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This chapter provides an overview of the evolution of the beaver as a single organism. The beaver is the second largest rodent after the South American capybara. Beavers belong to the family Castoridae in the suborder Sciuromorpha of the order Rodentia. Thy are more closely related to squirrels and marmots than to mouselike rodents (Muridae). Beavers split from their closest living relatives 90–100 million years ago. Two species of beaver live today: the North American Castor canadensis and the Eurasian Castor fier. The two beaver species differ in their number of chromosomes. This chapter discusses the beaver's genetics and geographic range, as well as fossils that illuminate its ancestry.

This chapter presents a comparative study of the structure and function of chemical senses in aquatic mammals. It first discusses the chemosensory systems of extant species of Monotremata, Didelphimorphia, Insectivora, Carnivora, Sirenia, Cetacia, Artiodactyla, and Rodentia. It then examines the evolution of chemical senses through a comparative analysis of the olfactory organs of semiaquatic mammals and terrestrial mammals.

This chapter summarizes the geographic distribution and ecology of the extant families of African rodents. It focuses on the fossil record and discusses the systematics, biochronology, and paleobiogeography of Rodentia. Most discussion is confined to the family level; notable exceptions include the subfamilies of the extremely diverse Nesomyidae and Muridae. The chapter concentrates on general geographic regions: northern Africa (countries bordering the Mediterranean Sea and including Chad), eastern Africa (Sudan, Ethiopia, Uganda, Kenya, Tanzania, Democratic Republic of the Congo [= Zaire; although situated in Central Africa], and Malawi), and southern and south-central Africa (Angola, Zambia, Mozambique, and countries farther south). The chapter also presents a chronologic history of African rodents, from the earliest records in the early to middle Eocene to the latest Pleistocene.

The mammal fauna of California was significantly shaped by Beringian migrations that occurred episodically from the Eocene through the late Pleistocene. Wide-dispersing mammals show evidence of continental divisions across North America, with western clades retreating variously to the coast or ice-free regions during glaciation periods. For small mammals, deep phylogeographic divergence is common due to vicariant events, and due to their limited dispersal, they often remained isolated. Severe habitat loss and degradation in the past few centuries has decimated many populations that occur in riparian corridors from east to west, further isolating historically contiguous populations and preventing migration. Barriers to dispersal existed in the Monterey Bay region from about 8–2 Ma, because the southern Sierra Nevada drained into the Pacific Ocean from this location and presented a significant obstacle for the dispersal of small mammals. The central Sierra Nevada, southern Sierra Nevada (particularly around the San Joaquin River), the outer Coast Ranges (particularly around the San Francisco Bay south to Monterey), San Bernardino Mountains, Tehachapi Mountains, Peninsular Ranges, and Inyo Valley are identified as focal areas for endemic taxa. A number of studies now verify that divergence for a number of taxa from the warm deserts began in the Miocene east of the Mojave.

This chapter introduces and describes the two extant species of beavers, the Eurasian Castor fiber and the North American Castor canadensis, including their classification from the order Rodentia, family Castoridae. The Castoridae family represents species that can produce, store, and distribute castoreum. This chapter describes the appearance of these two species in the fossil record and their evolution into two distinct species. Differences between the two species are discussed including chromosome number, appearance, behaviour, and ecology. Background information on extinct beaver species including Castoroides ohioensis—a giant beaver, the size of a bear—is provided. Also considered are the latest genetic findings on subspecies division of the Eurasian beaver which, as for its American counterpart, have been controversial and largely based on fur trade refugia populations. Similar species, which may be confused by some for beavers, are discussed and key differences noted.