280 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



reaches a peak, there is one of the famous "lemming years," when tho 

 animals swarm all over the countryside, moving of necessity in a 

 westerly direction. Thousands are drowned in the rivers and fjords; 

 and then the population crash comes and they die out, leaving com- 

 paratively few survivors on the hills to start building up toward the 

 next peak. Similar cycles, though on a smaller scale, are known in 

 other rodents, for example our own short-tailed vole and the American 

 gray squirrel. The journeys of vast hordes of springbok that used to 

 occur in the appropriately named "Trekkbok" years in South Africa 

 were also emigrations, and not true two-way migrations. 



But none of these terrestrial mammals can undertake migrations 

 similar to those that are so well known in birds, where a whole popula- 

 tion transfers itself in a few days or weeks over a great distance, often 

 hundreds of miles, to a seasonal territory, and where the dates of 

 arrival and departure vary only within narrow limits. It is only tho 

 mammals whose movements are unrestricted, because they fly in the air 

 or swim in the sea, that can make migrations in any way comparable 

 with those of birds. 



A few species of small bat make regular migrations from summer 

 quarters in the north to winter ones in the south, although most bats 

 seem to get over the difficulty of having no winter supply of insects 

 to feed upon by giving up the struggle and hibernating. Some, how- 

 ever, particularly in America, do not hibernate, but migrate far to the 

 south to warmer regions where food is abundant. Two species in 

 particular are known to migrate, the red bat and the hoary bat. They 

 spend the summer in the northern United States and migrate down 

 the Atlantic seaboard of America to the southeastern States in the 

 winter. There is no doubt about the southern migration, for the bats 

 have been both seen and captured on their passage, not only on land 

 near the coast but far out at sea. But hardly any have ever been seen 

 on their return journey, and beyond the fact that they are present 

 in the north again during the summer, there is little direct evidence 

 to demonstrate their northern journey. Some of the European bats, 

 too, are said to migrate but the known facts prove only that a local 

 movement takes place, and no long-distance travels have been shown 

 to occur. None of our British bats migrate to other countries for the 

 winter; they all hibernate. But the hibernation of some of them is 

 not so complete a winter sleep as has been sometimes thought. Mem- 

 bers of the Devon cave club have recently marked a very large number 

 of horseshoe bats with aluminum rings so that they can follow up 

 their movements. The result of this work shows that although the 

 bats hibernate, they keep on waking up at intervals during the winter, 

 and sometimes travel distances of many miles from one cave to another 

 during these wakeful periods. Horseshoe bats live in caves during the 

 winter, but they migrate to other quarters in the summer; no one 



