Lemmings, small rodents of the arctic and subarctic regions of the Northern Hemisphere, have long been a byword for mass mindlessness and mass suicide. Thanks to a deeply problematic 1958 Walt Disney documentary, White Wilderness, it is widely supposed that the little creatures are programmed to fling themselves from tall cliffs into the sea, where they drown, presumably because they cannot swim.
Some have indeed jumped, as it happens (see the video in this Britannica article), and some have drowned. But when they do, it is for good reason: lemmings are known for boom-and-bust, Malthusian swings in population. When their colonies become too large and competition for food becomes intense, lemmings migrate to find new territories—readily swimming, if need be, in order to get from one place to another, but exposing themselves to drowning, predation, and other dangers.
Times are hard for lemmings, as they are for most other creatures on Earth. Writes Sharon Begley of Newsweek, that boom-and-bust cycle has not been seen for several years, and the number of lemmings is declining. Loss of habitat is to blame, specifically, the lack of the pocket of air between snow and ground called the “subnivean space.” There lemmings winter and give birth to prodigious litters of young. Thanks to global warming, however, this insulated gap, which gives the lemmings access to moss and other foodstuffs, is disappearing; instead, snow melts during the day and freezes at night, producing an impenetrable layer of ice and depriving the lemmings of cold-weather habitat.
Given the loss of that ancient way of life, a cliffside dive may not look so bad.
译文:
旅鼠:又一次从繁荣到萧条
旅鼠是一种生活在北半球北极区和亚北极区的小型啮齿类动物，一直以来人们流传着很多关于他们愚笨和自杀的笑话。1958年沃特·迪斯尼做了一个带着深入疑问的记录片，白色沙漠，人们广泛认为这些小生命是计划好了要自己从悬崖上跳进海里，在那里它们被淹死了，原因很可能是它们不会游泳。
有一些真的是跳下去了，它们跳下去之后（点击这里查看这些大英百科全书网上的文章的视频），有一些淹死了。然后，他们这样做，是有原因的：我们都知道，旅鼠的种群是繁荣与萧条交替循还的，也就是数目呈马尔萨斯式的曲线。当它们的领地变得太大，食物竞争变得越来越激烈的时候，旅鼠们不得不迁往新的领土，为了到达新的寺方，当需要的时候，它们会欣然游过去，代价是把已置身于被淹死，被掠杀和其它的危险之中。
时间对旅鼠们来说是冷酷无情的，对其它的生物也一样。莱特斯·莎伦·贝克雷在新闻周刊上说，旅鼠种群数量的循环已经有好几年没有出现了，旅鼠的数量在不断地减少。生存环境的缩小是一个原因，更主要的是它们在冰雪和地面之间被称为“雪下空间”里得不到保护，有些旅鼠会在冬天生下数量惊人却无法存活的小旅鼠。因为全球变暖的出现，为旅鼠的提供了一些赖以生存的苔藓和其它食物，然而，这个缺口也在逐渐消失。取而代之的是，冰雪白天融化晚上冻结，给旅鼠制造了一层不可穿越的冰层同时也夺去了他们生存的寒冷栖息地。
相比在古老的生命之旅上的损失，悬崖边上的旅程看起来并没有那么糟糕了。