The central dilemma in studying intelligence is to come up with a universally acceptable definition of intelligence. Many researchers define intelligence as the ability to use language and create tools to manipulate the environment. While this definition may seem fairly logical, especially because humans seem to be the axiomatic example for this definition, many researchers argues that its anthropocentric nature is too limiting. By placing parameters on intelligence which only humans meet, and lower primates fit to varying degrees, it is inherently impossible to find "intelligence" in any other species. A second problem with this humanist definition of intelligence is that it is based largely on human introspection and the knowledge that we are conscious, rational, linguistic animals(1). Kenneth Marable argues "if the same criterion that are used to rule out non-human intelligence were applied to humans without the benefit of introspection, we would doubt even our own intelligence"(1).
One of the first goals of inter-species intelligence studies was to create a quantitative scale to measure the intelligence of animals. Since the administration of IQ test to many different species seemed illogical, researchers turned to using scales which compared the size of an animal's brain to the size of its body; this value was known as the Encephalization Quotient. The Encephalization Quotient (EQ) "allows researchers to ask the question: 'Is the brain of a given species bigger or smaller than would be expected, compared with that of other animals its size?'(2)". The scale is designed so that the expected score should be 1, and anything above that would be indicative of a larger then predicted brain which could possibly correlate with greater intelligence. There seems to be some conceptual validity to this scale because humans have the highest score of any known animal with a 7; this could of course reflect the biases of the species who designed the scale. An interesting aspect of this scale is that primates score around a 2.34 and dolphins score a 4.5 (2).
Although Encephalization Quotient seems to make a lot intuitive sense, mostly because it ranks humans well above other animals, it has proved difficult to establish a strong correlation between it and intelligence(2). One of the main problems with this method is that animals vary extensively in their body and brain weights at various time of the year, periods of life and even between sexes (3), making it hard to make a claim to an average brain to body ratio. Also it may be the case that some animals such as cows don't actually have small brains for their bodies, as indicated by their low EQ, instead their bodies may be heavier than another animals their size due to the extensive digestive track needed to break down grass(4). Thus they should not be considered unintelligent based on their low score.
In addition to the practical concerns with the scale there has been little experimental evidence to support a connection between brain size an behavior. Pilleri, Gihr and Kraus have done extensive experiments comparing behavioral differences in rats based on differences in brain size without any success (3). Comparative vertebrate research has also proved to be equally fruitless: because there are too many anomalies. A particular example is the spiny anteater (an egg laying mammal, related to the duck-billed platypus), with a neocortex (the so-called 'modern' part of the brain, which is greatly developed in primates and humans) relatively much larger than that of a human. Despite this endowment, nobody has so far put forward any claims for superior "intelligence" in spiny anteaters.(3) These problems make it clear that the connection between the brain and intelligence cannot be as simple or straight forward as was once hoped.
With the apparent failure EQ as an absolute measure of intelligence many researchers turned to cortical folding as a possible index. It is believed that cortical folding is indicative of an increase in cortical surface area, which is thought to be necessary for greater intelligence. Interestingly humans are not at the top of this scale, dolphins brains demonstrate the most cortical folding(1) (see picture1 and picture2)(5).
This method of comparison has other drawbacks besides concluding that dolphins are smarter than humans. There has been little evidence so for indicating that there is indeed a relationship between the degree of folding and the level of "cerebral processing abilities"(1). There has also be recent research which demonstrates that many of the basic assumptions of cortical folding, such as that it results increase volume, are not true (3). In the case of dolphins their greater folding does not actually result in greater cortical surface area than humans because their cortex is much thinner (3). This research demonstrates that looking at cortical folding without examining the total structure of the brain is too simplistic of an approach to yield any useful results.
