Evolution__3rd_Edition

516 PART 4 / Evolution and Diversity
Several hypotheses exist to explain
the greater proliferation of North
American mammals
Table 17.3
Relative brain sizes (expressed as an encephalization quotient, EQ (Section 22.6, p. 632),
which increases with increasing brain size) of North and of South American ungulates in the
Cenozoic. From Jerison (1973).
Ungulate brain size (EQ)
Time
(Myr BP) South America North America
65–22 0.44 0.38
n = 9 n = 22
22–2 0.47 0.63
n = 11 n = 13
continued. The North American mammals showed their superiority, therefore, not in
the original invasion, but in their relative success afterwards.
Why did the North American mammals prove superior? The increase in number of
originally North American genera can be seen across a wide range of mammal types,
which suggests they had some general advantage. There are several ideas why. One is
that the North American mammals had lived a more competitive life, in a larger
continent with more species, than the isolated southern mammals. The “arms race” of
competition had moved further in the north. The idea can be illustrated by Jerison’s
(1973) study of brain size (Section 22.6, p. 632).
In North American mammals, brain sizes, relative to body size, increased with time
in both predators and prey in the past 65 million years. Jerison’s interpretation is
that brain sizes increased as predators and prey grew increasingly intelligent, in an
escalating improvement of offensive and defensive behavior, and the pattern of brain
evolution fits his interpretation (Figure 22.11, p. 633). However, in South American
mammals, no such increase seems to have happened (Table 17.3). Arguably, then,
when the North American mammals invaded the south they had been prepared by 50
million years or so of more demanding competition. They possessed advanced armaments,
probably not only in intelligence, that enabled them to overrun the southern
mammals.
Alternatively, as Marshall and his coauthors suggest, the North American mammals
may have enjoyed some advantage in the environmental change of the past 3 million
years. The Andean upthrust sheltered the Americas from the Pacific, creating a rain
shadow east of the mountains. In South America, dryer pampas or even semidesert
replaced the moist savannah and forest. Quite why such a change should benefit the
North American mammals at the expense of the South American forms is unclear; but
such a change would be likely to benefit one of the two groups more than the other. It
was a large change, and was therefore probably influential in the faunal replacements of
the time.
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