Intelligence itself is hard thing to measure and knowledge about cognitive capabilities of the dolphin brain and dolphin intelligence is still limited. A number of different approaches have been adopted to try and quantify the intelligence of dolphins, whales and other marine animals but it should be noted that there are many different species of dolphin and we should be careful about making generalisations- differences between dolphin species may be as marked as differences between humans and the great apes.

Knowledge about the capabilities of the dolphin brain is limited because of major research difficulties. Research of cetacean behaviour in the wild is among the most expensive and difficult to carry out, owing to the nature of the environment they inhabit. There have therefore been relatively few scientific studies of dolphins in the wild, and most direct observations are anecdotal. Studies based on captive dolphins have limits, because it is not clear how natural their behaviour is under those conditions.

One method that has been employed is the ease with which an animal can be trained. Dolphins and killer whales that learn tricks could be said to have intellect. The problem with assessing intelligence in this way is that we only have a human perspective. Tests on dolphins (or any other creature) are carried out to measure responses or reactions, which can be compared to humans. Using this method it would also follow that dogs should be more intelligent than cats! Therefore these experimental results are biased by the technique used, and may not accurately measure the original criteria of intelligence at all.

Brain Size and Mass

Some attempts to resolve the issue of dolphin intelligence have focused on various indicators concerning the size of the dolphin's brain.The pictures below show the comparative brains of a human, dolphin and cow.

The human brain is typically 3 times heavier than that of a cow, while sperm whales have brains more than five times the size of a human. As a percentage of body weight the human brain accounts for 2.1% and the bottlenose dolphin 0.94% however brain size alone does not signify intelligence.

The relationship between brain mass and intelligence is a shaky one, at best. Cognitive ability, according to most scientists, is dependent on the quantity and quality of connections between brain cells, and not on mere brain mass. Research into the human brain has revealed that the development of the cerebral cortex is also an important factor. This is where analysis of sensory information takes place and conscious responses to stimuli originate. The cerebral hemispheres in human and dolphins are both large and posses convoluted folding.

However, the cerebral cortex is much thinner in dolphins than in humans and more closely resembles those found in cows and sheep. Whilst studies of dolphin brains have shown that they are less developed than a human brain they are more developed than other mammals such as chimpanzees, although there are parallels in the organization of dolphin and primate brains.

Most dolphin species have brains that are roughly equal in weight to the average human brain: for example, the average human brain weighs 1300-1400 grams, while the average bottlenose dolphin brain weighs 1500-1600 grams. Chimpanzee brains by contrast are only 400 grams.

However, many researchers believe that brain mass of itself is a poor measure because it makes no allowances for body size.

Negative evidence against dolphins being as intelligent as humans is the fact that the dolphin brain to body ratio is less than half that of humans on average, however a bottlenose dolphin is second only to humans in the ratio of brain size to body size. It is not clear that direct comparisons of species that occupy such different habitats is appropriate, given these differing habitats make hugely different demands on bodily functionality. For instance, cetaceans have a high percentage of body weight in blubber, which principally helps them deal with the effects of water temperature. In the case of bottle-nosed dolphins blubber takes up 18-20% of body weight.

Other researchers have asserted that an important measure is the size and complexity of brain at birth. This is an extremely positive indicator for dolphin intelligence. Bottlenose dolphins begin life with very large brains: at birth they have a brain mass that is 42.5% of an adult human's brain mass, in comparison with humans, who at birth have 25% of adult brain mass. By eighteen months, the brain mass of Bottle-nosed dolphins is roughly 80% of that of an adult human. Human beings generally do not achieve this figure until the age of three or four.

The true value of various comparisons of brain mass between dolphins and human is debatable. Comparisons of humans to closely related species like the Great Apes would seem appropriate, since our original habitats and thus bodily functionality are very similar. However, one needs to be careful of directly comparing a land based species and water based species, because their habitats make hugely differing demands. It should be noted, however, that no other species seems to compare so favorably with humans across the indicators of pure brain mass, brain to body ratio, and comparative percentage of size at birth.

