Evolution__3rd_Edition

..
Brain size
. . . are used to measure
intelligence
Body size
Figure 22.11
The distribution of relative
brain sizes for (a) ungulates
(prey) and (b) carnivores
(predators) through the
Cenozoic. Brain size increased
over time and at any one time
carnivores had bigger brains
than ungulates. Redrawn, by
permission of the publisher,
from Jerison (1973).
Deviation from allometric line
Allometric
line
CHAPTER 22 / Coevolution 633
Figure 22.10
Relative brain size may be measured as relative encephalization
by the deviation of a species’ brain size from the allometric line
for many species. Relative encephalization measures whether a
species has a brain larger or smaller than would be expected for an
animal with its body size. The species indicated in the figure has a
relatively small brain, and an encephalization quotient below one.
below the line, the encephalization quotient is less than one; if it is above the line, the
encephalization quotient is more than one. Encephalization quotients are sometimes
thought of as a crude measure of “intelligence” in a species. The more intelligent
animals, in a loose sense, are those that deviate further above the line; they have
greater relative encephalization.
Let us provisionally accept here that the encephalization quotient is an index of
intelligence. We can then consider, from the classic work of Jerison (1973), a possible
example of coevolution between prey and predator. (We met a related part of
Jerison’s general study when we discussed the Great American Interchange in Section
17.8, p. 512.)
In Cenozoic mammals, predators typically have relatively larger brains than their
prey. This relation can be seen in Figure 22.11; but the same figure also shows another,
more interesting fact. The relative brain sizes of both predators and prey have increased
Frequency (arbitrary units)
Frequency (arbitrary units)
(a) Ungulates
0.4
0.3
0.2
0.1
0
0.2
0.1
(b) Carnivores
Archaic
Paleogene
Archaic
Neogene
Paleogene
Neogene
Recent
0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Relative brain size
Recent