brain, the cetacean optic chiasm is completely crossed, and there is behavioral evidence for hemispheric dominance for vision. Brain evolution The evolution of encephalization in cetaceans is similar to that in primates. Though the general trend in their evolutionary history increased brain mass, body mass, and encephalization quotient, a few lineages actually underwent decephalization, although the selective pressures that caused this are still under debate. Among cetaceans, Odontoceti tend to have higher encephalization quotients than Mysticeti, which is at least partially due to the fact that Mysticeti have much larger body masses without a compensating increase in brain mass. As far as which selective pressures drove the encephalization (or decephalization) of cetacean brains, current research espouses a few main theories. The most promising suggests that cetacean brain size and complexity increased to support complex social relations. It could also have been driven by changes in diet, the emergence of echolocation, or an increase in territorial range. Problem-solving ability Some research shows that dolphins, among other animals, understand concepts such as numerical continuity, though not necessarily counting. Dolphins may be able to discriminate between numbers. Several researchers observing animals' ability to learn set formation tend to rank dolphins at about the level of elephants in intelligence, and show that dolphins do not surpass other highly intelligent animals in problem solving. A 1982 survey of other studies showed that in the learning of "set formation", dolphins rank highly, but not as high as some other animals. Behavior Pod characteristics Dolphin group sizes vary quite dramatically. River dolphins usually congregate in fairly small groups from 6 to 12 in number or, in some species, singly or in pairs. The individuals in these small groups know and recognize one another. Other species such as the oceanic pantropical spotted dolphin, common dolphin and spinner dolphin travel in large groups of hundreds of individuals. It is unknown whether every member of the group is acquainted with every other. However, large packs can act as a single cohesive unitobservations show that if an unexpected disturbance, such as a shark approach, occurs from the flank or from beneath the group, the group moves in near-unison to avoid the threat. This means that the dolphins must be aware not only of their near neighbors but also of other individuals nearby in a similar manner to which humans perform "audience waves". This is achieved by sight, and possibly also echolocation.