Seminar 1 Neuro Roth 2005 Brain Size An

Review

TRENDS in Cognitive Sciences

Vol.9 No.5 May 2005

Evolution of the brain and intelligence
Gerhard Roth1,2 and Ursula Dicke2
1
2

Hanse Institute for Advanced Study, D-27753 Delmenhorst, Germany
Brain Research Institute, University of Bremen, D-28334 Bremen, Germany

Intelligence has evolved many times independently among vertebrates. Primates, elephants and cetaceans are assumed to be more intelligent than ‘lower’ mammals, the great apes and humans more than monkeys, and humans more than the great apes. Brain properties assumed to be relevant for intelligence are the (absolute or relative) size of the brain, cortex, prefrontal cortex and degree of encephalization. However, factors that correlate better with intelligence are the number of cortical neurons and conduction velocity, as the basis for information-processing capacity.
Humans have more cortical neurons than other mammals, although only marginally more than whales and elephants. The outstanding intelligence of humans appears to result from a combination and enhancement of properties found in non-human primates, such as theory of mind, imitation and language, rather than from
‘unique’ properties.

Introduction
Animals differ in intelligence, and humans are usually considered to be by far the most intelligent. However, it is unclear which brain properties might explain these differences. Furthermore, the question of whether properties such as a theory of mind, imitation or a syntactical language are uniquely found in humans is hotly debated.
Finally, recent reports on high intelligence in animals with relatively small brains, such as corvid birds and dogs, has raised once again the discussion about the relationship between brain and intelligence [1,2].
In this context, we will address the following questions:
(1) How can we define and measure animal intelligence?
(2) What differences in intelligence are supposed to exist among animals and between animals and humans?
(3) When we relate differences in

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