This region comprises North America up to northern Mexico and Greenland. Ten orders are present, including 37 families, and around 643 species. Two families are endemic, each containing a single species. These are the Antilocapri-dae (pronghorn) and the Aplodontidae (sewellel, or mountain beaver), endemic to western North America. There are a large number of endemic rodents. These include the woodrats (genus Neotoma), 17 species of ground squirrel (Citettus), three antelope squirrels, 16 chipmunks, 10 squirrels and flying squirrels, 12 pocket gophers, and 37 species of heteromyid rodents (pocket mice, kangaroo mice, and kangaroo rats). Characteristic larger endemic species include bison (Bison bison), mountain goat (Oreamnos americanus), bighorn sheep (Ovis canadensis), and thinhorn sheep (O. dalli).

The Nearctic shares many aspects of its mammal fauna with the Palaearctic and Neotropical regions. More than 80% of Nearctic families and 60% of the genera also occur in the Neotropical region. There are relatively few species of Neotropical origin in the Nearctic fauna. Only the southern opossum (Didelphis marsupialis), the nine-banded armadillo (Dasypus novemcinctus), and some bats have survived of those that entered the region from the south, in contrast to the much larger number of Nearctic species that entered the Neotropical region following formation of the Panamanian land bridge. Nearly half the families are shared with the Palaearctic region, a reflection of the length of time that the two regions have been connected across the Bering Strait either by a land bridge or a chain of islands. Twenty-one genera arrived from the Palaearctic at the end of the Pleistocene. Shared groups include several families of Carnivora (Felidae, Canidae, Mustelidae, Ursidae), deer (Cervidae), shrews (So-ricidae), and moles (Talpidae).

A number of species in high latitudes have a circumpolar distribution in the Palaearctic and Nearctic regions, such as caribou (Rangifer tarandus), wolverine (Gulo gulo), gray wolf

A Galápagos sea lion (Zalophus californianus wollebaecki) basks in the sun near a brown pelican. (Photo by Harald Schütz. Reproduced by permission.)
Pronghorn antelope (Antilocapra americana) inhabit the Great Plains of North America. (Photo by Joe Van Wormer. Bruce Coleman, Inc. Reproduced by permission.)

(Canis lupus), Arctic fox (Alopex lagopus), and moose (Alces alces). Musk ox (Ovibos moschatus) also once ranged all around the tundra zone but is now extinct in Eurasia apart from some small introduced populations.

Many species in the boreal zones of both regions have a near counterpart in the other, for example pine marten (Martes martes) and stone marten (M. foina) in the Palaearc-tic and American marten (M. americana) and fisher (M. pen-nanti) in North America; European otter (Lutra lutra) and river otter (L. canadensis); American mink (Mustela vison) and European mink (M. lutreola).

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