The Neotropics includes South America, Central America from southern Mexico southwards, and the West Indies. This region contains a very diverse fauna. Twelve orders are represented, two of them endemic. The Paucituberculata, shrew opossums, consists of three genera and five species, all found in the Andean region. The Microbiotheria has a single species, the monito del monte (Dromiciops australis), distributed in southern South America. The order Xenarthra (sloths, armadillos and anteaters) is mainly restricted to the Neotropical region, with one species occurring in the southern part of the Nearctic. The marsupial order Didelphimorphia has 63 species, all but one of which are restricted to the Neotropics.
Nineteen of the 50 families and about 80% of the almost 1,100 species that occur are also endemic. These include ten endemic families of rodents, including the guinea pigs, chinchillas, and agoutis; several families of bats; and two families of primates. Two genera of wild camelids (Vicugna and Lama) are endemic. The guanaco (L. guanicoe) is evidently the progenitor of two domesticated varieties, the llama and alpaca.
The Neotropical mammal fauna consists of three main strata. The ancestral fauna consisted of some very distinctive extinct orders, the order Xenarthra and early marsupials. These were augmented by intermittent "invaders" from the north, such as primates, rodents, and some carnivores. Then about 2 million years ago during the Pleistocene, formation of the Panamanian land bridge allowed the immigration of many new forms from North America. These included peris-sodactyls (tapirs and horses), artiodactyls (camelids, deer, and peccaries) and carnivores (felids, canids, and mustelids). This wave of mammalian invasions had a drastic effect on the existing fauna and resulted in many extinctions, including the large herbivore orders Notoungulata and Litopterna, and ground sloths. Ultimately, a unique array of mammals, perhaps as distinctive as those of Australia, disappeared alto-
gether. This interchange has been mainly, but not exclusively, one-way. Many more species moved south than in the opposite direction. Northern species entering South America have also proved more successful colonizers, penetrating to the southern tip of the continent.
The high degree of mammalian diversity is mainly a result of South America's isolation from other major land masses for long periods of geological history. Climate changes during the Pleistocene and alternating wet and dry periods caused the rainforest to contract and fragment, perhaps becoming in effect a series of forest islands separated by dry savanna or scrub that acted as barriers to dispersal and further leading to the development of new forms and evolution of new species.
There is considerable habitat diversity within the Neotropical region. In addition to the tracts of rainforest there are also extensive areas of dry scrub woodland, tropical savanna, temperate grasslands, desert, and high mountains. The Andes run almost the length of the continent of South America and reach an altitude of 22,830 ft (6,960 m) at their highest point. The rainforest does not cover a continuous extent but is divided into four main blocks by large rivers, mountains, and extensive areas of drier habitat types. The Atlantic rainforest of southeastern Brazil has been completely isolated from the Amazon rainforest for a very long time and has its own endemic genera and species. During the Pleistocene, alternating wet and dry periods heavily influenced the extent of rainforest. At times this contracted to smaller patches isolated by expanses of arid habitats that acted as barriers to the dispersal of rainforest mammals. Many species subsequently evolved in these isolated forest refugia. Large rivers also act as dispersal barriers and related but separate species occur on opposite banks. For example, the fauna north and south of the Orinoco in northern South America show a number of differences, and the Amazon and Rio Negro also isolate some species.
Much of the characteristic fauna of the West Indies has disappeared; in fact, a disproportionate number of the mammals that have become extinct in recent times were endemic to the West Indies. These include an entire family containing eight species of shrews (Nesophontes), a species of raccoon (Procyon gloveralleni) formerly found on Barbados, 21 species of rodents, and the Caribbean monk seal (Monachus tropicalis). There is still one endemic family of large insectivores—the Solenodontidae—with two extant species, one each on the islands of Cuba and Hispaniola (Haiti and the Dominican Republic). These are thought to be island relicts of a formerly much more widespread group.
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