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After the two continents joined, a few South American forms moved north, for

example the armadillos and sloths, but most died out as a result of competition and

predation from North American invaders. The present-day deer, camels, bears, cats,

and wolves of South America are all derived from northern forms.

The ice ages: historical effects of climate

During the Pleistocene (from 2 million to 10,000 years ago) the earth went through

a series of cold and warm periods. Ice-caps developed over Canada, the northern pale-

arctic (Europe and Asia), and on the main mountain chains such as the Alps,

Rockies, Andes, and Southern Alps of New Zealand. Sea levels dropped as much as

100 m and “land bridges” were formed across the Bering Strait between Asia and North

America, and across the English Channel between Britain and France. The cold and

warm periods in temperate regions were paralleled by dry and wet periods in the

tropics.

The ice ages had a significant influence on the present-day distribution of animals.

The Beringian land bridge across the present-day Bering Strait allowed an earlier

invasion of North America by mammoths (Mammuthus primigenius,M.columbi),

mastodons (Mammut americanum) and sabertooth cats (Smilodon fatalis), and later

invasions by more modern forms such as beaver (Castor), sheep (Ovis), muskoxen

(Ovibos), caribou (Rangifer), elk (Cervus), moose (Alces), bison (Bison), brown bear

(Ursus arctos), and wolf (Canis lupus). There was a smaller reverse migration from

North America into Asia of horses and camels, both of which subsequently became

ANIMALS AS INDIVIDUALS 25

Ostrich

Rhea

Moa

Elephant bird

Emu

Fig. 3.4Present-day
evidence of
Gondwanaland can be
seen in the distribution
of ratite birds on the
southern continents.
The moa in New
Zealand and elephant
bird of Madagascar
became extinct only in
the last few centuries.
(After Diamond 1983.)