Cetacean intelligence: Knowledge about cognitive capabilities of the dolphin brain (and its related issue: the nature and magnitude of dolphin intelligence) is still limited. This article addresses some of the verifiable facts about the dolphin brain. It should be noted that there are many different species of dolphin (see the cetacea article for a full list), and so one should be careful about generalisations, because differences between dolphin species may be as marked as differences between humans and the great apes.
Research difficulties: 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.
In addition, the United States Navy has allegedly carried out a substantial amount of research which has not been put in the public domain. The US Navy does acknowledge that its dolphin programme has trained dolphins to search and tag mines and warn of divers approaching installations. Rumours circulate about less benign uses, but these are unsubstantiated.
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 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. But if dolphins were equipped with brains notably smaller than those of humans, it would make a powerful case against their having intelligence that approached that of humans (a certain amount of mass is necessary, after all, to allow for sufficient neural connections to be made). One of the major differences between humans and their nearest cousins the chimpanzees is that the human brain as compared to the chimpanzee brain is much larger in size, size proportionate to body, and proportionate size at birth.
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 bottle-nosed dolphin (Tursiops truncatus) brain weighs 1500-1600 grams, according to A. Berta's book Marine Mammals, quoted at this site (http://faculty.washington.edu/chudler/facts.html). Chimpanzee brains by contrast are only 400g.
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 (Klinowska, Margaret (1994), quoted at (2) However, 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. Bottle-nosed 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 favourably with humans across the indicators of pure brain mass, brain to body ratio, and comparative percentage of size at birth.
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 -- analogous 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.)
Researching the behaviour of dolphins in the wild is a difficult task. However several researchers have examined the social behaviour of dolphins and tried to extract from an understanding of the level of communication between individuals, which in turn is interpreted as a measure of intelligence.
Dolphin groups sizes vary quite dramatically. Older male Orca tend to lead
quite solitary lives but this is the exception. River dolphins usually congregate
in fairly small groups, from 6 to 12 in number. Researchers expect that the
individuals in these small groups may well know and recognise each other.
Other species such as the oceanic Pantropical Spotted Dolphin, Heaviside's
Dolphin and Spinner Dolphin travel in vast crowds, sometimes thousands in
number. It is extremely unlikely that every member of the group is acquitanted
with every other, as this would require more social interaction than found
in humans. However there is no doubt that such large packs can act as a single
cohesive unit - observations show that if an unexpected disturbance such as
a shark approach from the flank or from beneath the group occurs, the group
moves in near unison to avoid the threat. This means that the dolphins must
not only be aware of their next-door neighbours but also other individuals
near by - in a similar manner to which humans perform "Mexican waves".
This is achieved by sight, and possible also echolocation. One controversial
theory proposed by Jerison (1986) is that the pack of dolphins are able to
share echolocation results between each other to create a better understanding
of their surroundings. In the Encyclopedia of Marine Mammals Bernd Wersig
compares this to a group of humans being able to share exactly what they can
see with each other and so create a better 3D visual representation for all.
Jerison goes to speculate that these "shared data" echolocation
maps might account for the relatively large dolphin brain discussed above.
This idea has not received much formal backing in the literature.