Biology 202
1998 Third Web Reports
On Serendip

A Comparison of Primate and Dolphin Intelligence as a Metaphor for the Validity of Comparative Studies of Intelligence

Jonathan Ball

Primates and cetacean have been considered by some to be extremely intelligent creatures, second only to humans. Their exalted status in the animal kingdom has lead to their involvement in many experiments which hope to gain a better understanding of the basis of human intelligence. These experiments coupled with analysis of primate and cetaceans brain structure has lead to many theories as to the development of intelligence as a trait. Although these theories seem to be sound, there is some controversy over the degree to which non-human studies can be used to infer about the structure of human intelligence.

By many of the physical methods of comparing intelligence, such as measuring the brain size to body size ratio, cetacean surpass non-human primates and even rival human beings. For example dolphins have a cerebral cortex which is about 40% larger a human being's. Their cortex is also stratified in much the same way as a humans(1). The frontal lobe of dolphins is also developed to a level comparable to humans. In addition the parietal lobe of dolphins which "makes sense of the senses" is larger than the human parietal and frontal lobes combined (1). The similarities do not end there, most cetaceans have large and well developed temporal lobes which contain sections equivalent to Broca's and Wernicke's areas in humans (1).

Another major difference between primate and cetacean brains is that the primate brain favors the motor cortex, while "the cetaceans greatly favor the sensory region (and are not very balanced at all between the two)" (1). In the final measure of brain complexity, neural density dolphins also measure up quite favorable to humans. In certain areas of the brain concerned with "emotional control, objectivity, reality orientation, humor, logically consistent abstract thought and higher creativity" dolphins have an higher ratio of neural density(1). This seems to be correlated with dolphins ability to maintain a healthy emotional state while in captivity; humans in analogous situations often don't fair as well emotionally.

Despite the complex structures discovered in the brains of dolphins, primates have been the main focus of intelligence research. One of the main reason for the focus on primates is that they seem to be so similar to humans in many ways, the most important being genetically. Besides basic genetic similarities, or perhaps because them, primates have many advanced brain structures some of which are similar to human brain structures. Most primates have and EQ the around 2.34 (2) while humans have an EQ around 7. This may seem like are large difference, but when you consider that very few species have an EQ above 1, the difference is less profound. In addition to similarities in size and structure "primates devote more of their energy resources to their brains than do most other mammals", which is another important measure of intelligence(3).

Before non-human animals can be seen as intelligent a model must be created which can explain how intelligence would have developed through evolution. These theories must clearly establish how having a larger more sophisticated brain, could increase and organism's chance of survival. Once this question is addressed, then the question of whether the larger brain and the new behaviors it supports represents intelligence can be asked. Two of the hypotheses that deal with these issues are the foraging hypothesis and the social hypothesis.

The foraging model has been established mostly through research with non-human primates. The foraging model proposes that primate brains evolved in order to better allow them to remember the locations of sources of food across a wider range(3). In addition to the pressure to create better maps as to the location of food, some primates especially frugivores, also had to remember when fruits at various places would be ripe and worth collecting(4). This created another selection pressure towards more sophisticated brains. A third source of evolutionary pressure on primate brains was that primates had to become more efficient at collecting food over a wider range. In order to accomplish this primates had to develop "complex extractive techniques requiring extensive sensorimotor coordination presumably subject to cortical control"(3). These three selection pressure could have shape the evolution of primate brains individually, or more they all worked together to produce the primates we know today(3).

To support this hypothesis scientists have examined the differences between frugivore and foliovore monkeys. Frugivores generally have a much larger home ranges than foliovores and their resources are not as constant as foliovores', therefore according to the gather hypothesis frugivores should have a larger EQ. Research has demonstrated a correlation between the size of a home range and the EQ of the species, which would appear to support the gather hypothesis' prediction about furgivores and foliovores. But a more recent study found no relationship between the percentage of fruit in the diet and the EQ of the species. This study did not take into consideration the importance of fruit in the animals diet, so it still possible that the predicted difference may exist(3).

The second major theory about the evolution of intelligence is the social model. To live in a complex social group an organism must be able to form complex mental maps which represent the social hierarchy of the group. The organism must know where it stands in relation to all other members of its group as well as where other members stand in relations to each other(4). In order to perform all of the comparisons and remember them, according to the social hypothesis, an organism would need a more complex brain. Besides keeping relationships straight, organisms within a society also form alliance with other member of the group to create a more beneficial situation for themselves. This example of using other members of a groups as tools is another indication of intelligence required to maintain a social group(3).

The social hypothesis seems to be partially supported by dolphins and chimpanzees. Both dolphins and chimps live in complex societies in which there is constant alliance formation especially during mating season (4). And these two species also seem to posses brain complexity near that of humans, which would follow from the social theory. The problem with the theory is that the complexity of social systems has not be as well examined in other species, so it is difficult to conclude that complex social structure cannot exist without a complex brain. Another problem with the theory is that orangutans and gorilla's which don't have a complex social groups as chimps or dolphins also have high EQ's and complex brain structures(4).