Based on the lack of success of EQ scales and cortical folding, some experimenters argue for a detailed analysis and comparison of all the structures in the brain. The structure most often cited as being important to intelligence is the neocortex (3). In comparing the neocortex of land mammals with that of the dolphins it was discovered that despite their increase folding dolphin cortex's had only five of the six layers that exist in the cortex of land mammals. "In some views (e.g. Kesarev et al.,1977) this means that cetaceans have no true neocortex, or only a preneocortex" (3). If this is the case then this method of comparison seems to better fit the human notion of the intelligence hierarchy. But according to some researchers the neocortex does not has an exclusive claim to intelligence, this may mean that the structure that makes a human "intelligent" is not the same one that could make a dolphin "intelligent".
The one index that appears to have any correlation with intelligence is the number of connections made by nerve cells. This scale places humans far above any other organisms, in fact "human brains maintain nearly 100 times more contacts per neuron than rat brains and most vertebrates"(2). This measure seems valid because intelligence does rely on the comparison and coordination of many different inputs from many different structures; a process which would be facilitated by an increased number of connections per neuron.
One of the problems with all these method of comparison is the assumption, semantic discrepancies in definition aside, that there is an underlying intelligence which is constant throughout the animal kingdom. This debate is very similar to one currently in cognitive psychology as to whether humans should be studied and compared based on a general level of intelligence or based on specific categories of intelligence. If the notion of only one kind of intelligence among humans seems to limiting the notion of only one type of intelligence among all animals seems even more so. For example, pigeon's are able to discriminate many more visual images than humans beings, which demonstrates intelligence on some level, but few would argue that they are more intelligent than humans. In this instance a limited definition of intelligence would cause a researcher to devalue and incredible interesting aspect of another creatures intellect. Perhaps it would be better to define and study intelligence as an index of how well an organisms functions and adapts to its own specific environmental niche. With this scale it would be fair to say that underwater dolphin's are more intelligent than humans because to their echolocation abilities, and humans would still be considered more intelligent on land.
The debate over the validity of using an aspect of the brain as a measure of intelligence is a microcosm of the larger brain equals behavior question. And like that debate there appears to be no easy answer. If the differences in intelligence were as easily ascribed to differences in brain structure as differences in locomotion can be ascribed to different appendages then there would be strong evidence supporting the conclusion that the brain is behavior. But the lack of clear evidence indicates that if the brain is responsible for behavior, it does so in an extremely complex fashion and it is not a matter of possessing a certain structure or achieving a certain size. Although there is no doubt that the addition of certain brain structures do have some effect on behavior, to say that anyone of them is the sole cause of behavior is dangerously limiting. Instead it is better to view the brain as a compound structure in which many different internal and external environmental stimuli come together to cause behavior.
The notion that the brain is indeed affected by environmental contingencies is supported by the anatomical differences discovered between species. It is clear that the differences in such species as dolphins and humans that evolution their brains based on the environment in which the organisms existed. If the environment can have such broad temporal effects on a species it seems logical that it could also have an effect on an individual development.
The lack of clear anatomical correlation's with intelligence maybe actually be one of the best "failures" in the history of attempts to understand the brain. Since no one field of research has been able to answer the brain behavior question using its methods alone perhaps these failures will lead to a cooperative effort between scientist in which knowledge is shared and techniques used in conjunction to finally gain a true understanding of the brain. This convergent approach seems the best suited for finding significant results because since the brain is so multifaceted in is function and integrates some many aspects of the internal and external environments the methods of studying it should be equally diverse.
1)A Comparison of Primate and Cetacean Mentality-by Kenneth Marable
2) Does Size Really Matter? -- A new paper article about research by Deric Bownds, a UW-Madison on brain size.
3)Brains,Behavior and Intelligence of Ceteceans -- An article discussing the methods of brain comparisions and extending into primates and dolphins.
4)Brain Size and Cognition inPrimates -- Notes on a Lecture by Dr. Andrew Richards of UM who has researched and published on primates, but the focus of his research is on dolphins who are the only other animal up there with primates in terms of brain capacity. He has studied mostly bottle-nosed dolphins.
5) The Wisconson/Michigan State Brain Collection -- a collection of photographs of disected brains from various species.
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This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.