278 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



There are, however, mammals that make regular migrations similar 

 to those of birds. In these there is a seasonal movement of the whole 

 population, with a return at a later date: the movements are either 

 to reach good feeding grounds, a more congenial climate, or a suitable 

 place for breeding. But unlike the free-flying birds, land mammals 

 have to make extensive and possibly difficult journeys on foot — rivers, 

 mountains, and arid regions are insuperable barriers to travel, so it is 

 not surprising to find that comparatively few land mammals migrate. 

 They are mostly the large and long-legged sorts that can travel fast 

 and so can protect themselves from predators and other hazards of 

 migration. It is especially the mammals that live in a uniform medi- 

 um that are able to migrate on a scale comparable with that of birds. 

 They are the mammals that live in the sea and can swim, or live in the 

 air and can fly : the whales and seals on the one hand and the bats on 

 the other. 



Let us consider first the land mammals. The mammals that migrate 

 on foot must obviously be those that live on the large continental land 

 masses, if their travels are to extend to any considerable distance. 

 Land migration, too, must obviously be over ground that is reasonably 

 good for traveling ; apart from topographical barriers such as moun- 

 tains or deserts, tropical forests are almost impenetrable for anything 

 but local movements. 



It is in North America particularly that we find several species of 

 land mammals that migrate; by contrast, in Asia, Africa, and Aus- 

 tralia there are few. There is good reason for this. It is probably cor- 

 related with the topography and climate of the continents. In North 

 America there are great differences of climate in different latitudes, 

 and in addition the main mountain masses run north to south ; in Asia 

 the hotter south is divided from the cooler north by mountain ranges 

 running east and west : in Africa the climate varies from subtropical 

 to tropical but there are no arctic regions from which a winter migra- 

 tion might be necessary. 



In North America the larger migratory mammals traverse lands 

 that are open steppe or savannah, or are covered only with compara- 

 tively open forests. The American mammal whose migrations have 

 attracted most attention is probably the caribou, a species of deer like 

 a large reindeer. But the movements of the herds are by no means 

 constant and it is therefore very difficult to form a clear idea of their 

 migration routes. In some years the herds mass together in great 

 numbers so that the migration is an impressive sight; in others the 

 herds are broken up and the deer pass by in small bands that give 

 the impression of a scarcity. And the herds of different regions take 

 different routes, so that they do not appear to obey any uniform rule. 

 All that one can say is that in general they seem to follow a circular 