Although both of these evolutionary theories seemed sound, there are some basic theoretical questions that may cast some doubt on their validity. One of these is the question of directional causality. It quite possible that the pressures to find food and form social groups did not select for organisms with larger brains, but instead as creature brains developed the were able to utilize more complex gathering strategies and adept and handling social relationships(3). It also may be that one new ability is a by-product of the system created for other purposes. For example it may be as the selection of organisms who could be remember the location of the food better changed the complexity of brain structures of the species, the species began to develop the ability to map social hierarchies(3). This in turn allowed for the formation of more complex societies. These new societies could have added selection pressures of their own or served to maintain the traits selected for by the need to find food.

Throughout this paper and much of comparative evolutionary theory, a basic assumption has been made; that the complexity of an animals brain coupled with the presence of certain behaviors if good evidence of an intelligence which can be on some level compared to humans. In fact just the presence of complex brain structures is often used as a clue to look for behavioral indications of intelligence. Despite this common assumption there are many researchers who feel that it is too early to make a statement either way about animal intelligence. This caution is probably best summed up in

Morgan's Cannon of interpretation which states:

In no case may we interpret an action as the outcome of the exercise of a higher psychical faculty, if it can be interpreted as the outcome of the exercise of one which stands lower in the psychological scale (5)

This statement is extremely relevant to comparative psychology because it calls for great skepticism in attributing intelligence to animals other than humans just because there appears to be no other explanation for an observed behavior or because the proper neurology seems to be present.

Another philosophical argument for caution in comparative studies comes from Scott Rifkin (3). He believes that not enough is known about human and primate brains to assume that similarities in overt behavior have the same neurological base. The reason behind his trepidation at accepting current evolutionary theories on intelligence is that "there is no a priori reason to expect that brain evolution has proceeded regularly without reorganization of structures or redistribution of functions". He goes on to argue that before we can begin to make comparative statements a better understanding of the functioning of others species brains needs to be achieved.

Perhaps the most thought provoking question raised in comparative psychology is the question of generalized intelligence. It has been difficult in modern psychology to get researchers to agree on a general human intelligence, to extend that intelligence across species may be to great a theoretical leap. This idea is supported by Scott Rifkin who believes that "to assume a continuum of intelligence across today's species is incompatible with an evolutionary perspective, and this preconception must not be allowed to guide studies of brain evolution"(3).

It is an example of human hubris to assume that way in which we display the trait of intelligence is the best and only way to display that trait. This bais may blind us to a better understanding of intelligence as a universal trait. The best way to see intelligence as a universal trait is to view it and evaluate it as a factor within the environment in which it evolved. By using this method, human and non-human intelligence are considered to be inherently different because they evolved to facilitate the survival of an organism with two separate environmental histories. In this light it is impossible to say one is better than the other because it is a question of comparing how well and organism is adapted to its lifestyle not how well it is adaptive on any one scale. For example it is common to say that a dolphin is not as smart as a human because it doesn't use tools, but using this definition of intelligence it would also be valid to say that humans are not as smart as dolphins because they can't examine the internal organs of their other group members using natural ultrasound.

There is also the question of co-evolution of different structures to contend with when considering making cross species comparisons. It is possible that different structures in the brain could have evolved different functions in different species. For example if the skeletal structure of a bat wing and the human hand are compared it might be assumed that they have the same function because they have the a similar structure. This would obviously be a big mistake because evolution has shaped each skeletal structure to serve each species with a unique function. Following from this example, just because a species has an underdeveloped brain structure relative to a human it does not necessarily mean that it is less intelligent. It is quite possible that another structure has developed to serve the purpose that the particular structure serves in humans.

The arguments for caution that have been presented are extremely important to the field of comparative psychology and should not be easily forgotten. But one the basic tenants of the philosophy of science seem to be in support the notion of non-human intelligence. Occam's razor is a scientific principle which states that all things being equal the simplest solutions is often true. When applied to the evidence of non-human intelligence, the questions becomes whether it is simpler to assume that somehow human beings have transcended their biology and have developed a unique intelligence or that animals with similar anatomy to humans have some intelligence. It would seem illogical to assume that the complex structure of a dolphin brain, which in many ways rivals a human's, evolved only for hunting fish and getting mates and the rest is just useless space. The entire debate over the intelligence in non-humans and the usefulness of comparative psychology is a metaphor for the larger brain-behavior debate. In order for brain complexity to be an acceptable measure of intelligence the brain must be accepted as the source of all behavior. And if the brain cannot be accepted as the seat of behavior then there is no use in comparing species because the one connection that all complex organisms have is that the have a similar brain structure; behavior is way to species specific to be studied without the common thread of the brain.

The study of non-human primates and dolphins has lead to many profound questions as to the nature of intelligence. And thought the answers provided to date have been disputed, the questions are not any less worth of being asked. But in order to get beyond the disputes, researchers must be willing to shed there antrocentric view of intelligence and accept that it is an trait which can evolve like any other trait. When this is done it may be finally possible to recognize the remarkable abilities that some many people seem to find in animals as evidence of animal intelligence not lesser human intelligence.


1. A Comparision of Primate and Cetacean Mentality (May 1998)

2. Does Size Really Matter? (May 1998)

3. Untitled (May 1998)

4. Brain Size and Cognition in Primates (May 1998)

5. Historical Information about Animal Cognition (May 1998)

6. Grobstein, P. Insights From Complex Systems.

7. Grobstein, P. From the Head to the Heart.

8. Grobstein, P., Variability in Brain Function and Behavior.

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