动物迁徙是为了躲避恶劣的天气条件。一般来说，迁徙是动物用以增加其生存和繁殖机会的一种策略。