Self-awareness

The ability to possess self-awareness shows a highly developed abstract thinking. Self-awareness is the precursor to more advanced processes like meta-cognitive reasoning (thinking about thinking) that are typical of humans. Scientific research into self-awareness has suggested that Bottlenose Dolphins possess self-awareness. Dolphins differ markedly so an assessment can not be made for all species, some of which have much smaller brain sizes and presumably different structures.

The standard test for self-awareness in animals is the mirror test, developed by Gallup in the seventies, in which a temporary dye is placed on an animal's body, and the animal is then presented with a mirror. Most animals react to a mirror as if it is another animal. However, like great apes, dolphins have been shown to recognise the mirror image as themselves, by examining the marking on their body. Evidence for mirror recognition by dolphins was anecdotal until the nineties, but the scientific studies carried out by researchers Marten and Psarakos (1994, 1995) and Reiss and Marino (1998) confirmed it.

Some scientists still disagree with these findings arguing that the results of these tests are open to human interpretation. This test is far less definitive than when used for primates because primates can touch the mark or the mirror, while dolphins cannot, making their alleged self-recognition behaviour less clear. Critics argue that behaviours that are said to identify self-awareness resemble existing social behaviours, and so researchers could be mislabelling social responses to another dolphin. The researchers counterargue that the behaviours shown to evidence self awareness are very different to normal responses to another dolphin, including paying significantly more attention to another dolphin than towards their mirror image. Dr. Gallup called the results "the most suggestive evidence to date" of mirror self-recognition in dolphins, but "not definitive" because he was not entirely clear that the dolphins were not interpreting the image in the mirror as another animal.

Differences from other mammalian brains

Although dolphins are themselves mammals, their brains are constructed and act differently than those of most mammals. Unlike most mammalian brains, which have six neocortical layers, dolphins have five. While most sleeping mammals go through a stage known as REM sleep, dolphin studies have not shown any brain wave patterns associated with REM sleep. Unlike terrestrial mammals, dolphin brains contain a paralimbic lobe, which may possibly be used for sensory processing.

Dolphin brain stem transmission time is faster than that normally found in humans, and is roughly equivalent to the speed found in rats. As echo-location is the dolphin's primary means of sensing its environment (comparable to eyes in primates) and since sound travels four and a half times faster in water than in air, scientists speculate that the faster brain stem transmission time, and perhaps the paralimbic lobe as well, support speedy processing of sound. The dolphin's dependence on speedy sound processing is evident in the structure of its brain: its neural area devoted to visual imaging is only about one-tenth that of the human brain, while the area devoted to acoustical imaging is about 10 times that of the human brain. (Which is unsurprising: primate brains devote far more volume to visual processing than almost any other animals, and human brains more than other primates.)

The question remains "what is meant by intelligence?" and how can human concepts of intelligence be applied to an animal that lives in a medium so different to our own?

In its broadest sense intelligence is said to imply some comprehension of cause and effect i.e. the ability to think and often involves the mutual conveyance of information i.e. communication. Some species have demonstrated a greater capacity to learn than others and some are more resourceful. Some researchers say that in our enthusiasm to animate dolphins, we give them powers they just don't possess. A closer look suggests that much of the dolphin's large brain is taken up with echolocation and handling acoustical information processes at which they excel. But dolphins tend to rank at about the level of elephants in some "intelligence" tests and haven't shown any unusual talent at problem solving. They are excellent mimics of sounds and clearly communicate with one another, but does that mean they "talk?" No one less than Aristotle once wrote, "The voice of the dolphin in the air is like that of the human in that they can pronounce vowels and combinations of vowels, but have difficulties with the consonants." But a more scientific analysis of dolphin sounds suggests that for all their communication skills, dolphins lack the repertoire to have anything approaching language as we know it.

Today's research provides tantalizing clues pointing to significant intelligence, but few final answers. Dolphin intelligence is a subject that is certain to be debated until humans and dolphins are capable of communication with each other.