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The dolphin brain
Dolphins have very large and folded brains, which (besides the large body mass) suggest exceptional intellectual capacity. According to brain researcher Pilleri, it can be called into question whether or not the brains of Homo Sapiens(human) are the highest in the rankings. Dolphins are easily trained and they learn exceptionally fast.

However, recent research has shown that the amount of braincells found in dolphins is comparatively small. Scientists are coming to the conclusion that the dolphin is probably less intelligent than once thought. Humans have many more braincells and can therefore learn and think a lot quicker.
  
          Brain of a human                   Brain of  the bottlenosed dolphin
 
If "intelligence" was simply determined by absolute brain size, there would be no difficulty in deciding which species was top. This table shows the approximate brain weights and body weights of mammals in order of brain weight:

SPECIES:                           BRAIN WEIGHT:        BODY WEIGHT:
                                          (approx.)                   (approx.)
                                          grams                        tonnes

SPERM WHALE (MALE)        7,820                       37.00
AFRICAN ELEPHANT            7,500                         5.00
FIN WHALE                          6,930                       90.00
KILLER WHALE                    5,620                        6.00
BOTTLENOSE DOLPHIN       1,600                        0.17
HUMAN                                1,500                        0.07
COW                                       500                          0.6

But as the species with the biggest brains also tend to be the ones with the biggest bodies, it might be that large animals just need larger brains to control and maintain their larger bodies. Even when we talk of "intelligence" in a general way, we mean something more than the sum of body control systems. A simple way to make allowance for different body weights is to express brain weight as a percentage of body weight, as you can see in the table below:

SPECIES:                          BRAIN WEIGHT AS % OF BODY WEIGHT:

HUMAN                             2.10
BOTTLENOSE DOLPHIN    0.94
AFRICAN ELEPHANT         0.15              
KILLER WHALE                 0.09
COW                                 0.08
SPERM WHALE (MALE)     0.02
FIN WHALE                       0.01

In this list humans are seen to have a great advantage over the others, and we also have a very different view of the large whales. Of course, these are only very limited lists to illustrate this type of approach to the problem.

Researchers have made far more extensive and sophisticated attempts to investigate comparative intelligence in this way. There is, however, a basic problem in compiling lists of this type, and that is deciding which weights to take as typical of a species. For example, normal humans can have brains weighing anything between 900 and 2,000 grams. The weight of an individual brain will also vary depending on whether it is fresh or preserved, and on exactly which parts are included. Body weight varies greatly between individuals, and in some whale species the weight of individual animals can vary by about 40% over a year because of their seasonal feeding habits. The brain weight to body weight relation-ship varies with age young mammals have proportionally smaller bodies and larger heads, and brain size decreases significantly in old age. There can be marked sex differences in body size, for example adult female baleen whales in many species are much larger than males, while in sperm whales it is the adult males which are much larger than the females. Normal variations in brain and body size have only been well studied in a, few species, and usually a researcher seeking to compile extensive brain and body weight lists has no choice but to take whatever specimens are available, regardless of whether the material is really representative of the species as a whole.

Some of the most extensive modern comparative studies have been made by Jerison (e.g.1978), who has developed an index, the encephalization quotient (EQ), to express the brain weight/body weight relationship. His studies do show some cetaceans (e.g. toothed whales like the killer whale and sperm whale) with an EQ similar to humans. However, other studies conclude that relative brain size is not necessarily related to "intelligence". Pilleri, Gihr and Kraus (1985) made an exhaustive study of rodent brain size in relation to behavior and concluded that "intelligence", whether human or animal, is not a unified brain function, but one which is too complex to be characterized with a single numerical index. They found that cerebral quotients (various ways of expressing relative brain and body size) are generally inconclusive as criteria for mammalian "intelligence".

The degree of convolution or folding of the cortical brain surface has often in the past been taken as an absolute indicator of "intelligence". However, more recent work regards this as simply a mechanical reflection of an increase in neocortical volume. Jerison (1979), for example, regards degree of convolution and absolute brain size as equivalent measures, speculating that the extra volume is required to accommodate increasingly complex connections between the brain cells. Ridgway (1986) presents evidence from a variety of sources to show that bottlenose dolphins have a much higher index of folding than humans. However, as Ridgway (1986) also explains, the neocortex of the cetacean brain is relatively thin - about half that of humans - giving a total average dolphin neocortical volume about 80% of that of humans. Also the cetacean neocortical structure is generally very much simpler than that of land mammals, and does not therefore conform to the assumptions that more convoluted neocortices are necessarily more voluminous or more complex. This is not the only anomaly, for example compare the convoluted appearances of the horse and chimpanzee brains.
 
Although there are a lot of experiments done, still a lot of questions remain unanswered....