Scientists have tried to capture the rich cognitive life of dolphins through field and laboratory studies of their brain anatomy, social lives, communication and perceptual abilities. Encopheliration quotient data sugest a level of intelligence or cognitive processing in the large-brained dolphin that is closer to the human range than are our nearest primate relatives. Field studies indicate a fission-fusion type of social structure, showing social complexity rivaling that found in chimpanzee societies. Notably, cetaceans are the only mammals other than humans that clearly demonstrate vocal learning and parallels in stages of vocal learning have been reported for humans, birds and dolphins. The dolphin's vocal plasticity from infancy through adulthood, in what is probably an 'open' communication system, is likely to be related to their fission-fusion social structure and, specifically, to the fluidity of their short-term associations. However, conflicting evidence exists on the composition and organization of the dolphins whistle repertoire. In general, the level of dolphin performance on complex auditory learning and memory tasks has been compared with that of primates on similar visual tasks; however, dolphins have also demonstrated sophistrcated visual processing abilities. Laboratory studies have also provided suggestive, evidence of minor self-recognition in the dolphin, an ability previously thought to be exclusive to humans humans and apes.