Geographical Distribution of Terrestrial Mammals in Middle America

Abstract

The 370 species of nonflying, nonmarine and noninsular mammals of Middle America (Mexico, excluding Baja California, south through Panama) include 156 monotypic and 214 polytypic species. The distribution of these mammals is discussed with relation to their geo- graphical affinities and their occurrence in principal areas. Major distri- butional barriers and areas of intergradation between subspecies in Mexico northwestward of the Isthmus of Tehuantepec are defined. Middle America, defined herein as being Central America includ- ing Mexico, forms a narrow connection between North and South America. From the broad expanse of-,North America the continental mass narrows southeastwardly through Mexico and ultimately to the present minimum width of approximately 40 miles at the Isthmus of Panama. The restrictive nature of this corridor has been a major deter- rent through time to inter-American movements by terrestrial organ- isms. Furthermore, movements were greatly discouraged -if not stopped - throughout most of the Tertiary when water gaps existed in this corridor (Mayr, 1946: 8-9). Insofar as mammals are concerned, Simpson (1950) has shown that these animals moved southward from Holarctica at various times, prob- ably beginning in the late Mesozoic, to populate South America. He classes these immigrants into 3 groupings. Those considered by Simp- son as old-timers reached South America in the late Mesozoic and early Tertiary and include present-day marsupials and edentates. Those considered as old island hoppers reached South America in mid- Tertiary (late Eocene to Miocene) and include primates, caviomorph rodents and procyonids (allies of the raccoon). Those that Simpson considered as newcomers reached South America in the late Pliocene and Quaternary and include Recent pan-American species of rabbits, squirrels, myomorph rodents, dogs, raccoons, weasels, cats, tapirs, peccaries and deer. Following diversification in South America some mammals grouped as 'old-timers and old island hoppers moved northward beginning in the Pliocene and more abundantly in the Quaternary, when the present land bridge was re-established, until today Middle America and even parts of North America are inhabited by mammals having a direct affinity with the South American fauna. Species peculiar only to the