Ursula Dicke and Gerhard Roth (2015)
Abstract
Many attempts have been made to correlate degrees of both animal and
human intelligence with brain properties. With respect to mammals, a
much-discussed trait concerns absolute and relative brain size, either uncorrected
or corrected for body size. However, the correlation of both with
degrees of intelligence yields large inconsistencies, because although they
are regarded as the most intelligent mammals, monkeys and apes, including
humans, have neither the absolutely nor the relatively largest brains. The
best fit between brain traits and degrees of intelligence among mammals
is reached by a combination of the number of cortical neurons, neuron packing
density, interneuronal distance and axonal conduction velocity—factors
that determine general information processing capacity (IPC), as reflected by
general intelligence. The highest IPC is found in humans, followed by the
great apes, Old World and New World monkeys. The IPC of cetaceans
and elephants is much lower because of a thin cortex, low neuron packing density
and low axonal conduction velocity. By contrast, corvid and psittacid birds
have very small and densely packed pallial neurons and relatively many
neurons, which, despite very small brain volumes, might explain their high
intelligence. The evolution of a syntactical and grammatical language in
humans most probably has served as an additional intelligence amplifier,
which may have happened in songbirds and psittacids in a convergent manner.
[h/t Emil Kirkegaard]
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