But when they do, it is for good reason: lemmings are known for boom-and-bust, Malthusian swings in population.

• 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。