Evolution of the brain and intelligence

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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 intelligence to brain properties, which properties should we look for? (4) Are differences, especially those between humans and other mammals, quantitative or qualitative in nature?

Section snippets

How do we define and measure intelligence?

There is no universally accepted definition of animal intelligence, or procedure to measure it. Intelligence may be defined and measured by the speed and success of how animals, including humans, solve problems to survive in their natural and social environments (see also [3]). These include, for example, problems related to feeding, spatial orientation, social relationships and intraspecific communication. However, what animals must learn in their environments and how they accomplish this can

Supposed differences in intelligence

Using mental and behavioral flexibility as a criterion for intelligence, among tetrapod vertebrates, mammals and birds appear (on average) to be more intelligent (cf. [8]). Among birds, corvids, parrots and owls are considered more intelligent [11], and among mammals, primates and cetaceans [12]. Among primates, apes come out as more intelligent than monkeys, and monkeys more than prosimians; and among apes, chimpanzees, bonobos and humans appear to be particularly intelligent [8]. Thus,

Neural correlates of intelligence

The question arises of how to relate these supposed differences in intelligence to brain properties, and which properties we should compare. These could either be general properties, if we assume that the evolution of intelligence is based on an increase in ‘information processing capacities’ 13, 14), or specialities of anatomy or physiology of different brains, if we conceive intelligence as a number of special adaptations to a given environment [3].

The search for cortical specialties

So far, we have dealt with possible correlations between general problem-solving ability and general brain traits. The alternative is to look for adaptive specializations within the brains of vertebrate or mammalian taxa. In cognitive ecology, some experts view responses to challenges from the environment as a basis for an increase in cognitive capacities and brain complexity, whereas others emphasize a correlation with the complexity of social relationships.

A much cited example for the first

Do humans and their brains have unique properties?

There is a long tradition that ascribes properties to humans that are supposedly not found in other animals. The most cited are causal understanding of mechanisms of tool use, tool-making, syntactical–grammatical language, consciousness, self-awareness, imitation, deception and theory of mind [9]. There is evidence, however that great apes possess at least some states of consciousness found in humans (reviewed in [42]; see Figure 4). Deception has been widely observed among monkeys; great apes

Conclusions

If we define animal intelligence as the degree of mental or behavioral flexibility resulting in novel solutions, either in the wild or in the laboratory, we can conclude that among tetrapods, mammals and birds are more intelligent, and among mammals, humans are more intelligent than members of other taxa. Differences in intelligence among the great apes, cetaceans and elephants are not sufficiently tested, but these taxa all appear to be more intelligent than monkeys, and monkeys more

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