Geoscience Reference
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Figure 3.5 During the Great American Interchange, mammals and other organisms
were able to move freely between Central and South America. (Illustration by Jeff Dixon.)
rose above sea level, allowing for a great interchange between South America and
North America. While armadillos, caviomorph rodents (like capybaras and agou-
tis), primates, and marsupials moved south, squirrels, deer, peccaries, murid
rodents, carnivores, and others moved north (see Figure 3.5). The exposure of this
land bridge varied with fluctuating sea levels. The present fauna of the Neotropical
rainforest is a product of ancestral taxonomic connections, temporary land bridges,
recent uplift, and climatic changes. It includes families that evolved in both North
and South America, and some whose origins may be Gondwanan.
The mammals families of the Neotropical rainforest include both marsupials
and placentals (see Table 3.2). Marsupials are mammals with pouches to carry
their young. All the marsupials in the Neotropical rainforest are opossums (family
Didelphidae), a family restricted to the Americas, with one species in North Amer-
ica and about 70 in Central and South America. All 10 opossum genera are present
in the Neotropical rainforest, including common opossums, woolly opossums,
black-shouldered opossums, bushy-tailed opossums, short-tailed opossums, mouse
opossums, water opossums, and four-eyed opossums.
Sloths spend most of their time in the middle and upper canopy, hanging upside
down on branches or nestled into the fork of the tree eating leaves and shoots of
trees (see Plate VII). They rest up to 20 hours a day.
Sloths have long, thick, coarse, brownish-grayish hair. The hairs are grooved lon-
gitudinally and—since the animal is usually hanging upside down—hang down from
a center line on the belly (the opposite of other mammals on which the hair parts
along the back). This allows sloths' fur to shed water while hanging. Under this
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