Survival depends on the conservation of energy and big brains are calorie hogs. Therefore big brains are rare. They do things that (as far as we can tell) smaller brains don't have the computing power for.
Dolphins have the capacity for abstract thought and complex memetic communication, complete with regional dialects. How did such a brain develop?
Through the fossil record and genetics, we are able to piece together the development of Dolphins, like forensic scientists recreating a crime scene. Thirty-nine million years ago, odontocetes (an order inclusive of toothed whales, beaked whales, sperm whales dolphins and porpoises) diverged from the ancestral Archaeoceti group. For the ancestors of modern dolphins, body size decreased and brain size increased. Echolocation emerged which helped locate food and networked individuals more effectively. The interplay of selective pressures and the survival advantage created by these new survival strategies spurred the growth of the Dolphin brain. This interplay led to another growth spurt around Fifteen million years ago. The more a dolphin depended on communication to survive, the more environmental pressures selected for big brains.
Researchers are still trying to "crack the code" of Dolphin language, which due to its complex relationship with the ocean environment and Dolphin experience, has not been as easy to decipher as researchers in the 1960s had hoped. But researchers are increasingly aware that when a problem seems unsolvable, it often means we need to approach it in a new way, asking different questions and questioning assumptions.
Dolphins are creatures like us, who clearly have memetic communication built on top of protomemetic communication. Understanding the protomemetic framework which creates structure for memetic communication is our first step in decoding Dolphin language.